Overburden, usually unconsolidated mineral matter on a rock consists of material from rock weathering or glacial deposits.
In the Baltic region weathered rock is rare since most of it has been removed from its original site and mixed into glacial till. Glacial deposits are usually somewhat sorted by flowing water and will therefore contain less clay than the weathering product, since the clay accumulates in lacustrine and marine deposits. Glacial deposits also contain coarse material obtained during the physical force of the moving ice.
In the Baltic region the overburden consists mainly of glacial deposits in:
Basement rocks can be:
Geologically the Baltic Shield - the area of Sweden, Finland and north-western Russia - is covered with crystalline basement rock and glacial till as overburden. In the rest of the Baltic region the basement rocks are sedimentary with appreciable potential aquifer storage.
The glacial till is relatively thin, on an average 3 to 5 m. The major part is sandy or silty till. The productivity is highly variable but is considered to be less than 1 l/s. Dug shallow wells are abundant in rural areas.
Eskers and glaciofluvial river plains are productive with yields varying between 10-100 l/s. They can therefore be the supply for towns and smaller cities. The water quality is usually good.
Finland has an extensive system of terminal moraines consisting of more or less parallel ridges, up to 100 km long. The moraine consists of glaciofluvial sand and gravel with inter-bedded layers of silt, till and clay. A number of eskers run perpendicular to the ridges, resulting in very high productivity. The most important aquifer is 55 km2 in area and has an estimated sustainable yield of 450 l/s.
The Baltic Shield also covers a fair part of the area north of Russia's Lake Ladoga, which drains into the Baltic Sea. This area has features similar to those in Finland, particularly the terminal moraines.
Drilled wells on crystalline rock are common in the Baltic Shield region. Although drilled wells are costly they are much less sensitive to dry years than dug wells. In Finland and Sweden the wells in crystalline rock are of significance. In Finland the range of yields in crystalline rock is given as 0.03 to 3 l/s and the average depth of is 65 m.
In the rest of the Baltic Region, i.e. in Estonia, Latvia, Lithuania, Poland,
Germany and Denmark, the crystalline rocks are covered by sedimentary
rocks of various ages:
Estonia: The most important aquifer is the Ordovician-Silurian limestone
which extends across the entire country, with fresh water down to a 300-m depth.
Latvia: A productive aquifer in the eastern part of the country consists
of upper Devonian carbonate which is about 150 m thick and contains fresh water.
Lithuania: The northern part has also the upper Devonian as the
productive aquifer. In the south the quaternary aquifers are the most important
source of water.
Western Russia and Belarus: Sedimentary rocks appear to be the major water
sources.
Poland: The major aquifers are Cretaceous limestone deposits as well as
younger rocks - sandstones and limestones.
Germany: Groundwater conditions in the land draining to the Baltic Sea
are a continuation of the geology of North Poland. Cretaceous rocks may also be
of value provided salinity is reasonable
Denmark: For the Baltic, the most important part of Denmark is Zealand,
which has a deep Cretaceous aquifer.