Geology of Estonia

Except for artificial outcrops in oil shale and phosphorite strip mines, the only outcrops of Paleozoic rocks are found in a few river valleys, coastal cliffs on the Baltic and the shores of lake Vortsjarv and Peipsi.

During the Ordovician and Silurian, a shallow marine environment predominated in Estonia, depositing organic-rich black carbonate shale which later generated oil shale. Reef limestones and backreef dolomite contributed material to the carbonaceous shale. Silurian sedimentation took place during a retreat in sea levels, bringing a break in sediment deposition.

In the Early Devonian, sedimentation continued. The Old Red sandstones deposited in the Middle Devonian in a nearshore marine environment from sand and silt, shed during the Caledonian orogeny onto the Fennoscandian continental plain. A few carbonate rocks capped the Devonian in the southeast.

The sedimentary rocks preserve extensive fossils and tens of thousands of specimens have been gathered at the University of Tartu and Institute of Geology. Brachiopod, mollusk, trilobite, ostracod, bryozoan, graptolite and stromatoporoid fossils are especially common.[3] Most natural resources in Estonia formed from fossil remains, including kukersite and phosphorite deposits formed from shells.

Through the Devonian, the continent Baltica, which included Estonia, drifted from the South Pole to north of the Equator and was influenced by glacier-related changes in sea level. Deformation of rocks from weak, regional tectonic stresses leads to some anticline folds and fluid migration in the rocks, which generate metasomatic dolomite limestone, zinc mineralization and lead-sulfide deposits. The Kärdla astrobleme on Hiiumaa Island formed from an asteroid impact 455 million years ago, and Estonia has three other small meteorite craters, less than 110 meters in diameters in the bedrock.[4]

The Mesozoic and much of the Cenozoic are poorly attested in Estonia. However, extensive sediments and landforms remain from the last 2.5 million years of the Quaternary.

A thin blanket of Quaternary sediments covers the plains of western Estonia. The Baltic Sea and Gulf of Finland are relatively deep erosional features in the bedrock, as are the lake basins of Vortsjarv and Peipsi, which were deepened by glaciers during the Pleistocene. Before and in-between glaciations, a network of deep valleys formed, up to 145 meters below sea levels, connecting the Baltic and the Gulf of Finland. However, more recent sedimentation has disguised old valleys which do not appear as a major part of present-day topography.

Glacial debris is less than five meters thick in northern Estonia and parts of the country underlain by limestone and dolomite have karst processes and hardly any remnants of glaciation. The Haanja Heights and Otepaa Heights have up to 100 meters of glacial sediment, or up to 207 meters in the Abja Valley in the south. Quaternary geologists define five major till layers, separated by interglacial pollen assemblages.

Moraines are often up to tens of kilometers long and more than 50 meters in height, such as the Laane-Saaremaa terminal moraine. Kame fields and eskers are also common, particularly on the Pandivere Upland and the West Estonian Lowland. Glacial retreat began around 13,000 years ago and ended by around 11,000 years ago. However, ice-dammed lakes and isostatic rebound in the region played an important role in geomorphology for several more millennia. The Baltic Ice Lake gave way to the Yoldia Sea, Ancylus Lake and Littorina Sea, followed by the Limnea Sea. The land of Estonia rose 65 meters in a span of only 2450 years and has risen an additional 50 meters in the past 10,000 years.

Deglaciation led to the formation of river valleys and 20 to 40 percent of downcutting in rivers in the south happened during a short span after glaciation. Estonia has 1500 lakes and 20,000 peat bogs, although the number of lakes was three times as many at the start of the Holocene.[5]