Post-Piney Creek alluvium and Piney Creek Alluvium, undivided

What the map says

Mostly sand and gravel along major streams in piedmont that head in mountains, and mostly sand and locally sandy silt and silty clay along tributary streams that head in piedmont east of mountain front. Locally cobbly and slightly bouldery along major streams near mountain front and sandy and silty in mountain meadows. Post-Piney Creek alluvium commonly lies within stream channels cut in Piney Creek Alluvium and consists chiefly of silty to pebbly sand and pebble gravel that is subject to periodic stream flooding. Piney Creek Alluvium commonly consists of sandy silt, silty sand, lenses of silty clay, and detrital organic matter in upper part, and mostly sand and lenses of pebble gravel in lower part. Forms one or two terraces less than 5 m above present streams, and locally forms alluvial fans and sheetwash aprons along margins of major valleys. In valleys of major streams that head in glaciated drainages, Piney Creek Alluvium forms much of flood plain where it overlies Broadway Alluvium (Qb). Grain size and content of organic matter in upper part of Piney Creek Alluvium suggests that it was eroded from soils formed in loess (Qlo) and other fine-grained deposits. Soils formed in upper part of post-Piney Creek alluvium have A/Cu profiles. Soils formed in upper part of Piney Creek Alluvium are better developed and have A/ Bw/C profiles where formed in sand and A/Bw/Bk profiles with stage I carbonate morphology where formed in finer grained sediments. Some of the soil A horizons formed in Piney Creek Alluvium are over-thickened (cumulic) where A-horizon formation kept pace with sedimentation. Radiocarbon (14C) dating of organic material helps to constrain ages of post- Piney Creek alluvium and Piney Creek Alluvium. Wood from logs in post-Piney Creek alluvium near Fort Lupton, Colo., yielded radiocarbon ages of a few hundred years (Lindsey and others, 1998), and charcoal at an archeological site (stratigraphic position not specified) near Kassler, Colo., just south of map area, yielded an age of about 1,500 14C yr B.P. (carbon-14 years before present; Scott, 1962, 1963a). Radiocarbon ages of organic material in Piney Creek Alluvium near Colorado Springs range from about 1.1 to 2.2 ka (Madole and others, 2005). Gravelly alluvium overlying Broadway Alluvium near Longmont, Colo., was deposited between 1,900 and 3,900 14C years ago (Madole, 1976). Unit may locally contain pre-Piney Creek alluvium (Scott, 1960, 1962, 1963a). Low-lying deposits are prone to periodic stream flooding. Thickness about 1.5–6 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.

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