The first, and oldest type of rock found in Castlewood Canyon forms a wide, slanted edge that falls into the Cherry Creek bed. This is the Dawson Arkose formation made from conglomerate, sandstone, and shale (USGS ). This rock also contains biologic elements including fossils and petrified wood left over from a tropical rainforest that once thrived on the land (CO State Parks).
This rock was covered 36.7 million years ago when a super volcano near what is now Salida, Colorado, erupted (CO State Parks). A large layer of extrusive rhyolite (known also as Wall Mountain Tuff in this area) was formed and hot volcanic ash and magma lithified its surroundings.
The upper portion and caprock of Castlewood Canyon's walls is formed by enormous, distinctive conglomerate boulders. These conglomerate rocks are described as "the coarsest of all the Cenozoic conglomerates and gravels in the Colorado Great Plains sequence," and dot the canyon because they were once swept by water from the Rocky Mountains into the canyon (Field Guides). They are the first noticable feature of the canyon's geologic profile and the most accessible; many have fallen from the top of the weathering canyon walls and into the creek bed below. The water that has been flowing through what is now Cherry Creek has cut through the resistant, cement-like conglomerate, rhyolite, and into the softer Dawson Arkose.
US Geological Survey: http://tin.er.usgs.gov/geology/state/sgmc-unit.php?unit=COTdu%3B0
Colorado State Parks: http://www.parks.state.co.us/Parks/CastlewoodCanyon/Publications/Pages/CastlewoodPublications.aspx
Field Guides: http://fieldguides.gsapubs.org/content/3/59.full
