Pg. 13, 14, 16, 17-22. Bony Falls member of Black River formation. Section at Bony Falls consists of about 40 feet of limestone some of which is argillaceous and some finely crystalline dolomitic; a 2-inch layer of bentonite occurs about middle of section, and this may mark top of Black River. Evidence of subaerial erosion appears at several levels throughout section, and a very obvious disconformity near middle of section may represent a time break of considerable duration. Underlies Chandler Falls member (new) of Trenton formation. Separation of Black River rocks from Trenton above presents difficult problem; rocks of the two formations are commonly distinguished on basis of faunal changes that are not always very significant; deposition was practically continuous at some localities from Black River to Trenton time, and dividing line between the two formations becomes an arbitrary one. Age is Middle Ordovician.
Type locality: at Bony Falls on Escanaba River, in sec. 1, T. 41 N., R. 24 W., Delta Co., northern MI.

Source: US geologic names lexicon (USGS Bull. 1200, p. 429).