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Entries / Geological location

Geological location

Categories / City Topography/Nature and Natural Phenomena

GEOLOGICAL LOCATION. St. Petersburg along with its outskirts is located on the convergence of two major tectonic structures: the southern Baltic Plate and the Northwest Russiаn Plate of the East European plain. The strata of the plate consist of corrugated metamorphosed rocks of granite-gneiss structure with intercalations of crystalline schist, which developed 1.7-3 billion years ago. They crop out north of St. Petersburg, lying 175-250 metres below the surface within the limits of the city. Granites, especially rapakivi granites are well-known as the material for numerous architectural monuments of the city, such as the pillars of St. Isaac's Cathedral, embankments of the Neva, the base of the Bronze Horseman etc. These rocks are also known for many boulders which are bedded on the surface in the outskirts of the city and at the higher undeveloped locations of St. Petersburg. The higher stratum is formed by sedimentary rocks of the mantle of the Russian Plate. Their foundation is made up of greyish argillaceous rocks with sandstones and gravel pertaining to the late Proterozoic Era (Riphean and Vendian periods). Above, the sedimentary rock layers of soft blue clay of the Early Cambrian period are deposited; the most notable fact about these layers is that the underground lines permeate here. The upper layer is formed by Ordovician formations which consist of resilient sandstone, schist and limestone; these rocks are found, for example, in Sablino caves on the Tosna River and were employed for the building of the pillars of Kazan Cathedral, the Stock Exchange and the foundations of many other St. Petersburg constructions. The outcropping limestone is karst (Ordovician Plato). The rocks pertaining to Ordovician period crop out in the vicinity of the river valleys of the Slavyanka, the Popovka, the Tosna, the Izhora, as well as in some parts of the Glint. Southward of Pavlovsk and near Krasnoe Selo, there are Devonian dolomites, marls, sandstones and reddish clays lying above the deposits of Ordovician period. Starting from the Late Palaeozoic Era, the territory of present-day St. Petersburg had been exposed to the conditions of continental development for nearly 300 million years. This time saw the formation of the basic roughness of relief, now buried under Pleistocene sediments, which originated in the last hundreds of thousands of years. The Pre-Pleistocene relief shows many aspects of the internal structure of the Earth's crust. Furthermore it predetermined the distribution of recent sediments and the features of the modern relief. On the north of the area, crystalline rocks served as the base for the formation of undulating plain of subaerial denudation up to 100 metres high; on the south, the stepped plateau-like relief of Ordovician Plato, with a height of 120-170 metres, together with the surrounding lower surfaces, partially karst, 60-80 metres high and about 40 metres high, formed behind the bench of the glint. The central part represented a low flat peneplain, with the depression of the present-day Gulf of Finland in its western part. The formation of this depressed part of the junction of the ancient plate was determined by tectonic processes. Even now, the depression adjoins to the border of the neotectonic upheaval of the northern part of St. Petersburg and the border of the relative subsidence of its southern part. Verging to this lowering, the network of erosional channels developed: its pattern, determined by incutting, gives evidence of the presence of valleys formed by erosion to a depth of up to 100 metres from the modern sea-level. There was a heavy dissection northward of St. Petersburg; it converged into a major artery north of present-day St. Petersburg and headed west. In St. Petersburg, the depth of these valleys varied from 87 metres to 101 metres. The crossing of one of them resulted in the "washout" of the underground line. The Pra-Tosna River used to flow into this valley from the south; its incutting amounted to 93 metres at the intersection with the escarpment, coming up to 98 metres within the vicinity of St. Petersburg. Consequently, by modern times, estimated at 0.5 million years, the surface relief was quite dissected; the relative heights reached up to 250 metres, which made them larger than the modern heights are. The surface is covered with strata of Pleistocene sediments coming to a total size of 150 metres; these sediments mostly consist of various ice-borne sediments, fluvio-glacial sediments, fluviatiles, lacustrine sediments, marine sediments and peat-bog formations. In the city, some significance should be attached to anthropogenic alluvions and technogenic alluvions. The sediments dating to the Pleistocene, mainly in the ancient hollows, is made of bouldery loams from the Oka glaciation 480,000-380,000 years old, overlaid by 380,000-240,000 year-old Likhvinian interglacial sands. On these sands, a mass of varying width and different composition is deposited: it consists of four levels of bouldery loams which pertain to the Dnieper glacial epoch (240,000-180,000 years), Moscow glacial epoch (125,000-110,000 years), Early Valdai glacial epoch (80,000-50,000 years) and Late Valdai glacial epoch (18,000-10,000 years). In between the moraines are interlayers and lenses of interglacial and interstadial marine and continental deposits of clay and sand, with the remains of flora and fauna. The absolute dating of the last interglacial period varies from 24,000 years to 35,000 years, while the absolute dating of the stages of the Late Valdai time is estimated in the range from 17,000 years to 12,000 years. During the degradation of the last glaciation, in addition to the moraines, some fluvio-glacial formations were deposited; they resided in kames, esker and outwash plains. At the same time some periglacial basins were formed: the first basin was the one of Lake Ramzaya with a water level of approximately 55 metres, then the basins of Neva ice lake and Baltic ice lake were formed, retaining levels of 40 metres, 30 metres, 20 metres and 15 metres. The basins are set by glaciolacustrine sediments: clays, sand loams and sands with a depth of 3 metres to 15 metres. The barrier beaches and bar sediments consist of pebble-and-sandy formations. The level of Baltyskoe freshwater lake had been reduced by the beginning of Holocene, and this depression was filled by a saline intake coming in from the Baltic; thus the Yoldia Sea was formed (10,000 -9,200 years). During this time silts and aleurites up to 5 metres deep were laid down. The short-time regression, during which peatbogs and soils settled down, was followed by the freshwater Ancylus Lake transgression (9,000 -8,000 years), which left sands, aleurites and clays with a depth of up to 3 metres. The further development of the area was determined by the incursion of saltwater from the Littorina Sea (7,000 -4,000 years), which resulted in the laying down of sand-clay sediments with the depth up to 4-5 metres. This transgression, as well as some of the previous ones, was confined to the western Neva land side exceptionally, while on the east, the basin of Lake Ladoga with the average rise of the level up to 17 metres developed quite independently. The last Holocene processes went under the continental conditions, when the area was dissected by rivers, and peat-bog formations were laid down in the interfluvial areas. The recent river incutting reaches 20-30 metres, while the depth of peats reaches 4-5 metres. During this period, the terraces in the vicinity of St. Petersburg developed, and the Neva River emerged. The sediments of the latter river are confined to the limits of the Neva Sand-Bar, at the place where the river flows in the Neva Bay, while the other part of the river valley is free of terraces and flood-lands, observed at other rivers. Anthropogenic (natural identical in composition) and technogenic alluvions cover nearly all the area of St. Petersburg with the depth up to 10-12 metres. Originally, the horizontal bedding of Holocene marine and lacustrine sediments was broken by recent tectonic movements, which enables the estimation of the speed of the rise of the area of Karelian Isthmus in the range from 0.9 millimetres per year in the first half of Holocene to 0.2 millimetres per year in the second half of the period. This rise is most likely to have been caused by the melting of the glacier, which imparts a glacioisostatic character to the process. The melting of the glacier also determines sinking and swamping of the southern banks of the Neva Bay and Lake Ladoga.

References: Природа Ленинградской области и ее охрана. Л., 1983.

Y. P. Seliverstov.

The subject Index
St. Isaac's Cathedral
Bronze Horseman
Kazan Cathedral