Development of the basin of Lake Ladoga from the standpoint of glacial theory. Ladoga lake

In the concept of “lake”, a hollow and the water mass that fills it are an inseparable whole. For the formation of a lake, it is necessary to form a basin and fill it with water for a long time. The basin is filled more often with river and groundwater, precipitation, less often waters of marine origin.

The formation of lake basins occurs under the influence of endogenous (internal) and exogenous (external) processes. Usually, several processes are involved in the formation of the modern appearance of lake basins, but one or a group of these processes are leading. The best known is the genetic classification of lake basins proposed by M.A. Pervukhin (1937), the basic principles of which were taken as a basis for the development of classifications by other authors. The main genetic types of lake basins, or, otherwise, lakes, according to the nature of their origin, are as follows:

Tectonic- are formed in troughs of the earth's crust on the plains (Ladoga, Onega, Ilmen), in troughs in the mountains (Markakol, Sonkel, Issyk-Kul, Alakol), in foothill depressions (Balkhash), in rift depressions (Baikal, Tanganyika). Most tectonic lakes are large in area and depth.

Volcanic- arise in craters and calderas of extinct volcanoes (Lake Kronotskoye, Kurilskoye in Kamchatka, lake turquoise on Simushir Island), in deepenings of lava covers (Lake Komarino in Iceland), in maars (Laker Laherskoye, Germany).

meteorite- are formed in depressions that arose during the fall of meteorites (Lake Kaali in Estonia).

Glacial- their occurrence is associated with the exaration-accumulative activity of ancient and modern glaciers. Numerous lakes in Karelia and Finland owe their origin to the exaration activity of the glacier. They are often elongated in the direction of glacier movement. This group also includes cirque and trough lakes. Karovye arose in kars and cirques - niche-like depressions on the upper slopes of the trough mountains - in the trough valleys (Lake Geneva, Baduk lakes in the Caucasus, etc.). Distributed in the Alps, the Caucasus, the Tien Shan and other mountainous countries.

Failed- lakes, the basins of which arose as a result of leaching of soils and rocks by surface and mainly underground waters, as well as during the thawing of permafrost soil or the melting of ice in it. The sinkhole lakes include: a) karst, b) suffusion and c) thermokarst lakes (in Yakut - alas). The latter are common in the tundra and taiga zones of the permafrost region. The basins of suffosion and thermokarst lakes often have an oval shape, poorly indented shores and shallow depths.

eolian- lakes that have arisen in the hollows of blowing, as well as between dunes and dunes. With rare exceptions, they are small in size and shallow (lakes Selety, Teke in Kazakhstan).

Podprudnye- the emergence of these lakes is associated with mountain landslides, landslides blocking river valleys, damming of rivers by lava flows, moraines of glaciers. Thus, dammed lakes are formed under the action of several processes. So, as a result of landslides caused by an earthquake, Lake. Sarez in the valley of the river. Murgab in the Pamirs, lake. Gekgel - in the valley of the river. Aksu in Azerbaijan, lake. Sevan, which arose in a tectonic depression, dammed by a lava flow. Organogenic- intramarsh lakes and lagoon lakes among coral structures (atolls).

16. Lake districts in Russia.

There are more than 2 million lakes on the territory of Russia. Basically it's not big lakes with a water surface area of ​​less than 1 km2. There are few large lakes. Two lakes in Russia - Baikal and Ladoga - are among the 18 largest lakes world (the area of ​​each of them is more than 10,000 km 2), Lake Onega is close to them. by the most deep lake world is Lake Baikal (maximum depth 1637 m). The lake content of Russia is 2.1%.

On the territory of Russia, lakes are distributed extremely unevenly. While in some areas they are relatively rare or completely absent, in others, on the contrary, the number of lakes is very large, and they occupy a significant part of the surface, in some places up to 10-50% of the total area of ​​the region. On the territory of Russia, the following lake areas can be distinguished, which are characterized by a large accumulation of lakes:

1. Northwestern Lake District- one of the largest lake areas. In literature, it is known as the Lake District. This vast region covers the territory of the Karelian-Finnish SSR, the Kola Peninsula, the Leningrad, Pskov, Novgorod and Velikolukskaya regions. On the territory of the Karelian-Finnish SSR alone, there are about 42,000 lakes, which occupy an average of up to 10% of its surface. Within Northwestern region, along with a large number of small and medium-sized lakes, there are such large lakes as Ladoga, Onega, Beloe, Ilmen, Chudsko-Pskovskoye, Vygozero, Segozero, Kovdozero, Pyaozero, and many others. etc. The abundance of lakes in the North-Western region is closely related to the Quaternary glaciation, and the origin of the basins is closely related to the accumulative and erosive activity of the glacier. It is characteristic that the boundary of this region coincides rather closely with the boundary of the last glaciation. Along with lakes of glacial origin, tectonic lakes are also common. This type includes most of the lakes of Karelia and the Kola Peninsula, developed in cracks and faults of hard stone rocks and having a characteristic orientation (their shape is elongated in the direction of the main faults of the earth's crust). Lake basins tectonic origin subsequently, they were largely reshaped by the erosive activity of the glacier, which is especially clearly seen on the example of the northern shores of Lake Ladoga and Onega. Among swamps and marsh massifs, there are often numerous lakes of secondary origin, formed during the development of swamps. There are many such secondary lakes in the territory of this region, especially among the swamps of the Lovatsky lowland (Polistovsky swamp massif, etc.). In places of shallow occurrence of easily soluble rocks (limestones) there are karst lakes. These include many lakes of the Valdai Upland, lakes of Obonezhya (between lakes Onega and Bely), the Onega basin, and others. Some of them periodically disappear.

2. Azov-Black Sea lake The area includes a large number of peculiar lakes located along the coast of the Black and Azov Seas. The origin of these lakes is connected with the activity of the sea, and most of they are estuaries. The most famous estuaries are Khadzhibeysky, Kuyalnitsky, Tiligulsky, Molochny, etc.

The origin of the estuaries here is due to the advance of the sea on land and the flooding of the mouth sections of the rivers. Their characteristic feature is that they are usually elongated in the direction of flooded river valleys, and are separated from the sea by sandy spits. In cases where the estuary is formed at the mouth of a large abounding river, then communication with the sea is free, since excess water is discharged into the sea in a wide stream. In cases where estuaries are formed at the mouths of relatively small rivers, the spit almost completely separates such a reservoir from the sea, leaving only a narrow strait, called the girl; An example is the Dnieper Estuary. The estuaries, into which the rivers, insignificant in terms of water flow, are separated from the sea tightly and lose contact with it; filtration through the barrier is usually preserved.

In addition to estuaries, a significant number of lakes on the Azov-Black Sea coast belong to the lagoon type. Lagoons are formed as a result of the separation of shallow bays from the sea by spits. Some of them, like the estuaries, retain a connection with the sea through the vent, while some are cut off tightly and sometimes subsequently desalinated. A typical lagoonal body of water is the Sivash, separated from Sea of ​​Azov long Arabat arrow. Other examples of lagoons are some lakes of the Crimea, for example, the famous Evpatoria lakes (Sasyk-Sivash, Saki). Most of the lakes in this area are saline or mineral and have great importance for the chemical and salt industries. Silt deposits (mineral mud) in many of these lakes have healing properties.

3. Caspian lake region covers a large group of lakes in the Caspian lowland. Most of the lakes in this region were formed from the overflow of steppe rivers during spring floods. Typical for the area are shallow Kamysh-Samar lakes. In the Caspian lowland, temporary reservoirs are also widespread, called estuaries, which usually form in low depressions and are accumulations of melt water; with the onset of summer, they quickly dry out.

4. West Siberian Lake District includes numerous lakes of the steppe and forest-steppe zones of the West Siberian lowland. There are several tens of thousands of lakes here; in most cases they are small and are flat, saucer-shaped depressions. There are several lake groups in this area: 1) the lakes of the Baraba steppe, headed by Lake. Chany, 2) lakes of the Kulunda steppe, among which the largest is Lake. Kulunda, 3) lakes of the Ishim steppe, 4) lakes of the Trans-Urals. Their food is exclusively due to melted snow water. During the period of snowmelt, the lakes increase significantly in size, and in the summer they are greatly reduced, and at this time many of them completely dry up.

5. Altai Lake District characterized by the presence a large number lakes, developed mainly in cirque basins, characterized by rounded outlines and small sizes. The largest lakes in the region are one of the most beautiful mountain lakes - Teletskoye and Lake. Mark-Kul. The group of lakes in the region can conditionally include a large shallow lake. Zaisan, located in the Irtysh valley.

6. Zabaikalsky lake district. The lakes here are mostly remnants of disappeared larger reservoirs. Among them are the vast, now almost dried-up hollows of Zun-Torey and Barun-Torey.

7. Lower Amur Lake District. Within the lowland that accompanies the lower course of the Amur, there is a significant number of large reservoirs, and the area of ​​some of them reaches 100-750 km 2. Such, for example, are the lakes: Petropavlovskoye, Bolen, Evvo, Kizi, Kadi, Orel, Chlya, Chukchagirskoye, etc.

8. Yakutsk Lake District covers the territory of the Leno-Vilyui lowland and the Leno-Amga watershed. There are several tens of thousands of small lakes here. The origin of the lakes is connected with the phenomena of thermokarst.

Which was located in the north and protected the approaches to Veliky Novgorod. The ancient name of the lake - Nevo - was gradually forgotten, remaining only in the name of the Neva River flowing from Ladoga.

Lake Ladoga is of glacial origin. About 12 thousand years ago, the edge of the glacier, which extended south of the Gulf of Finland, retreated to the north and the large depressions that it occupied filled with water. Then Ladoga and appeared.
The shores of Ladoga are very diverse. Northern - rocky, composed of crystalline rocks- indented by narrow bays with a mass of small islands - skerries. The remaining shores are predominantly low and gentle, strewn with soft sand or swampy. Lake Ladoga at first had a flow in a northerly direction. But when, along with the rise of the Karelian Isthmus, the northern shores lake, the water from it began to overflow through the southern watershed and gradually developed a runoff channel for itself - the bed of the Neva River.
Fogs are frequent and very windy on Ladoga, strong storms often occur, therefore, according to the terms of navigation, the lake is equated to the seas. On the southern coast, due to frequent storms and shallow depths, a system of bypass channels has been built for the passage of ships.
Winds, temperature difference and water density cause a kind of circulation in the lake basin. Water masses rotate counterclockwise, moving at a speed of 200-350 m/h. Sometimes, in the summer, the speed of water movement reaches 2-2.5 km/h. In December, Ladoga begins to freeze gradually and only by mid-February is it completely covered with ice, which reaches a thickness of 1 m. Already in March, the ice begins to melt, the full opening of Ladoga ends in the first decade of May. Part broken ice carried by the Neva to the Gulf of Finland.

lake of life

Lake Ladoga has long been of great economic importance. For a long time, fishermen lived on the shores of Ladoga, who hunted for catching lake fish, of which 58 species live here. The overwhelming majority of them are permanent inhabitants of the lake, and only Baltic salmon and sturgeon, Neva lamprey and sea eel are guests coming here from the Neva, the Baltic and the Gulf of Finland. Valuable commercial species include salmon, pike perch, trout, whitefish, vendace, etc. The role of Ladoga is great as highway. Through the Neva, the lake is connected with, and through the Volga-Baltic Canal, Vyshnevolotskaya and Tikhvinskaya water systems- With . Ladoga is connected to the White Sea through the Svir River, Lake Onega and the White Sea-Baltic Canal. The shores of the lake are quite densely populated. Novaya Ladoga, Sortavala, Priozersk, Shlisselburg - these former small settlements have turned into industrial regional centers. Ladoga played a great role during the Great Patriotic War. The siege of Leningrad lasted 900 long, hard days and nights. People were dying of hunger and cold. The northern and southern shores of the lake were captured by the Nazis, but part of the eastern and western shores were held by Soviet troops. And when the frosts bound the lake, several paths were laid on the ice, along which, from November 22, 1941, columns of motor vehicles walked around the clock. They brought food to Leningrad, and from the city - sick and wounded Leningraders. It happened that the ice could not withstand the load, broke through - and the precious cargo went to the bottom. Today, the bottom of Lake Ladoga is a continuous cemetery, where the skeletons of boats and ships, weapons of different times and calibers, and much more play the role of monuments. Exploration work is constantly taking place on the lake. From the bottom they raise a lot of various objects, evidence of different eras and historical events.

Numbers

Length: from south to north 219 km, from west to east 138 km.

Max Depth: 230 m
Volume of water: 908 km3.

Number of islands: about 660, the most famous of them -.

Curious facts

A1. What animals can live in this zone?
Among the plants, mosses and lichens dominate. Bird colonies are located on the rocky shores in summer. The sea feeds the animals.

1) polar bears
2) reindeer
3) arctic foxes
4) lemmings

A2. Define the steppe zone according to its description.
The "forest" is knee-deep, even ankle-deep. The trees are a little larger than a mushroom. The dwarf trees "kneeled down", crawled along the ground, clung to it, hiding under a cloak of light.

1) arctic desert
2) tundra
3) taiga
4) steppe

A3. In what part of the taiga do cedar forests join spruce-fir forests?

1) to the taiga of the Russian Plain
2) to the taiga of the West Siberian Plain
3) to the taiga of Eastern Siberia
4) to the Ussuri taiga

A4. What breed occupies the largest areas in the Russian taiga?

1) spruce
2) fir
3) pine
4) larch

A5.What is main reason lack of forests in the steppe?

1) low rainfall
2) insufficient moisture
3) high air temperature in summer
4) infertile soils

A6. Indicate the incorrect statement.
1) Corn, wheat, sunflower are grown in the steppe
2) Trees grow in river valleys and along the beams
3) Chernozems are widespread in the steppe
4) Steppe plants have a powerful ground part and a superficial root system

A7. What birds are typical for the taiga?
1) bustard, little bustard
2) hazel grouse, capercaillie
3) little bustard, capercaillie
4) hazel grouse, bustard

A8 What natural area extends from the western borders to the Pacific coast?
1) taiga
2) mixed forests
3) steppe
4) semi-desert

A9 What tectonic structure underlies the East European Plain?
1)young platform
2) area of ​​ancient folding
3) an area of ​​​​medium folding
4) ancient platform

A10 What factor influenced the relief of the north of the Russian Plain?
1) tectonic movements
2) ancient glacier
3) water erosion
4) aeolian processes

A11. What is the origin of the basins of lakes Ilmen, Peipus, Pskov?
1) tectonic
2) karst
3) residual
4) glacial

A12. What causes frosts and a sharp drop in temperatures in autumn and spring on the territory of the Russian Plain?
1) westerly winds
2) cyclones
3) arctic air masses
maritime temperate air

A13 What features of nature are not characteristic of the Russian Plain?
1) the continentality of the climate increases from northwest to southeast
2) In the Far North of the Russian Plain there is a zone of Arctic deserts
3) the steppes are plowed almost everywhere
4) the largest lakes of the Caspian lowland - Elton and Baskunchak

A14 On the territory of what "all-Russian interfluve" of the Russian Plain is Lake Seliger located?
1) Northern Ridges
2) Central Russian Upland
3) Valdai Upland
4) Timan Ridge

A15 What natural complex of the Russian Plain has been most strongly changed by man?
1) tundra
2) taiga
3) mixed and broad-leaved forests
4) semi-desert

A16 What river does not originate on the Valdai Upland?
1) Western Dvina
2) Volga
3) Dnepr
4) Northern Dvina

A17 What cultural monument is located on the White Sea?
1) Valaam
2) Kizhi
3) Solovetsky monastery
4)Szdal

A18 What city North Caucasus is not part of the Caucasian Mineralnye Vody resort area?
1) Pyatigorsk
2) Essentuki
3) Kislovodsk

The history of the formation of the lake

Ladoga lake

Ladoga lake(Also Ladoga; historical name - Nevo listen)) is a lake in Karelia (northern and east coast) And Leningrad region(western, southern and southeastern coast), the largest freshwater lake in Europe. Refers to the Baltic Sea basin of the Atlantic Ocean.

The area of ​​the lake without islands is from 17.6 thousand km² (with islands 18.1 thousand km²); the volume of water mass - 908 km³; length from south to north - 219 km, maximum width - 138 km. The depth varies unevenly: in the northern part it ranges from 70 to 230 m, in the south - from 20 to 70 m. On the shores of Lake Ladoga, there are the cities of Priozersk, Novaya Ladoga, Shlisselburg in the Leningrad Region, Sortavala, Pitkyaranta, Lahdenpokhya of Karelia. 35 rivers flow into Lake Ladoga, and only one, the Neva, originates. IN southern half lakes - three large bays: Svirskaya, Volkhovskaya and Shlisselburgskaya bays.

Etymology

In the ancient Russian Nestor chronicle of the 12th century, it is referred to as “the great lake Nevo”(there is no doubt a connection with the name of the Neva River (also cf. Fin. neva swamp, swamp). In the ancient Scandinavian sagas and agreements with the Hanseatic cities, the lake is called Aldoga(cf. fin. aalto- wave).

From the beginning of the 13th century, the name Ladoga lake derived from the name of the city Ladoga, in turn, named after the tributary of the same name in the lower reaches of the Volkhov River (fin. alodejoki- a river in a low area). Other variants of the origin of the name of the lake: from the Karelian word aalto(Karelian aalto- wave; hence Karelian. aaltokas- wavy); from the dialectal Russian word alod signifying open lake, vast water field.

Name Ladoga bears the river, the lake and the city. At the same time, until recently it was not quite clear which of the names is primary. The name of the city was derived from the name Lake Ladoga(from fin. *aaldokas, aallokas"worried" - from aalto"wave"), or from the name of the river Ladoga(now Ladozhka, from Finnish. *Alode-joki, Where alode, aloe- "low terrain" and jok(k)i- "river").

As T. N. Jackson writes, “by now it can be considered almost proven that the name of the river first arose, then the city, and only then the lake.” Therefore, she considers the primary hydronym Ladoga, from other Finn. *Alode-jogi (joki)"lower river" From the name of the river came the name of the city of OE. Aldeigja, and it was already borrowed by the Slavic population and transformed with the help of metathesis ald → lad in other Russian. Ladoga. The Scandinavian mediation between the Finnish and the Old Russian word is fully confirmed by archeological data: the Scandinavians first appeared on Ladoga in the early 750s, that is, a couple of decades earlier than the Slavs.

E. A. Khelimsky, on the contrary, offers a Germanic etymology. In his opinion, the name of the lake is primary - from other Scandinavian. Aldauga"the Old Open-Sea-Like-Source" This hydronym is associated with the name of the Neva (which follows from Lake Ladoga) in the Germanic languages ​​\u200b\u200b- "new". Through an intermediate form Aldaugja this word gave OE. Aldeigja"Ladoga (city)".

The history of the formation of the lake

The basin of Lake Ladoga is of glacial-tectonic origin. In the Paleozoic, 300-400 million years ago, the entire territory of the modern basin of Lake Ladoga was covered by the sea. Sedimentary deposits of that time - sandstones, sands, clays, limestones - cover the crystalline basement, consisting of granites, gneisses and diabases, with a thick layer (over 200 m). The modern relief was formed as a result of the activity of the ice sheet (the last, Valdai glaciation ended about 12 thousand years ago). After the retreat of the glacier, the Littorina Sea was formed, the level of which was 7-9 m higher than the modern level of the Baltic Sea. In the north of the Karelian Isthmus, the Littorin Sea was connected by a wide strait to Lake Ladoga. The Mga River at that time flowed to the east and flowed into the lake in the area of ​​the modern source of the Neva.

In the area of ​​Lake Ladoga, the land rose faster, and the lake eventually turned into a closed reservoir. The water level in it began to rise, and when it exceeded the watershed level, the lake waters, having flooded the valley of the Mga River, broke into the valley of the Tosna River. Thus, 4 thousand years ago, a strait arose between Lake Ladoga and the Gulf of Finland, which became the valley of the Neva River. The relief has hardly changed for the last 2.5 thousand years.

The northern part of Lake Ladoga lies on the Baltic Crystalline Shield, the southern part lies on the East European Platform. In the areas closest to Ladoga, the southern border of the shield runs approximately along the line Vyborg - Priozersk - the mouth of the Vidlitsa River - the source of the Svir River.

Climate

The climate over Lake Ladoga is temperate, transitional from temperate continental to temperate maritime. This type of climate is explained by atmospheric circulation and the geographical location characteristic of the Leningrad region. This is due to the relatively small amount of solar heat entering the earth's surface and into the atmosphere.

Due to the small amount of solar heat, moisture evaporates slowly. There are an average of 62 per year sunny days. Therefore, during most of the year, days with cloudy, overcast weather and diffused lighting prevail. The length of the day varies from 5 hours 51 minutes at the winter solstice to 18 hours 50 minutes at the summer solstice. The so-called "white nights" are observed over the lake, coming on May 25-26, when the sun drops below the horizon by no more than 9 °, and the evening twilight practically merges with the morning. The white nights end on July 16-17. In total, the duration of the white nights is more than 50 days. The amplitude of the average monthly sums of direct solar radiation on a horizontal surface with a clear sky is from 25 MJ/m² in December to 686 MJ/m² in June. Cloudiness reduces on average per year the arrival of total solar radiation by 21%, and direct solar radiation - by 60%. The average annual total radiation is 3156 MJ/m². The number of hours of sunshine is 1628 per year.

The lake itself has a significant impact on climatic conditions. This is characterized by the smoothing of extreme values ​​of climatic characteristics, as a result of which the continental air masses, passing over the surface of the lake, acquire the character of maritime air masses. The average air temperature in the area of ​​Lake Ladoga is +3.2 °C. The average temperature of the coldest month (February) is −8.8 °C, the warmest (July) is +16.3 °C. The average annual rainfall is 475 mm. The smallest monthly amount of precipitation falls in February - March (24 mm), the largest - in September (58 mm).

As a result of geological and geomorphological analysis with the involvement of specialized computer modeling systems, the general concept of the formation of the pre-Quaternary surface and, in many respects, the modern relief of the bottom of Lake Ladoga was discussed from the standpoint of glacial theory. Complex glacial and water-glacial denudation served as a determining factor in the development of structural-denudation forms of different orders. The allocation of an independent family of plain cirques of ice sheets is proposed. These, according to the authors, include the giant Severoladoga, today the largest in the world, as well as Landsort - the most deep depression the Baltic Sea. At the same time, the glacial cirque is interpreted as a basin in the form of an amphitheater with close values ​​of length and width, a steep frontal slope or ledge, pronounced lateral slopes and a rear threshold, usually located within the glacial flow, which created a characteristic pronounced contrasting relief profile in geologically and geomorphologically predetermined areas.

Key words : glacier , denudation , cirque , korri , Lake Ladoga , relief , geomorphology .

The general concept of the formation of pre-Quaternary surface and, in many ways, the modern landscape of Lake Ladoga is discussed as a result of geological-geomorphological analysis including specialized computer modeling systems, with implications of aspects of the glacial theory. Complex glacial and fluvio-glacial denudation served as a determining factor in the development of structural-denudational forms of various order. We propose existence and specification of the independent family of non-mountain cirques created by ice sheets. It includes giant Severoladozhsky (North Lake Ladoga) cirque representing the largest corrie in the world, as well as Landsort – the deepest basin of the Baltic Sea. In this case a glacial cirque is determined as an amphitheatre-shaped overdeepened basin, with close values ​​of length and width, steep headwall, side slopes and pronounced lip; the basin is usually located within the ice stream, which produces a typical specific contrast terrain profile in geologically and geomorphologically predetermined locations.

Keywords : ice sheet , glacial erosion , cirque , corrie , Lake Ladoga , terrain , geomorphology .

Lake Ladoga is a priority natural object for St. Petersburg. Some morphometric features of the basin bottom, such as the northern deep-water part with a contrasting relief, are typifying for the basin and, to some extent, specific for the region.

General characteristics. The basin of Lake Ladoga was formed within a fragment of the junction zone of the Baltic Shield and the Russian Plate, complicated by a large Riphean graben-syncline, called the Ladoga-Pashskaya [ Amantov, 1992; 1993]. This determines the specifics of the structure of the bottom of the pool. The features of the basin are associated with the distribution of non-metamorphosed Early Riphean and Late Riphean-Early Vendian (?) complexes that make up the graben-syncline and dominate within Northern Ladoga. Structural complexes of the Archean or Early Proterozoic age of the crystalline basement are typical of the Baltic Shield and are represented in the skerry zone. Plate (orthoplatform) mantle complexes, which began to form in the Late Vendian, are widely developed in the southeastern part of Lake Ladoga.

The expressiveness of the northern, deep-water, part of Ladoga in the modern relief, its contrast, and the features of the coastal zones formed the point of view about the neotectonic active zone [ Biske et al., 1974; Lake Ladoga, 1978; Usikova et al., 1970 ]. According to other ideas, the relief of bedrocks of the Ladoga basin and the nature of the distribution of Quaternary sediments are the usual result of selective denudation preparation of a fragment of the Riphean structure with the leading role of glacial gouging [ Amantov, 1988; 1992; History of Ladoga…, 1990 ].

Riphean sedimentary and effusive rocks hundreds of meters thick dominate within Northern Ladoga [ Amantov, 1992; 1993; Amantov et al., 1995; 1996]. Terrigenous formations are brought to the pre-Quaternary section mainly within the deep-water part of the basin. They are represented by red-colored and gray-colored sandstones, siltstones and mudstones. Several phases and episodes of the Riphean trap formation are noted. Somewhat earlier are lavas and sills known on the coast in the Salmi region. An example of younger magmatic manifestations is the large Early Riphean [ Lubnina et al., 2010] Valaam layered sill of gabbroids and syenites up to 150-200 m thick in the area of ​​about. Valaam [ Amantov et al., 1996]. The Riphean deposits of Northern Ladoga are characterized by a general centricline occurrence and somewhat large dip angles (a few degrees) in the marginal parts. In addition, wide and gentle complicating folds are characteristic, dipping up to 1-2° with a dominant northwest strike. More complex plicative dislocations mark some zones of pre-Late Vendian faults, which significantly completed the structural plan.

Terrain analysis. In a morphometric analysis of a model that includes relief grids, roofs of pre-Quaternary formations, pre-Late Vendian peneplain, pre-Riphean basement, faults [ Amantov, 2002], we used elements of an automated system prepared and tested by us in solving geological and geomorphological problems in platform areas, including those subjected to glaciation [ Amantov, 2007]. In particular, a complete analysis of the percentage distribution was carried out with the construction of abstract surfaces (vertex, 75% of heights, average heights, medians, 25% of heights, basic, natural classes [ Jenks, 1967] with an asymmetric distribution), their ratios (range, degree of dissection as the ratio of the Euclidean and three-dimensional distance between sample points) and other statistical parameters in moving square search windows of various dimensions. Some of them are shown in fig. 1.

Recall that the size of the window is a significant factor, especially if we recall one of the indicated features of a polycyclic relief (the older the surface, the higher sections it occupies, covers a smaller area and is less common) [ Amantov, 2007]. For areas subjected to intense glaciation, it is important to assess the nature of the distribution of Quaternary formations in order to exclude the influence of specific accumulative forms, especially those associated with terminal moraines. This is facilitated by the conjugated processing of the relief and the pre-Quaternary surface. In addition, it is necessary to take into account the selective effect of glacial and fluvio-glacial denudation, even if they have a lesser effect on relative elevations [ Amantov et al., 2011].

Although modern analysis of surfaces is undoubtedly informative and some of its results can even be used without modification in geomorphological zoning, it only complements traditional methods that focus on studying the age and genesis of morphostructures of various ranks. However, its correct application according to the proven methodology [ Amantov, 2007] allows us to draw preliminary conclusions about the possible genesis and history of development. For example, a sharp mismatch - as the search window increases to acceptable limits - of the calm pattern of the summit surface (Fig. 1) (and adjacent natural classes) with subsequent ones (in a row to the base surface) suggests a significant role of denudation dissection with the development of a contrasting lithomorphic relief, rather than tectonic movements of individual blocks. The subsequent automated correlation of the pattern of the former with interpolation of actually observed ones (such as the pre-Late Vendian peneplain) can, for example, provide information on the possible age of surface fragments at command heights, their degree of preservation, etc. Calculation of deviations from the median surface with an increasing window facilitates the selection of various generations of relative elevations and depressions with their further independent processing.

Northern deep-sea basin. The surface of pre-Quaternary formations in the depressions of the northern deep basin is located at elevations from -140 to -270 m. It can be conditionally divided into slightly deeper central, eastern and western (Fig. 1). These large forms are confined to the areas of development of Riphean sandstones, siltstones and mudstones, due to the sustained thickness of Quaternary deposits, which are similarly expressed both in the pre-Quaternary (primary) and in the modern relief. The Quaternary section is also monotonous, which is characterized by two seismic complexes identified with a cloak of moraine formations of the last glaciation and late-postglacial sediments 20-40 m thick, most of which, apparently, reflects a narrow time interval immediately after deglaciation, which is characterized by hurricane speeds sedimentation in isolated relief depressions.

Complicating ledges and asymmetric uplands can be traced within the basin, with a trend in many cases consistent with that of the Riphean complexes. Their representative Valaam (Fig. 2) is the most large island within the deep-water basin, dividing the basin into the central and eastern.

In addition, extended linear northwestern uplands stand out, the most clearly expressed is Vossinansaar, marked by the island of the same name. It is according to it that we conditionally divide it into the western and central depressions. There are also small (usually a few hundreds of meters in diameter) isometric, but contrasting positive landforms, distributed mainly to the north of Valaam.

All the mentioned morphostructures are united by their origin; they are armored by strong subvolcanic formations of predominantly mafic composition compared to non-metamorphosed sedimentary rocks. With high dissection, the scope of the contrasting lithomorphic relief on the surface of pre-Quaternary formations is up to 210 m in the narrowing of the axial part of the eastern basin between the islands of the Valaam ridge and the ridge of about. Mantsinsaari with a search window width of 7.5 km (Fig. 3). Somewhat smaller, but close values ​​are reached west of Valaam and along the northern and northwestern coasts. With an increase in the sampling window to 25 km, the values ​​of the relief range increase to 310 m, and the maximum shifts to the aforementioned coast, acquiring the shape of a characteristic horseshoe, typical of complex glacial erosion with a significant decrease in the bed slope angle.

The noted variety of contrasting forms, in our opinion, is due to the features of the manifestation of subvolcanic bodies and their geometry. A series of contiguous ledges and associated elevations along the northern coast, the Valaam and Mantsinsaar ridges, as well as the islands of the Western Archipelago (northwest and west of Vossinansaari Island) are associated with exposures of sills of predominantly mafic composition. The Vossinansaar Upland itself is a “tailed rock” [ Amantov, 1993]. Within the archipelago, in zones of reduced glacial activity, there are significant increases in the thickness of Quaternary deposits with the probable preservation of remnants of pre-Kulin deposits that survived from exaration, which are displayed as a layered seismic complex, correlated with those cut off in the coastal ledge of the southwestern coast and forming large positive landforms. Small isometric pillar-like hills, which are common north of Valaam, are either dissected stocks and fragments of dike bulges, or, in some cases, sill remnants. Here, due to the simulated change in the nature of ice movement, some expected increase in the thickness of glacial formations is also observed.

Speaking about the linear complications of the relief associated with the Riphean fault zones, we note the dominant northwestern ones, especially near the island. Vospominanii (Fig. 2), where the fault zone indirectly determined the orientation of the axis of the eastern basin. Here, according to the mentioned hill, we conditionally separate it from the central one. Also significant is the role of the northwestern and complementary northwestern sublatitudinal zones, passing directly to the north of about. Kilpisaret. Here they also controlled the outcrops of the Riphean complexes and, possibly, increased fracturing, which led to variations in the pattern of denudation, primarily due to the influence of the Valaam sill immediately southwest of Valaam on the contours of the outcrop. We also note the zones of disturbances of a wide and extended dike swarm close to meridional (330-350°). Their manifestation in the relief of pre-Quaternary formations is local and has different signs. Negative shapes are developed, probably, by fracture zones and accompanying plicative dislocations, which are often characteristic in the interpretation of seismoacoustic records in the case of insignificant dike thickness. Positive forms (in the form of small hills) were created in the case of dike bulges or local changes in the geometry of igneous bodies. In particular, one of the largest Valaam-Staraya Ladoga zone, with which the main supply channel of the Valaam sill magma is presumably connected, is intensely manifested within the skerry zone to the north of about. Memories, where it defines the configuration of several long fjords. Speaking about the system of azimuth disturbances in the skerry part of 350°, we note that in some cases this is the dominant direction of fracturing, combined with a system of fractures of azimuth 45-50, 110-120, 260-280°.

To the southeast, the northern deep-water basin is closed by the extremely flattened Konevets zone of inflection (extremely gentle swell) of the bedrock relief, which can be traced in a detailed analysis to the northeast of the island. Konevets and passing into a subhorizontal flattened plain of the central part of the basin. Thick (locally up to 70 m) glacial deposits are developed on the slope of the basin and in this zone, forming distinct characteristic hills of complex shape (Fig. 4, 5).

Geomorphological zoning according to other principles is given by D.A. Subetto [ Lake Ladoga, 2002 ].

Northern deep basin as the North Ladoga circus. To explain the origin of both the Konevets zone and the basin as a whole, let us recall some typical and in many respects obligatory features of classical glacial cirques (korri), to which, in our opinion, it belongs to genetic and morphological (with a similarity coefficient) features. Further, we refer to it as the Severoladoga Circus, which, if this interpretation is recognized, is the largest circus in the world.

This is a basin in the form of an amphitheater, confined to a glacial valley and having a pronounced asymmetric or radial chair-like profile, often significantly overdeep [ Lewis, 1960]. It also differs in the presence of a steep, often sheer frontal slope (FS) (headwall in English terminology), steep side slopes and a low closing marginal threshold of the inflection zones (IP) (lip), which is sometimes incorrectly called a crossbar, since this term is more applicable to circuses to complicating uplands of a distinctly lithomorphic relief. The WP is characterized by enhanced glacial accumulation, and pronounced marginal moraines are often present. Many patterns of cirque morphometry and their longitudinal profile can be quantified. The most important, in our opinion, is the ratio of the length, determined between extreme point FS to the axis of the ZP, to the width, estimated as the maximum distance between the side slopes. It is close to 1 (0.8-1.2 for 80% of morphostructures) in classical forms with a normal distribution of representative samples [ Graf, 1976; Nelson & Jackson, 2003] with some specifics of size classes and regional distribution [ Evans & Cox, 1974; Evans, 2006; 2009]. In other words, an ideal fully developed cirque tends to a circle (Fig. 2), although in cirques worked out in sedimentary rocks, like the Northern Ladoga basin, the width often somewhat exceeds the length [ Evans, 2006].

As the size of classical mountain cirques increases, the range of heights grows less than the length [ Evans, 2009], and in general it experiences significant variations. The size of circuses usually does not exceed a few kilometers, although Walcott in Antarctica was considered the largest [ Hambrey & Alean, 2004], which is a meteorite caldera reworked by glacial erosion with a diameter of about 60 km [ Harvey & Schutt, 1997]. The width of the North Ladoga circus is about 80 km with an average length-to-width ratio of 0.875 (0.95 without filtering the Mantsinsaar hollow, which we will discuss below).

It is quite natural that, despite the similarity (Fig. 2), each scale level is characterized by specific properties. We attribute to it a significantly lower ratio of length to relief elevation, which is explained by the limited possibility of glacial and water-glacial denudation, as well as potentially more significant internal variations. Circuses of huge size can only arise with a favorable combination of a number of factors. At the very least, long-term development of glaciation and an obligatory structural predisposition, which consists in the presence of arcuate or annular elements defining the FS, are required to limit geological bodies. In these elements, the orientation of the axis should be consistent with the direction of ice movement, which has the spatiotemporal ability to actively denudate them. In our case, we are talking about the presence of a potentially stable radial Bothnian-Ladoga glacial flow, the zone of which extended to a significant part of the Riphean Ladoga-Pashian graben-syncline, which led to selective denudation with the formation of a cirque.

Despite the possible revision of the degree of applicability of the Glenn relations describing the stresses of viscoplastic deformations in the near future, the current degree of knowledge of the physical processes of glaciers is well covered in the literature [ Cuffey & Paterson, 2010]. In our opinion, the tendency of cirques to a ring shape is caused by a directed change in the velocity of a viscoplastic glacial flow both in a horizontal section, where a normal known gradual increase from the edges to the center is predicted, and in a vertical section with a decrease in velocity towards the base. The combination with a different slope gradient of the bed and a theoretical change in the temperature field of the base, starting from the FS site, as well as the redistribution of the water component of the velocity variation, led to a change in the erosion pattern. The shape of the FS set the convergence of the flow to the zone of the ZP. At some moments, depending on the maturity of the form, the angles of inclination of the FS and the general gradient, one can speak of a rotational movement [ Ritter et al., 1995], although this apparently does not apply to all stages [ Amantov et al., 2011]. Very simplified, during pronounced glacier pulsation cycles, with sufficient power and favorable geological and geomorphological conditions, certain analogies arise with landslide processes that create circuses similar to glacial ones [ Geological Dictionary, 1973 ].

Complications of the general classical distribution of velocities are due to structural features of the substrate, such as outcrops on the pre-Quaternary section of igneous complexes that are much more resistant to denudation. They led to the division of the basal part of the flow into northwestern segments of different dynamics with a decrease in velocities and (or) a violation of the rotational motion vector within the island uplands, primarily such as Valaam and Mantsinsaari. As a result, the previously described differentiation with the isolation of large forms was outlined, and the WP somewhat deviated from the classical one in the direction of a decrease in the length of the cirque in the shady part of the uplands in relation to the dominant northwestern movement of the glacier. Deviations to the southeast to the west of Konevets and especially to the south of Mantsinsaari are associated with the expected ways of episodic discharge of pressure subglacial waters, which we will dwell on in more detail when describing the buried tunnel valleys of the central and southern parts of Ladoga.

As the dominant mechanisms of denudation impact, we note the plowing by splitting of large fragments (placing), glacial abrasion (korraziya) and the action of basal water flows, including abrasion, under-ice thermal abrasion, and erosion. By thermal abrasion, in this case, we mean the destruction of the base by the combined effect of freezing-thawing cycles and pulsating relaxation oscillations of the glacier, which are taken into account in computer simulation at certain stages [ Amantov & Fjeldskaar, 2013; Amantov et al., 2011] as an independent process, including accelerating the effect of plaking in certain areas. Fundamental for the deepening of the basin is the enrichment in the inflection zone of the FS profile with fresh strong abrasive, “supplied” to the body of the glacier by supracrustal framing rocks [ Amantov et al., 2011]. Here in the FS zone, by analogy with cirques [ Ritter et al., 1995] plaking processes dominated, including those that formed characteristic low-order relief forms in the skerry zone, which depended not only on the composition of metamorphic complexes, but even to a greater extent on the nature of jointing. Theoretically, they also dominated in some zones of inflection of the longitudinal profile marking the heights, although in all cases it is more correct to speak of complex denudation. It seems that the latter was more significant in connection with earlier glaciations [ Amantov et al., 2011]. Moreover, a change in the general direction of glacier movements during the Pleistocene is planned, the earlier was an unexpressed northwestern - north-north-western and northern. In particular, this is connected with the fact that in the region of the northwestern coast, the classical form of the cirque is hidden in the surface of the modern relief by remnants of older Pleistocene deposits.

Theoretical model of development in time with a significant role of regressive (backward) erosion of the FS in combination with a general deepening of the profile [ Gordon, 1977] cannot be transferred to the North Ladoga circus, where it was initially controlled by the northern side of the Riphean graben-syncline. However, one can speak of some gradual retreat of the slope inflection zone, which did not change the fundamental pattern of development, which requires the presence of some initial ledge. Its formation in the contact zone of rock complexes with sharply different erosion resistance, however, is quite natural. Supporters of a larger contribution of the Cenozoic preglacial denudation would like to explain the small value of the FS advance towards the shield, which is inconsistent with the average and even low rates of retreat of the scarps. The systems we have developed [ Amantov, 2007; Amantov & Fjeldskaar, 2013; Amantov et al., 2011] allow us to propose models of relief transformation in time slices, but this issue is beyond the scope of this article. However, it will not be a significant exaggeration to say that, under the existing conditions, the abstract summit surfaces reflect some possible features of the relief of pre-Quaternary formations, and under the condition of the future development of ice sheets without significant changes in the regional tectonic scenario, the base ones outline its subsequent changes (Fig. 1).

Anticipating objections that glacial cirques are genetically related precisely to mountain-valley glaciations and develop without pronounced predestination, let's try to discuss this issue. Indeed, classical circuses are characteristic of many mountain landscapes. Their origin is often referred to as glacial erosion in mountain troughs, but the issue is far from over. Turnbull & Davies, 2006]. Domestic scientists have always fairly divided glacial and landslide cirques [ Geological Dictionary, 1973 ]. The question, however, is whether the former can develop on their own. It seems to us that, from the point of view of modeling, the initiation of the development of a cirque in the highlands is difficult, at least without a primary ledge and a significant thickness of the glacier. It is highly probable that in many cases glacial and nival processes triggered landslides in already established zones with subsequent development and modification by complex glacial denudation. In connection with the above and the existence of a variety of definitions of glacial cirques in the world literature, we offer a more universal one that meets our understanding, glacial circus - a hollow in the form of an amphitheater with close values ​​of length and width, a steep frontal slope or ledge, pronounced lateral slopes and a rear rapid, usually located within a glacial flow that has created a characteristic pronounced contrasting relief profile in geologically and geomorphologically predetermined areas.

This definition is met by both small classical mountain circuses and the giant North Ladoga circus. However, it is natural that it should belong to an independent family of lowland cirques of ice sheets. Its closest relative, the authors consider the deepest basin of the Baltic Sea, Landsort [ Amantov & Amantova, 2012] (Fig. 6), where during the plowing of the Riphean structure, the conditions were identical to the northern part of Ladoga. Despite the general similarity of the nature of the basins on the margins of the Baltic and Canadian shields [ Amantov. 1988], there are no direct analogues in this part of North America, although there are similar less pronounced forms, such as the South Chippewa depression of Lake Michigan.

Subhorizontal and gently sloping plains of Central and Southern Ladoga. Naturally, the active action of glaciers did not end in the convergence zone corresponding to the ZP of the North Ladoga circus, steadily continuing to the southwest with decreasing differentiation. The subhorizontal flat plain of the central part of the basin (Fig. 5) on Riphean formations does not show noticeable variations in denudation even when younger complexes of the Vuoksa syncline are exposed to the erosional section, which differ significantly in physical properties, judging by the nature of the wave pattern of seismoacoustic profiles. To the southwest, it borders on alternating subhorizontal and gently sloping plains formed on Late Vendian deposits. The relief here is distinctly lithomorphic, clayey complexes are characterized by somewhat large slope angles. Its general character does not differ from that usually manifested on the slope of the general White Sea-Baltic negative form, folded by a slab cover [ Amantov, 1992; Amantov, 1995].

The main complication of the plains is buried valleys (incisions), which are a typical feature of the cover area (Fig. 5). The incisions shown in the southwestern part of the bottom of the basin continue further south and form a common system with those known in the pre-glint lowland, which determine the position of the river. Neva. These contrasting linear features of the pre-Quaternary and, to a lesser extent, modern surface were developed by episodic discharge under hydrostatic pressure of subglacial waters of the peripheral part of this ice sheet zone as a whole, and primarily of the North Ladoga circus, as they accumulated in depressions. Supraglacial waters entering through the system of fissures also took part, especially in the FS and WP zones.

The proposed formation scenario is as follows. In the outcrop zone of the Upper Vendian deposits, a certain growing counter gradient arose, which impeded the movement of ice in the basal layer. At the same time, at some stages, a short-term freezing of the glacier to the bed could begin with an increase in water pressure and its subsequent “rise” with a breakthrough. The cycles could be repeated many times. The buried valleys were worked out in easily denuded rocks of the cover, possibly being formed along the elements of the pre-glacial river network, although at different hypsometric levels. Many, but not all, channels once created were used by subsequent glaciations. As usual, the most pronounced valleys were formed along the edges of the glacial flow, adjacent to the framing uplands of the primary relief with a long-lived glacier with a colder, slow-moving basal layer.

The most pronounced is the extended Volkhov-Mantsinsaar valley, incised along the eastern coastal slope. Its thalweg was controlled by more stable complexes of the Late Vendian or to the north of the pre-Late Vendian basement. No faults controlling the valley with visible displacement of the Vendian layers were found (Fig. 7), which is generally typical. The control of the position of individual fragments by zones of increased microseismic fracturing is not excluded.

It seems that the intensification of valley incision was associated with the position of the edge of the ice sheet and the change in permafrost zoning, for which we used the corresponding author's software module (Geoglacier 2007–Permafrost 2010). After estimating the thickness of the ice sheet and its dynamics in time and space, with the significant role of automated analysis of relief forms of various orders, a diagram of the distribution of basal temperatures was prepared. Then, taking into account the heat flow, differences in the thermal conductivity of geological complexes and the nature of the occurrence of layers (to correct possible thermal anisotropy), the well-known Stefan problem was solved. It is possible that at certain stages in front of the glint zone removed from water exchange by the processes of continuous deep permafrost, under the warming effect of the more dynamic Botnica-Ladoga flow, a pre-glint zone of discontinuous permafrost developed in the marginal zone of the glacier with increased linear-focal migration of subglacial and underground artesian waters. Created best conditions creation of hydrostatic pressure during pulsating emissions of glacial waters into the system with vigorous erosion of the most compliant deposits. A secondary role was played by the change in the section of clayey and sandy strata (aquifers and aquicludes in the normal state) with slightly different temperatures and freezing rates.

As a result of a complex geological and geomorphological analysis with the involvement of specialized computer modeling systems, the general concept of the formation of the pre-Quaternary surface and, in many respects, the modern relief of the bottom of Lake Ladoga was discussed from the standpoint of modern glacial theory. Complex glacial and water-glacial denudation served as a determining factor in the development of structural-denudation forms of different orders. A broader interpretation of glacial cirques is proposed with the allocation of an independent family of plain cirques of ice sheets. These, according to the authors, include the giant Severoladozhsky - today the largest in the world.

The authors express their sincere gratitude to G.A. Suslov, M.A. Spiridonov and other employees of the Department of Marine Geology and Geoecology of VSEGEI, as well as V. Fjeldskaar and L. Kaslsu for fruitful discussion of a number of issues related in one way or another to this article.

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