From the relation of Delaware County to the great Niagara escarpment it would scarcely be expected that rocks older than the Upper Silurian would be found within its limits. Upper Silurian sediments indeed constitute the rocks that may be said to be normal to the entire county. They cover probably ninety-nine per cent of its area. Without exception all the higher lands are occupied by them, and most of the valleys have a floor of Niagara dolomite. Still some of the valleys have cut through the Niagara limestone down into the underlying shales of the Maquoketa age, and very many of the valleys come much nearer to exposing Maquoketa shales than has hitherto been supposed. The normal dip of the Iowa strata to the southwest is evidently reversed along a line that passes obliquely near the northeast corner of the county, so that the Maquoketa shales are covered with a relatively thin veneer of Upper Silurian dolomite, even as far as the western border of the county in the northwestern corner.
Proceedings of the Iowa Academy of Sciences
©1894 Iowa Academy of Science, Inc.
"Maquoketa Shales in Delaware County,"
Proceedings of the Iowa Academy of Science, 2(1), 40-42.
Available at: https://scholarworks.uni.edu/pias/vol2/iss1/13