Arkose is known to be young because of its content of feldspar, a mineral that usually degrades quickly into clay. Its mineral grains are generally angular rather than smooth and rounded, another sign that they were transported only a short distance from their origin. Arkose usually has a reddish color from feldspar, clay and iron oxides—ingredients that are uncommon in ordinary sandstone. (about sandstone)
Arkose is similar to graywacke, which is also a rock laid down near its source. But whereas graywacke forms in a seafloor setting, arkose generally forms on land or near shore specifically from the rapid breakdown of granitic rocks. This arkose specimen is of late Pennsylvanian age (about 300 million years) and comes from the Fountain Formation of central Colorado, the same stone that makes up the spectacular outcrops at Red Rocks Park, south of Golden. The granite that gave rise to it is exposed directly underneath it and is more than a billion years older.
For more photos see the Sedimentary Rocks Gallery.
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