Castle Rock formation in Rock Park for which the town of Castle Rock is named. The rectangular-appearing Castle Rock itself is composed of Castle Rock Conglomerate. Rhyolite from a volcanic eruption near Mount Princeton broke off canyon walls, accumulated in the stream bed below, and hardened into the conglomerate making up the present formation. The surrounding area eventually eroded away.  The large sloping base of the formation, on which the top rests, is Dawson Arkose, a form of sandstone.

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

By Jack Barkstrom

A moving cloud of gas and ash...

Mount Princeton, in the Sawatch Range, is about 85 miles west of Colorado Springs. A body of magma which formed in the region of Mount Princeton 36.7 million years ago suddenly found a weak area in the surface rocks and explosively broke through.  A mixture of molten magma, pumice, ash, glass, and rock exploded into the atmosphere and formed a cloud which began moving or "flowing" downslope.  Pyroclastic flows, as such moving clouds are known, can be very hot, with temperatures reaching between 500 and 1,000 degrees C (932 - 1,800 degrees F).  The cloud, perhaps moving at 50 to 100 miles an hour, probably reached what is now the city of Castle Rock, some 90 miles to the northeast, within two hours or less.

The ash which settles out of a pyroclastic cloud is known as tuff. Sometimes the pyroclastic globules, which are very sticky, pick up ash and the combination precipitates out of the cloud as welded tuff.  In other instances they cool quickly and fall in the form of powder or volcanic ash, a form known as soft tuff.  Welded tuff can also take the form of rhyolite, a fine-grained, compact, form considered the volcanic equivalent of granite.  It is usually light brown to gray in color. 

The pyroclastic cloud which reached the Castle Rock area was still very hot. As a result, much of what came out of the cloud was a form of welded tuff.  It has been called the Wall Mountain Tuff or, alternatively, Castle Rock Rhyolite.  At the time, the region was probably a flat plain.  The amount of material emerging from the Mount Princeton explosion was substantial, since the deposited layer was, in places, at least twenty feet thick.

The "Castle Rock" formation, for which the town is named, has the appearance of an island standing high above the surrounding plain, bearing the brunt of the volcanic eruption.  The reverse is true. The present formation was one of the lowest points around.  In addition, it is not even part of the original deposit of volcanic material, which somehow avoided erosion, but instead, a form of conglomerate, known as Castle Rock Conglomerate. Over millions of years water and streams began to cut paths through the rock, creating canyons.  The area seems to have been particularly hard hit by floods around 34 million years ago. The rhyolite broke apart and the chunks fell into a streambed as they eroded from the canyon walls, to be reformed and cemented together as conglomerate.  Below the Castle Rock Conglomerate is Dawson Arkose, a feldspar-rich sandstone.  The ash and tuff from the Mount Princeton eruption was falling on what had been an accumulation of sand, an alluvial fan created by the continued erosion of the then existing Rocky Mountains.  

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