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The bedrock under the townsite — 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 Mishawaka 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 boring stood in the valley of the St. Joseph, at 725 feet above tide, "fully one hundred and fifty feet lower than the uplands in the southern part of the county," which run 875 to 900 feet. 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 are carried here side by side because they 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 drift figure, the Niagara's 640 feet and the Trenton's 427 agree in both.
Mishawaka's own boring: The townsite has a drilled section of its own. Capps prints the record of the Kamm & Schellinger brewery well at Mishawaka, drilled in 1906 to 723 feet: sand, gravel and clay — the drift — 130 feet; blue shale 240 feet; limestone 353 feet, with some sulphur water near the bottom. The ground under the city therefore reads, at the one point it has been drilled through: about 130 feet of glacial deposit, then shale, then limestone. That single bore is the deepest direct knowledge of Mishawaka's ground in the published record.
The modern mapping: The Indiana Department of Natural Resources' bedrock aquifer mapping for the county recognizes one bedrock aquifer system — the Devonian and Mississippian shales, about 300 feet thick, subcropping beneath the drift throughout the county; of the system's three named shales (Coldwater, Ellsworth and Antrim), the two present in this county are the Ellsworth shale, a gray-green shale with limestone and dolomite lenses, lying above the Antrim, a brown-black shale. The United States Geological Survey's study of northeastern St. Joseph County (Water-Resources Investigations Report 95-4041, 1995) likewise identifies the electrically conductive bedrock under that part of the county as the Ellsworth Shale. The period record agrees in kind if not in name: 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 at depths such as 88 feet at Fort Wayne, 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 here as the period record it is; the deeper borings of 1906 and after superseded it.
Sources: Howard, A History of St. Joseph County, Indiana, vol. 1 (1907), Chapter I, §§III and VIII — the no-outcrop statement, the two-hundred-foot average, the South Bend boring section and the upland elevations; 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, and the Kamm & Schellinger brewery well at Mishawaka, p. 209; Indiana Department of Natural Resources, Division of Water, "Bedrock Aquifer Systems of St. Joseph County, Indiana" — the Ellsworth and Antrim shales and the 300-foot system; Bayless, Westjohn & Watson, USGS WRIR 95-4041 (1995) — the Ellsworth Shale identification in the county's northeast; 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), pp. 429–474 — the Devonian floor and the boulder-bed wells.