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After a short historical review of the correlation of Palaeozoic rocks between Estonia and Sweden, this paper focuses on the results of marine seismic studies, achieved during the cooperative Swedish-Estonian project since 1990. The most recent seismic correlation scheme of the Cambrian, Ordovician, and Silurian strata and their distribution at the seafloor across the northern Baltic Proper are presented. Thickness changes and trends, as well as the sedimentary structures, reef bodies, and erosional features of different seismic units, are treated in connection with structural and facies changes in the Palaeobaltic Basin. The immediate background of this project is outlined and the locations of the seismic lines shot during the joint expeditions to the Baltic Sea are shown.

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The erosional relief of Ordovician and Silurian deposits in Estonia was developed during the continental period in late Silurian and early middle Devonian times. The uplift of the area and marine regression were induced by compressional tectonics in the continental interior related to the closure of the Iapetus and Tornqvist Oceans.In the northern part of the Baltic sedimentary basin (Estonia), on the gentle southerly dipping slope between the Fennoscandian Shield (Finland) and Baltic Syneclise (Latvia), a pre-Devonian, slightly rugged erosional relief with few cuesta was developed. The pre-Devonian erosional landforms -hills, depressions and escarpments reaching 150m in height -were probably buried under the Devonian deposits and then partly re-exposed by pre-Quaternary erosion. These landforms are described in detail using data from several thousands of cores drilled in the course of oil-shale and phosphorite exploration and mining.

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