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The bedrock under the upland — shale over limestone, known only from the drill
No outcrop: There is no place in St. Joseph County where the bedrock shows itself. Timothy Howard's county history, in the topography chapter built on the state geologists' papers, states it flatly: "There is not an outcrop of primitive rock in the county, the entire surface… being covered with glacial drift which will probably average two hundred feet in depth." Everything known about the rock under this upland comes out of well bores, and the county's rock record is a record of drilling.
The deep boring: In Howard's day the drift had been pierced to the stratified rock at only one place in the county — at South Bend, in the boring for gas and oil — and the county history prints that well's full section "as given in the 18th report of the state geologist": drift 160 feet; Sub-carboniferous and Devonian 220; Corniferous limestone 60; Lower Helderberg 40; Niagara 640; Clinton 60, marked with the report's own query; Hudson River 200; Utica shales 200; Trenton limestone 427 — a total depth of 2,027 feet, and "No gas or oil." The United States Geological Survey's groundwater report for north-central Indiana (Capps, Water-Supply Paper 254, 1910) prints the same South Bend well, citing the 16th Annual Report of the Indiana Department of Geology and Natural History, page 260, with the same drift of 160 feet and the same total of 2,027 feet. The two printings diverge in one member: where Howard's table splits the Hudson River and Utica into 200 feet each, Capps' source carries them combined at 420 feet — and Capps' combined figure is the one that reconciles with the stated total, Howard's split subtotals summing twenty feet short of it. The boring stood in the valley, some 150 feet below this upland; it is the county's one full pierce, and it is carried here at that boundary.
This ground's own identification: For the rock under the northeastern county — this ground — the identification is direct. The United States Geological Survey's study of northeastern St. Joseph County (Bayless, Westjohn & Watson, Water-Resources Investigations Report 95-4041, 1995) identifies the electrically conductive bedrock beneath that part of the county as the Ellsworth Shale. The Indiana Department of Natural Resources' bedrock aquifer mapping gives the county one bedrock aquifer system, the Devonian and Mississippian shales about 300 feet thick — of its three named shales the Coldwater is absent here, and the two present are the Ellsworth, a gray-green shale with limestone and dolomite lenses, lying above the Antrim, a brown-black shale. The period record agrees in kind: the first state geological reconnaissance of the district (Levette, 1874) reported the underlying rock as Devonian, reached by bores elsewhere in the seven-county district while wells at South Bend of 92 to 103 feet in his day "stopped in a boulder bed without reaching rock" — the 1874 state of knowledge, printed as the period record it is.
Sources: Howard (1907), Chapter I, §§III and VIII — the no-outcrop statement, the two-hundred-foot average, the South Bend boring section; Capps, The Underground Waters of North-Central Indiana, USGS Water-Supply Paper 254 (1910), pp. 205–212 — the same well from the 16th Annual Report, p. 260; Bayless, Westjohn & Watson, USGS WRIR 95-4041 (1995) — the Ellsworth Shale identification under the northeastern county; Indiana Department of Natural Resources, Division of Water, "Bedrock Aquifer Systems of St. Joseph County, Indiana" — the shale system and its members; Levette, "Report of Observations Made in the Counties of Dekalb, Steuben, LaGrange, Elkhart, Noble, St. Joseph, and Laporte," 5th Annual Report of the Geological Survey of Indiana (1874).