Named as uppermost member of Hornbrook Formation for outcrops in Blue Gulch southeast of Hornbrook. Includes Rancheria Sandstone Beds and Hilt Bed (both new). Composite type section designated as: (1) SE/4 SW/4 sec.20; (2) SW/4 SE/4 sec.21; (3) NE/4 sec.33 and NW/4 sec.34; (4) SW/4 SE/4 sec.21; and (5) NE/4 NE/4 sec.34 and SE/4 sec.27, all T47N R6W, Hornbrook 15' quad, Siskiyou Co, CA. Consists chiefly of mudstone with thin interbeds of very fine- and fine-grained sandstone. Is 912.5 m at type which includes Rancheria Gulch Sandstone Beds. Thins to northwest and southeast because upper parts have been eroded. Unconformably underlies Tertiary nonmarine gravels in Bear Creek and Shasta valley areas and Tertiary volcanic rocks in Cottonwood valley. Conformably overlies Rocky Gulch Sandstone Member (new) except locally where it may rest directly on Ditch Creek Member (new) (both of Hornbrook Formation). Assigned Late Cretaceous (Campanian and Maestrichtian) age based on foraminifers identified by W.V. Sliter and ammonites (Peck and others, 1956; Jones, 1960) collected in strata equivalent to unit above Rancheria Gulch Sandstone Beds. No fossils found in lower part beneath Rancheria Gulch Sandstone Beds. Age is considered to be early Campanian to early Maestrichtian.

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

Lower age limit of Blue Gulch Mudstone Member of Hornbrook Formation remains somewhat problematical because no fossils recovered from lower part below Rancheria Gulch Sandstone Beds. Appears to range from late Turonian to early Maestrichtian. Measured type section, detailed lithology, lists of fossils, depositional environment, and mapping included in study.

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