A Geological Tour of Denver, Golden, and Colorado's Front Range

Pericles Press Home | Book | Contact Us/Ordering

A Theory | Seven Nations | Governments and Empires | Geological Past

Geologic Time Period

The Fox Hills Formation

Fox Hills Formation

Alternating yellow-brown sandstone and grey shale layers of the Fox Hills Formation along Rooney Road.

The retreating sea...

The retreat of the Cretaceous Sea occurred in fits and starts, not in one gradual subsidence. Sand deposits built up into islands, which served as protective barriers for shallow lagoons.  Ocean waves deposited large amounts of sand on the shores, which became the yellow-brown sandstone characteristic of the Fox Hills Formation. Silt, clays, and organic matter settled out, became mixed with the sand, and turned into the shale layers.

Along Rooney Road the sandstone layers can measure several feet in thickness, indicating a fairly fast sand accumulation.  Sandwiched between the sandstone are thinner layers of grey shale, an indication that sea levels may have recovered before receding again.  Barrier islands may have risen above the ocean surface, only to be covered as sea levels rose again.  The process would be repeated, even as the Cretaceous Sea was receding.