The Pliocene appears only in the coast region of the Black and Azov Seas, but it is widely developed in the Aral-Caspian region, where, however, the Ust-Urt and the Obshchiy Syrt rose above the sea.
The second-class fortresses are Kronstadt and Sveaborg in the Gulf of Finland, Ivangorod in Poland, Libau on the Baltic Sea, Kerch on the Black Sea and Vladivostok on the Pacific. In the third class are Viborg in Finland, Ossovets and Ust Dvinsk (or Dunamunde) in Lithuania, Sevastopol and Ochakov on the Black Sea, and Kars and Batum in Caucasia.
Both these rivers come from the south-west: the Argun, or Kerulen as it is called above Lake Kulun (Dalai-nor), through which it flows about half way between its source and Ust-Stryelka, rises in 49° N.
The principal water-divide in this highland region is, however, the range of Egri-dagh (Ararat), which j ust south of 40° S.
A little farther down it becomes completely navigable, and attains a breadth of 4200 ft.; but between the village of Ostrovki and that of Ust-Tosna it passes over a limestone bed, which produces a series of rapids, and reduces the width of the river from 1050 to 840 and that of the navigable passage from 350 to 175 ft.