Rivers flowing across
the basin from a renewed phase of uplift in the Front Range deposited a
sequence of rocks above the paleosol, now part of the Dawson Arkose. These
rocks are made of coarse-grained gravel and sandstone with smaller amounts
of mudstone and siltstonetypical remnants of a fluvial environment.
Fossils are present in this rock unit, but they are relatively rare. The
age of these beds is Eocene, although the exact time span represented is
not yet known. Typical thicknesses are 100 to 1,000 feet. These rocks form
the shallow aquifers in portions of the Denver Basin.