H. B. Willman and T. C. Buschbach
The Briton Member of the Mifflin Formation (Templeton and Willman, 1963, p. 83) is named for Briton School, 2 miles northeast of Dixon and 1.5 miles southwest of the type section.
The type section of the Briton Member is located in the same quarry as the Hazelwood type section on the east side of the Rock River Valley, 3.5 miles north of Dixon (SW SW NW 22, 22N-9E), where it is 10 feet thick.
Extent and Thickness
The Briton Member is the uppermost shaly member of the Mifflin and is widely present. It is 3-12 feet thick in the northern outcrop area, but it thickens southward to as much as 55 feet in the southern outcrop area in Missouri and probably has a comparable thickness in subsurface in southern Illinois.
In the northern outcrop area the Briton Member is largely light gray, white-weathering, fossiliferous, lithographic, argillaceous limestone in thin wavy beds that have strong green shale partings. It grades to dolomite in a few places. In the southern area it is less argillaceous and only slightly shaly but is distinctly more impure than adjoining members.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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