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

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Geologic Time Period

The Laramie Formation

Laramie Formation sandstone

The white sandstone of the Laramie Formation, left, blending into the shales and sandstones of the Fox Hills Formation.

Sand dunes and coal deposits...

Visible from the Rooney Road overpass, the white sandstone exposure is all that remains of a 69 million-year-old sand dune.  No longer under water after the retreat of the Cretaceous Sea, the Golden-Denver region had become semi-tropical.  It was also a land of contrasts.  Semi-arid dunes were accompanied by swamp lands.  Palm trees grew abundantly in the semi-tropical climate.  Rivers emptied sand and debris into large flood plains.

Walking west along Rooney Road, through progressively older time-periods, the white sandstone turns a light red or pink, as the Fox Hills shale is encountered, then to grey and yellow-brown, as the main Fox Hills sandstone comes into view.

The swampy conditions conducive to tropical plant growth, seem somewhat at odds with the end-product.  Coal results when water has been forced from the original plant material. With limited exposure to oxygen, dying and dead plants accumulated at the bottom of swamps without decomposing.  As layer after layer built up, the increasing pressure "fixed" the carbon in the remains. Sandstone or shale layers eventually covered the carbon material, preventing water from reaching what had become coal.

Although the Laramie Formation coal in the Golden area was of high quality, it was difficult to mine since the uplift of the Rockies created coal beds which were nearly vertical, necessitating the cutting of horizontal ledges to remove the coal.  Nevertheless, mines such as the White Ash Mine and others supported local Golden glass and brick-making industries.