[Named] for exposures on Butano Ridge, San Mateo Co, CA. Outcrops in triangular area, eastern point of which is San Lorenzo River, 4 mi north of town of Boulder Creek, Santa Cruz Co, CA. Consists of medium- to coarse-grained massive brown and buff sandstone with minor pebbly beds and dark-colored shale. Is about 2000 ft thick. Supposed Butano rocks overlie Chico sandstone about 0.5 mi south of mouth of Pescadero Creek which contain few fossils. No fossils found in typical Butano sandstone. Conformably underlies San Lorenzo formation. Age is Eocene or Oligocene.

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

Type section designated along Little Boulder Creek [in SW/4 sec.22, SE/4 sec.21 and N/2 sec.28 T8S R3W] Big Basin 7.5' quad, San Mateo and Santa Cruz Cos, CA. Has wider distribution than Branner and others (1909) realized. Underlies most of area between Montara and Ben Lomond Mountains and along LaHonda and Coal Creeks. At mouth of Pescadero rocks are here referred to Mindego formation. Is thought to be 6000 feet thick. Unconformably overlies Locatelli formation. Grades upward into San Lorenzo formation except in few places. Overlapped by Monterey shale in western part of district. Age is Eocene (Narizian) based on fauna (Schenck, 1963).

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

Consists of about 3000 m of light-gray to buff very fine- to very coarse-grained arkosic sandstone with dark-gray to brown mudstone and shale and with conglomerate present locally in lower part. Age is early and middle Eocene since Two-Bar Shale Member of San Lorenzo Formation which overlies unit is middle and late Eocene in age.

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