Valley-floor alluvium
Holocene, younger than 12,000 years. 3 crags on this site have their map point on it.
What the map says
Sand and gravel in stream channels as well as sand, sandy and clayey silt, and gravel underlying floodplains and adjacent low terraces less than 4.5 m above major streams and their major tributaries east of the foothills hogback belt of the Front Range. Unit includes post-Piney Creek alluvium and Piney Creek Alluvium (Colton, 1978). The younger alluvium commonly lies within stream channels cut in the older alluvium. Unit Qa locally includes small fan deposits (Qf) and sheetwash aprons (Qsw) along the margins of major valleys, and overlies Broadway Alluvium (Qb) in major valleys. The abundance of silty and clayey sediment, particularly along Boxelder Creek and its major tributaries, suggests that unit Qa was derived in part from sediments eroded from silty and clayey bedrock units such as the Pierre Shale and probably from loess deposits (Qlo). Low-lying deposits are prone to periodic stream flooding. Estimated thickness is 1–5 m Lithology: sand, gravel, silt.
1:100,000 sheet: Workman and others, 2018, U.S. Geological Survey Scientific Investigations Map 3399 (1 crag). A contact line at that scale is about 100 m wide on the ground.
Mostly poorly sorted coarse sand and pebbly to bouldery cobble gravel in stream channels, flood plains, and low terraces in mountain valleys. Flood-plain and terrace deposits of Holocene age are commonly about 1.3 m or less above present streams (Madole, 1996). Unit locally includes a minor amount of young stream-terrace alluvium (Qg1), colluvium (Qc) and other mass-movement deposits, fan deposits (Qf), and glacial deposits (Qtp and Qti) along valley margins and organic-rich sediments in bogs, marshes, and meadows. Locally may include minor lake sediments. Top of unit probably consists mostly of poorly sorted sand and gravel in nonglaciated valleys. Unit is commonly less than 5 m above the Fraser River and tributary streams near Fraser and Tabernash, Colo., and commonly less than 5 m above present streams elsewhere within map area. Low-lying deposits are prone to periodic flooding. Estimated thickness 5–10 m
1:100,000 sheet: Kellogg and others, 2008, USGS Geologic Investigations Series 3000 (2 crags). A contact line at that scale is about 100 m wide on the ground.
Served by Macrostrat (CC BY 4.0). The wording is the map's own, quoted as a fact about the mapped unit, not about any one wall.