Bedrock relief with the basics (subdivision and thickness) of the overlying Quaternary sequence was studied by means of seismo-acoustic profiling beneath the Suur Strait in the West Estonian Archipelago. Reaching ca 15-20 m below sea level (b.s.l.) in the watershed area in the NE corner of the Gulf of Riga, the bedrock surface beneath the strait drops along two channels, divided by the islet of Kesselaid rising ca 15 m above sea level (a.s.l.), into a large depression more than 60 m b.s.l. in the interior of the Väinameri. A similar setting indicates that the low-lying bedrock surface between the Estonian mainland and the islands of Muhu and Saaremaa was formed by combining erosions of preglacial river(s) and Pleistocene glaciers. The large depression NW of the islet of Kesselaid possibly represents a section of an east-west running preglacial river, heavily reworked by Pleistocene glaciers. Although supported by preliminary data from the Soela Strait, the latter opinion still needs additional profiling between the islands of Hiiumaa and Saaremaa. The thickness of the Quaternary sequence around the strait follows the general height and ruggedness of the bedrock surface; it is occasionally missing in the nearshore bedrock plateaus (submarine alvars) and reaches ca 50 m in the depression NW of Kesselaid.