Seismic reflection profiles and sonographs provide the means of mapping bedrock type, sediments, and faul's in Lake Thingvallavatn, SW-Iceland. The oldest part of the lake basin is formed in (Weichselian) hyaloclastites and a lava flow of probable interglacial (Eemian) age. Postglacial lavas (Thingvallahraun, Nesjahraun) flowed into the basin and modified its shape as did eruptions within the lake (Sandev, Skalabrekkusker?). Tectonic movements appear to have been important in forming and maintaining the depression occupied by the lake throughout its history. The greatest displacements have occurred within the graben extending from Thingvellir in the north to the Hengill area in the south. Sedimentation in the lake ranges from glacial sediments in the southern part of the lake to postglacial biogenic and allochthonous sediments which have accumulated mainly in the deeper parts of the basin. In places there is evidence of redistribution of bottom sediments by wave action (in less than ca. 15 m of water), by gravity (Sandev slope), by cold bottom currents, and by earthquakes (Midfellsdjup).
