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Land areas existed in the general position of the present southern Rocky Mountains in Paleozoic time and reached their maximum development in late Pennsylvanian time. These "Ancestral Rockies" conformed to the position of the present mountains in only a part of their extent. The evidence for non-deposition instead of erosion lies in changing lithology and thicknesses from the centers of the basins shoreward and from parallel faunal changes.
The most important stratigraphic breaks are post-Permian and post-Triassic. It is not believed that any widespread hiatus exists between the Permian and Pennsylvanian.
Mesozoic time was essentially a period of denudation, the ancient Rockies having been practically base-leveled by the end of the Jurassic. Marine conditions prevailed throughout most of the Upper Cretaceous period, upward movement beginning in Pierre time or earlier and culminating in the making of the present Rocky Mountains in the Laramide revolution.
The logical sequence of events is most effectually deciphered by a combination of faunal evidence, kind of materials and their source, areal distribution of the formations, correlation of homogenetic as well as time equivalents, and diastrophism. Fossil evidence is most important except in non-fossiliferous strata.
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