and
some lie atop grooved and polished rock surfaces or are associated with sandstones and conglomerates that show features indicating they were deposited as stratified drift. Two Precambrian glacial episodes have been identified in the geologic record, the first approximately2 billion years ago and the second about 600 million years ago. Furthermore, a well»d0cumented record of an earlier glacial age is found in late Paleozoic rocks that are about 250 million years old and that exist on several landmasses