Name applied to all Cambrian rocks in central TX beneath the Wilberns formation. Named for the Riley Mountains in southeast Llano Co., Llano uplift. The only complete section in the Riley Mountains is in the Moore Hollow area. Divided into Hickory sandstone, Cap Mountain limestone, and Lion Mountain sandstone members. The 3 members were formerly considered to be of formation rank. The contacts for all 3 are gradational and are thought of as members of a single formation. The Hickory is a non-calcareous brown sandstone 335 ft thick. The Cap Mountain has a lower dark brown limestone and calcareous sandstone, and an upper gray massive limestone that together are 421 ft thick. The Lion Mountain is a 24 ft thick unit of glauconitic sandstone with numerous coquina-like limestone lenses. The total Riley is 780 ft thick. Of Late Cambrian age. Underlies Welge sandstone member (first used) of Wilberns formations.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Riley formation in the study area (Llano uplift of central TX) ranges from about 600 to nearly 800 ft thick; it is thickest in southeast Llano Co. and thinnest in northwest corner of uplift; it overlies pre-Cambrian rocks; it underlies Wilberns formation; it is assigned a Late Cambrian age. Cross sections. Riley is divisible into the Hickory sandstone member, at base, Cap Mountain limestone member, in middle, and Lion Mountain sandstone member, at top. Upper boundary of Hickory modified; contact with overlying Cap Mountain placed at top of a noncalcareous sandstone beneath a zone of alternating impure dark brown limestone and calcareous sandstone; this contact is also a distinct topographic and vegetational break. The Hickory-Cap Mountain contact was previously placed at an indefinite point as thin the sequence. The upper contact of the Lion Mountain is revised in that Lion Mountain underlies the newly named Welge sandstone member of the Wilberns.

Source: GNU records (USGS DDS-6; Denver GNULEX).