Chugwater Formation
Overlying the Dinwoody is one of Wyoming's thickest and most distinctive formations, the Triassic Chugwater Formation. The Chugwater Formation is separated into three members-Red Peak, Crow Mountain, and Popo Agie. The basal Red Peak Shale is the thickest at 1,030 feet (Anderson and O'Connell, 1993). It contains a blood-red, interbedded siltstone and sandstone that accumulated within a paralic and nearshore marine complex (Picard, 1978). The upper portion of the Red Peak weathers into shades of tan, gray, red and purple. The Crow Mountain is 210-feet-thick of purple, very fine-grained sandstone, and gray to red siltstone (Anderson and O'Connell, 1993. The main depositional environment for the Crow Mountain Member was probably a tidal flat complex (Picard, 1978). The upper Chugwater member, the Popo Agie, is dark red to purple on the base, and weathers to a dark rust color or ochre color. The upper portion is a bright tan siltstone with sharp brittle edges. It sounds like glass when walked upon. The difference in color is probably from different amounts of iron oxides present. The Popo Agie Member represents dominantly fluvial conditions, whereas the ochre-colored unit was deposited in a lacustrine environment on a deltaic plain (Picard, 1978). At Circle Ridge, the Popo Agie Member measures 170 feet in thickness (Anderson and O'Connell, 1993).