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.