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 coastline of the Gulf of Mortvyi Kultuk on the north-east is, on the other hand, formed by a range of low calcareous hills, constituting the rampart of the Ust-Urt plateau, which intervenes between the Caspian and the Sea of Aral.
The eastern shore of this section of the sea is also formed by the Ust-Urt plateau, which rises 550 ft.
The Ust-Urt recedes from the Caspian and circles round the Gulf of Kara-boghaz or Kara-bugaz (also called Aji-darya and Kuli-darya).
Ust-Urt), which divided it into an eastern and a western portion, communicating by one or two narrow straits only, such as on the south the Sary-kamysh depression, and on the norththe line of the lakes of Chumyshty and Asmantai.