Not until the Cretaceous period of Earth history, about 100 million
years ago, did the geologic record again become clear. At that time a
great thickness of mud and sand was deposited in the geosyncline burying
deeply the ancient Belt and other rocks which had accumulated as
sediment during the preceding several hundred million years. Life had
made tremendous advances in this interval, and the abundance of fossils
in Cretaceous rocks indicates that the sea swarmed with shelled
creatures during that period.