Historical:Tonti Sandstone Member
H. B. Willman and T. C. Buschbach
The Tonti Sandstone Member of the St. Peter Sandstone (Templeton and Willman, 1963, p. 45), the middle member, is named for Tonti Canyon in Starved Rock State Park.
The top of the Tonti Sandstone Member is exposed in the canyon, but the type section is at Starved Rock, La Salle County (SW NW NW 22, 33N-2E), where the contact with the overlying Starved Rock Sandstone Member and the upper 40 feet of the Tonti Member are exposed. The Tonti is entirely exposed in the bluffs at Split Rock, 4 miles northwest of Starved Rock.
Extent and Thickness
The Tonti Member generally forms the greater part of the St. Peter Sandstone, and in many areas all the St. Peter is Tonti. Although commonly from 100-200 feet thick, it is locally over 500 feet thick.
The Tonti Member is chiefly fine-grained, well sorted, friable, highly porous sandstone, but locally in western Illinois it is even finer grained, silty, less porous, and partly cemented by secondary silica. It is noncalcareous, except in southern Illinois where it contains some dolomitic beds.
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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