Successive geologic changes by which visible Wisconsin was shaped are revealed in exposed rock masses in many parts of the State. Oldest of these, the Archean fire-born stones of the north, found at Wausau, Rhinelander, and Chippewa Falls, may be remnants of the earth's primeval crust. They are part of the foundational body of the early North American Continent, which extended from Alaska to Labrador and thence south to the present latitudes of southern Missouri and Tennessee. This land was a barren expanse surrounded by seas that held only the simplest of single-celled life. Wind, rain, frost, sun, and all corrosive chemistries slowly planed the heights. Then, with a downward movement of the earth's crust, the continent sank into the sea where surface waste had already been deposited.
Into the waters covering what is now Wisconsin poured sediments from southern and eastern shores; they settled and eventually became thick deposits of hard crystalline sandstones and conglomerates. Finally the sunken land began slowly to rise again, and, as it presented its new face to the elements, portions of the great mass folded and fractured to form mountains. During the process molten rock welled up from deep within the earth and invaded the roots of the mountains. Pressure and heat metamorphosed sandstone into quartzite, shale into slate, limestone into marble. This period of flood and subsequent land elevation and the period of like action which immediately followed are known as the Lower and Upper Huronian stages in Wisconsin's geologic history. The purple-grey rocks seen in the present Baraboo Ranges are quartzite of these periods. Huronian slate and marble are both revealed near Mellen. Some of the rock formations are rich in iron; good examples are found at Hurley.
The alternation of deposition and erosion continued. Still a third sea terminated millions of years of levelling. Differing from the previous oceans whose beds lay quietly gathering their sedimentary stores, the third presented a restless floor. Alternate short terms of erosion and deposition occurred as the ocean's bottom frequently emerged from the deep, then sank again. Volcanic activity accompanied these processes, and great surface areas were covered with the hot spread of lavas. Upon cessation of volcanic action sandstone was deposited. A final movement upheaved the earth's crust again, and mountains were made. Thereafter northern Wisconsin was depressed in a mighty downward fold of the earth, the Lake Superior syncline. St. Croix Falls, Mellen, and Superior afford good outcroppings of the Keeweenawan masses of lava, sandstone, and conglomerate. In places the formation reaches a thickness of 55,000 feet.
With the end of the Keeweenawan period more than half of the earth's estimated billion years of record had passed; living things had developed slightly in complexity but, without backbone or shell, were still confined to water.
Then began a long age of land rest, with the bulk of Wisconsin protruding above waters whose northern boundary lay close to the present Illinois-Wisconsin line. The elements continued to wear away the land until the only heights that remained were a few rock masses in the central part of the State and the early Huronian mountains, including the Baraboo Ranges. The waters gradually crept over the land until northwest Wisconsin became an extensive peninsula compartioned only by offshore islands of rock. With complete inundation came the deposition of sands, fine muds, and clays that form the Cambrian series of rocks, examples of which are present at Madison, La Crosse, Eau Claire, Camp Douglas, Trempealeau, and Lodi. Soft and porous, the Cambrian sandstones are the natural reservoirs that supply water to many Wisconsin cities.
While the ancient sea sometimes covered Wisconsin, sometimes claimed only parts, two more geologic periods passed. Animal and plant life had progressed materially. Shelled fauna were plentiful, corals built their limey colonies, and earliest fishes swam through the weeds. In southern and eastern parts of the State the stacked formations of Ordovician and Silurian times lie in ordered sequence save where complete erosion of a formation or insufficient deposition has caused a "lost interval"; in such places rocks of one series lie directly upon those of an older series, without trace of the missing term in the structural record.
First of the Ordovician deposits is the lower Magnesian limestone, a heavy rock quarried near Madison and La Crosse and used for road building and general construction. Frequently complete erosion of this particular limestone resulted in a "lost interval," indicated by the abutment of the second Ordovician formation immediately upon the Cambrian series.
St. Peter sandstone, a formation of the second Ordovician deposit, which appears, for example, at Viroqua, is a source of water supply for southern and eastern Wisconsin. Because of insufficient St. Peter deposition the next formation, Platteville limestone, rests in some places upon Lower Magnesian limestone.
Two limestone deposits, Platteville followed by Galena, were made in Ordovician time. Small quantities of lead and zinc are distributed throughout the Galena formation and are most concentrated in southwestern Grant, Iowa, and Lafayette Counties. A thickness of one hundred to five hundred feet of shale completes the Ordovician system in the State. Along the east side of Lake Winnebago near Fond du Lac, Richmond shale is dug out and used in the making of tile and brick.
Clinton iron ore is derived from deposits made in the Silurian period. Once mined at Iron Ridge in Dodge County, the low grade ore of the Clinton formation is not in sufficient demand to be profitably mined. Directly above the Clinton deposits is a series of limestone beds known as the Niagara formation. It is believed that these thick deposits are in part compounded of extensive coral reefs which were reduced to sedimentary muds by erosive water action. Appearing as a line of westfacing cliffs all the way from a point slightly north of Waukesha to the tip end of Door County, the formation is known as the Niagara escarpment and is a prominent physiographic feature of Wisconsin. Racine, Waukesha, Chilton, Sturgeon Bay, and Green Bay lie on or near it.
Devonian time was a signal period in the State's geologic evolution. Shallow seas deposited limestone and shale -- the Milwaukee formation -- in a small area along the Milwaukee shore of Lake Michigan. After Devonian time all the area that is now Wisconsin rose above sea level, and there, as far as can be ascertained, it remained. Millions of years passed; through erosion old mountains were again exposed; the Baraboo Ranges took their place as surface features.
Animals and plants became more detailed in bodily structure, more varied in kind.
Another radical altering of surface features began within the Pleistocene period, one million or more years ago, when great ice sheets advanced upon the continent; in their course they covered all but the southwestern quarter of Wisconsin. Moving cumbrously over ridges, deep valleys, and rounded hills, the glaciers ground away the hill tops and filled the valleys with the accumulation of their grinding and scouring. Sand, clay, gravel, and even huge boulders were all parts of the glacial load. In periods of warm temperature the forward parts of the ice sheets melted, but each time when the cold returned the glaciers again extended south. There were four major advances or forward movements of the ice.
When the continental glaciers melted away, probably only a few tens of thousands of years ago, they left a vastly changed Wisconsin, with only the "driftless" or unglaciated area of the southwest representative of previous surface features. Irregular humps and depressions were now characteristic configurations. The former are the kames, eskers, drumlins, and moraines -- mounds of glacial accumulation which the ice sheets dropped in temporary periods of melting or in their final recession. The valleys, which were dammed by glacial deposit, and the depressions left where buried ice masses melted are the basins for most of the lakes in the State.