The northern plain district of the Ukraine joins directly on to the Polish lowlands, and, indirectly, to the North German lowland. The first section of the northern plain district is called Pidlassye (Podlakhia, land on the Polish border). Its northern boundary consists of the southern limits of the White Russian Plateau; the western boundary of the flat elevations near Sidlez and Bilsk; on the south the plain borders on the spurs of the Rostoche; in the east the boundary is the Buh-Pripet divide, which is only 170 m. high. The surface of the Pidlassye is very even, only slightly undulating in places on the north and south borders. The river valleys are very broad and flat. Only the great forest (the well-known Biloveza forest lies here) and the water-courses bring variety into the monotonous country. The main stream of the Pidlassye, the Buh, as well as its tributaries have the character of genuine lowland rivers. They flow thru their over-great valleys in great turns, divide into many arms, and form innumerable old river beds. Besides these we find, in southern Pidlassye, a large number of lakes and many swamps and moors which mark the sites of former lakes. The chalk and tertiary substratum appears only in very few places, the rest being covered everywhere by sand and loam, which include boulders and rubble of Finnic-Scandinavian origin. These are traces of the great (second) [55]glacial period of Northern Europe, which covered the entire region of the Pidlassye with glacial ice. The lakes are morain-lakes. The ice of the glacial period did not reach Pidlassye. At that time a broad primeval river valley formed here as an extension of the primeval Vistula river valley. In this valley the water from the melting glacier flowed off to the east toward the lowland of the Polissye.