Among the widespread Devonian sediments on the Russian platform, predominantly clayey rocks are found only in its north-western part - within the Leningrad, Novgorod, Pskov, Velikoluki regions, Estonian and Latvian SSR. This area of Devonian sediments distribution is known as the Main Devonian Field, which stretches a wide strip from the shores of Lake Onega and Lake Ladoga southwestward to the Western Dvina. Studies have established that the Devonian rocks of this territory are mainly covered by glacial, late- and post-glacial sediments and crop out on the surface only along the river valleys, but in many places they are exposed by exploration workings. To the east and southeast they are submerged under Carboniferous deposits.