Rock n’ Rock
In somewhat more precise terms the Culebra uplands can be described as a disorderly combination of several geological formations, some sedimentary, others volcanic in origin- a generally unstable combination that was bound to mean a great deal of trouble. The oldest of these formations dated from the geological time period known as the Oligocene, making them roughly thirty million years old. Probably the Isthmus had its beginnings in the Oligocene as a string of islands in a shallow sea. A land bridge sank back below the sea at least four different times. In the late Pleistocene, the epoch of the glaciers- yesterday in geological time- the land was elevated to several hundred feet above its present level, then subsided again to perhaps thirty feet below the present level.
-- David McCullough
