The boundary between the Eocene and Oligocene epochs was characterized by a big drop of the global (eustatic) sea level and a sudden steep cooling of global climates. At the same time the Alpine orogeny, a tectonic phase by which the Alps, Carpathians, Dinarides, Taurus, Elburz and many other mountain chains along the southern rim of Eurasia were formed. The combination of a drop in sea level and tectonic uplift resulted in a large regression of the sea and a barrier was formed between the Tethys and Paratethys domains. Connections with the Arctic Ocean (the Turgai Sea), the North Sea Basin and Atlantic Ocean (in the form of a strait north of the Carpathians) also closed in the Early Oligocene. However, it is possible connections with the Rhônegraben (and the Mediterranean) and the Hessen Strait (that connected the Molasse Basin with the North Sea Basin) still kept open.