This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
During the Eocene nearly all Germany had been land, but in the Oligocene it was invaded by the sea, which broke in from the north and covered all the northern plain, extending into Belgium, and sending long bays to the south. One of these reached to the strait on the north of the Alps, expanding into a large basin near Mayence and Frankfort. Over Germany the sea was shallow, permitting the formation of extensive peat-bogs, where were accumulated masses of lignite or brown coal. The Oligocene is very extensively displayed in southern Russia, marine below and lignitic above. In the basin of Paris the sea had a greater extent than in Eocene times, though with lacustrine beds intercalated. The Lower Oligocene of the Parisian area contains thick bodies of gypsum, which were formed in very strongly saline lagoons. In England the beds are more of brackish- and fresh-water origin. In southern Europe the sea retreated from wide areas, and in its place were extensive bodies of fresh and brackish water, in many of which peat-bogs accumulated masses of lignite.
Such lignitic deposits occur at intervals in the south of France, Switzerland, and Bavaria. In the Alps, Apennines, Carpathians, Caucasus, Asia Minor, and southern Asia, the Oligocene is represented by the upper part of the Flysch, the formation of which began in the Eocene.
The Oligocene is found in north Africa, but in the other continents, beside those enumerated, it has not yet been separated from the Eocene below or the Miocene above.