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The glacial deposit itself — its bulk, its buried soils, its sealed waters
The wedge: The county history's geology, drawn from Prof. S. S. Gorby's paper in the state geological report for 1886, describes the drift of Indiana as a single vast mass "in the form of an irregular wedge, its thick end to the north, its edge, or thin end, to the south": bluish or smoky-gray boulder clay — till — as its chief component, growing thicker step by step northward, with bowlders of granite, gneiss, greenstone and other crystalline rocks "quite foreign to the area covered by the mass," ground from the Canadian highlands and carried south; beds of sand and gravel assorted by water along the courses; and, in the northern part of the state, deep basins in the boulder clay filled with water, the small lakes. The drift is not this county's soil in the sense of weathered local rock: it is transported ground, laid over a striated rock floor, and the townsite's 130 feet of it in the brewery well is one point in a body whose thickness varies with the buried surface it covers.
Buried forests and sealed waters: The county history's chapter, following Maurice Thompson's account of the northern Indiana drift, records what the wells and cuttings of the nineteenth century actually disclosed inside the mass: strata of ancient soil intercalated between sheets of boulder clay, "in which are found vegetable remains more or less preserved, consisting of tree-trunks, branches and roots, belonging to what have been large forest trees"; lenticular beds of sand and gravel gripped in the clay; and pockets of water "hermetically sealed up in the body of the blue boulder clay," which when tapped by borings would flow as artesian fountains. The 1907 explanation — repeated advances and retreats of the ice, forests growing on the drift in the intervals and buried uncrushed under the next advance, lake basins filled with ice and entombed the same way — is printed here as the period theory it is; the beds themselves, the buried soils and the sealed water-bearing sands, are the observed record, and they are the reason the drift of this county is a layered archive and not a single dumping.
The measured thicknesses: The period average of about 200 feet (Howard, above; Capps independently puts the county's till at "probably averages about 200 feet") is an average over strong variation, and the modern mapping prints the variation. The United States Geological Survey's northeastern-county study found the unconsolidated deposits — outwash, till and lake sediment together — running from about 15 to more than 70 metres thick, with the outwash sand and gravel lying mostly north and east of the river and forming the primary drinking-water source, while south of the river the sands and gravels thin and clay-rich deposits dominate. The state bedrock mapping puts the unconsolidated cover at 150 feet thick in the northeast and west of the county to over 300 feet in the northwest. Capps' valley figures give the local frame: the valley floors of the county lie under 700 feet of elevation and the till plain of the southeast stands above 850, and the St. Joseph's own valley, "during the retreat of the last glacier was filled with alluvial gravels. The stream now occupies a narrow valley which it has cut into the gravels" — a belt of alluvium two miles or more in width, with terraces standing on either side above the modern channel.
Sources: Howard (1907), Chapter I, §§III–V — Gorby's wedge description and Thompson's account of the buried soils, sealed water pockets and intercalated beds; Capps (1910), pp. 205–212 — the 200-foot till average, the valley filled with alluvial gravels, the terraces, the elevation frame; Bayless, Westjohn & Watson, USGS WRIR 95-4041 (1995) — drift thickness and the outwash/clay distribution in the northeastern county; Indiana DNR, Division of Water, bedrock aquifer mapping for St. Joseph County — the 150-to-over-300-foot range of the unconsolidated cover.