After a century and ft half it began to recede rather than to advance.
Islands are frequent; the banks recede and become lower until, after 50 m., they stand almost level with the water.
End the hills again recede and the plain widens to include Dweru.
The first two predominate earlier, and gradually recede before the last-named.
As we recede in time we find the extinct representatives of many of these orders approximating more and more closely to a common generalized type, so that in a large number of early Eocene forms it is often difficult to decide to which group they should be assigned.