Pg. 13 (fig. 3), 31-33, 44-47, 107-111, pls. Rio Dell formation of Wildcat group. Massive mudstone, alternating thin sandstone and mudstone, "phantom-banded" mudstone, and very fine grained sandstone. In Ferndale quadrangle, divided into: upper member, mudstone; middle member, alternating sandstones and mudstones; lower member, mudstone with sandstones; units not distinguished to the east where there is more very fine grained sandstone in upper part. Thickness 3,000 to 6,000 feet; thins northward. Underlies Scotia Bluffs sandstone (new) of Wildcat; top of Rio Dell arbitrarily taken as lowermost buff-weathering sandstone of Scotia Bluffs. Unconformably overlies Eel River formation (new) of Wildcat; base of Rio Dell chosen as a thin glauconite mudstone or fine-grained sandstone overlying a cut surface of upper Eel River mudstone. Fossils [listed], notably PECTEN (PATINOPECTEN) CAURINUS Gould, and Mohnian foraminifers. Age is middle to late Pliocene.
Type section: east bank of Eel River north of Scotia and west bank of river west of Scotia, Humboldt Co., northwestern CA.

Source: Publication; US geologic names lexicon (USGS Bull. 1200, p. 3280-3281).