The Pleistocene of Wisconsin
In 1839 Charles Lyell named the Pleistocene (Gr. pleistos most + kaines new) because the life of that epoch constituted the closing stage of the transition from the geologic past to the present. Usage by many geologists has equated the Pleistocene, however, with the last Great Ice Age. Inherent with both the biologic and glacial approaches, as a common denominator, is the concept that climates over most, if not all, of the globe were distinctly cooler during the Pleistocene than the many tens of millions of years of geologic time that preceded (Emiliani 1954, 1955).
To understand fully the Pleistocene of Wisconsin we need to be cognizant of problems of nomenclature, of local physical and biological minutae, and of global and even astronomic events. This makes for a somewhat disjointed summary in order to avoid undue length, but many topics need no elaboration because the standard textbooks on the subject provide the necessary background (e.g., Flint 1957).
The generally accepted classification of the Pleistocene for North America was developed over the last 70 years in the Upper Mississippi Valley, including Wisconsin. It is now under attack by investigators elsewhere using various approaches, particularly on ocean sediments. Changes are now almost a daily occurrence in our classifications and many supposedly firm cornerstones probably will be overturned soon.
To this day authorities differ on the duration of the Pleistocene, in part because of different criteria used in defining its boundaries, but also because of differences in interpretation of available data (e.g., Ericson et al. 1963, 1964; Dreiman is 1964; Akers 1965; Krinsley and Newman 1965; Damon 1965; Flint 1965). Biologic changes and glaciation are both manifestations of climate, but they are time transgressive and are not necessarily synchronous. Clearly the biologic assemblages of that part of typical early Pleistocene that was dominated by glaciation in the north temperate latitudes had markedly earlier origins (Evernden et al. 1964). With notable exceptions, where transition beds of the Pliocene-Pleistocene boundary are recognized, no abrupt change (Flint 1965) occurs in either faunal or floral remains (Deevey 1965) or in related paleo-temperatures (Emiliani et al. 1961).
Thus, physical events and biologic changes are not yet integrated on a world-wide basis acceptable to all. Several frameworks of marine and terrestrial events are emerging. The disparity of chronologies between states or within continents, from continent to continent, or continent to ocean basin is striking (Wright and Frey 1965). It is too early to know whether we will end up with a Pleistocene Epoch based on fossils whose first part is characterized by a cooling but non-glacial climate in areas (outside Antarctica) where glaciation dominated the latter part, or whether the Pleistocene will be restricted to that time when glacial events characterized the land areas in the north temperate regions. The Pleistocene could include well over one million years (Ericson et al. 1963, 1964; Deevey 1965) or be only a few hundreds of thousands of years (Emiliani 1955, 1961).
The manifold arguments for the use of climate, biologic changes, or glaciation in defining the Pleistocene are beyond the scope of this book. For convenience the appropriate terms in the classical chronology of fourfold major stages of glaciation and threefold major interglacial stages will be used. Nonetheless, the rumblings one hears more frequently (and rightly so) that this classification must be modified (e.g., Deevey 1965) should be kept in mind.
The glacial stages are named for particular glacial deposits in the respective statesNebraskan, Kansan, Illinoian, and Wisconsinan from oldest to youngest. The Wisconsinan Stage was originally named East Wisconsin by T. C. Chamberlin (Geikie 1894:763), shortened by him to Wisconsin in 1895, and altered by Frye and Willman (1960) to the adjectival form to make it consistent with common usage of the other stages.
The Wisconsinan Stage is aptly named for we cannot identify with certainty any non-reworked or non-buried Pleistocene deposits of pre-Wisconsinan age in the state (Black 1962).1 The area of west-central Wisconsin, where widespread pre-Wisconsinan deposits were shown for several decades on glacial maps, has only recently been reevaluated (Black 1959a) with all surface deposits being correlated with the Wisconsinan Stage. Whether the older materials were never deposited, or, if deposited, have been removed remains debatable although both processes seem to have operated locally. Reworking of older Pleistocene deposits by Wisconsinan ice is recognized in a few places, and overrunning without removal of older deposits by younger ice also seems to have occurred. Hence, some deposits in Wisconsin now correlated with the Wisconsinan Stage may prove to be older when dating methods improve or new evidence appears.
Conversely, no unmodified landforms in Wisconsin can be said truly to be older than the Pleistocene. Unquestionably the oldest surfaces in Wisconsin are those in the classical Driftless Area of the southwest. Clearly that area was not invaded by ice of the last two major substages, the older of which covered most of the rest of the state. However, the former correlations of upland surfaces in southwest Wisconsin with early Pleistocene and pre-Pleistocene peneplain remnants (Trowbridge 1921; Bates 1939; Horberg 1946) have been discredited (Martin 1932; Thwaites 1960; Palmquist 1965).
At this time the Reserve includes part of the border of the Driftless Area. However, the early events recorded in the Driftless Area not only are of local concern but, as we shall see, affected the rest of the state as well. For example, the dissection of the uplands of southwest Wisconsin can be assigned tentatively (Palmquist 1965) to an erosion cycle comparable to one in Illinois that Frye (1963) concludes followed the Nebraskan Stage but preceded the Illinoian Stage. This cycle affected all the state, even though the evidence largely lies buried under drift outside the Driftless Area. Such problems are complicated and will be cited only with respect to the areas concerned.
The glacial deposits of Wisconsin, classified as end moraines, ground moraine, pitted and unpitted outwash, and lake deposits are shown in Fig. 2. This is a very generalized map prepared by Thwaites from data available in the files of the University of Wisconsin Geological and Natural History Survey, augmented by published geologic literature and from his interpretation of soils maps. Thwaites was the first to admit that in places the map represented only "guess work", but it still is the best available.
End moraines are deposited essentially at right angles to the direction of flow of ice at that margin and can be used to depict the lobation of the ice that formed them. Ground moraine was left behind the fronts in part as ice advanced and in part during stagnation and destruction of portions of the ice sheets. Outwash was laid down by streams outside the ice fronts, except where pitted. Pitted outwash clearly demonstrates buried ice masses of the same or an earlier advance were buried under the stream deposits (Thwaites 1926).
A very marked lobe of ice occupied the lowland of Green Bay-Lake Winnebago following the strike of the Platteville-Galena Group of dolomite (Ordovician, Fig. 3). The deployment of weak shale under the resistant dolomite of the Niagara Formation of Silurian age, both dipping gently eastward, led to the excavation of the lowland in large part by ice action. Thus, ice deployment was determined in part by rock type which controlled topography and also in part determined what the topography was to be by virtue of its ease of removal. In all instances the bedrock over which the ice traveled determined the composition of the drift deposited later.
Ice followed the lowland routes, but still was thick enough to fan out normal to the concentric end moraines depicted in Fig. 2. Where ice from adjacent lobes butted against each other, interlobate moraines formed. The classic example is that between the Green Bay Lobe where it moved southeastward and the Lake Michigan Lobe where it moved westwardthe world-famous Kettle Interlobate Moraine.
The end moraines mark fronts which represent distinct spans of time during which loss of ice (ablation) along the front equalled the resupply by the various processes of ice motion (Kamb 1964). Additional debris thus was being brought forward by the ice movement and dumped at the same site along the front. This requires a very delicate balance between the ice and its environment. On the scale used in Figs. 1 and 2 it is not possible to distinguish segments of end moraines as single entities where time differences are small. We know that the outer limit of the Green Bay Lobe, as depicted in Fig. 2 by relatively continuous end moraines, is actually a series of short segments of different lengths representing different times or short-term local pulsations of the ice. Such pulsations leave cross-cutting end moraines which can be shown only on maps of much larger scale. Some pulsations probably represented many centuries.
My approximation of the sub stage classification of the deposits, ignoring short time spans and lake deposits which are marginal and at least in part synchronous with the adjoining ice sheets, is shown in Figs. 1 and 2. All the deposits are correlated with the Wisconsinan Stage, two contrasting subdivisions of which are shown in Table 1 which also shows the generally recognized time of events in thousands of years B. P. (before the present or more exactly before 1950). A combination of terms from both classifications will be used in this paper even though they pose a problem in nomenclature for which no solution is immediately at hand. An additional term, Rockian, is used for latest Altonian time (Black 1962). A discussion of these problems, some of the reasons for the interpretation of time of events in Figs. 1 and 2, and other facets of the Pleistocene of Wisconsin follow after the description of individual areas. It is hoped that this discussion will provide further back ground for evaluation of the geology of individual areas and for fitting them to the regional format. More than that, however, it is hoped that the reader will be intrigued to look further into the many problems himself for possible alternative explanations.
Nine specific areas (Figs. 1, 2) are recommended for inclusion in the Ice Age National Scientific Reserve of Wisconsin. These are discussed for convenience in the order recorded by representatives of the Wisconsin Conservation Department and of the U.S. Department of the Interior in their various progress reports. They are:
1. Two Creeks Forest Bed
Detailed descriptions of boundaries of these areas, acreage, ownership, cost of acquisition, etc., have been covered in reports by others. In this report the boundaries shown in maps and sketches accompanying the description of each of these areas are predicated, within reasonable bounds of practicality, to preserve the most important features rather than all the desirable features. As such, these are minimal boundaries. Minor variations in order to follow property lines might be made locally without harm, but it is emphasized that general constriction in the boundaries can be made only to the detriment of the Reserve.
As stated in the preface, not all kinds of features attributable to the Ice Age are to be found in these nine areas. To include them all would mean an enlargement of the Reserve that would carry it to all parts of the state. It is hoped that this can be done ultimately. The recommended areas have features considered representative of many in the state, even though they are not necessarily the best features to be found there. Again, the bounds of geographic location, cost of ownership, and completeness of the geologic story to be told in a local area all played a role in the final selection of the specified areas. From the point of view of use by tourists, the Northern Kettle Interlobate Moraine, Devils Lake, Bloomer, and St. Croix Dalles areas are considered the four most important because of their size, accessibility to large metropolitan areas, and variety of features. Of these Devils Lake is by far the most important on all countsthe variety and uniqueness of its geology, scenery, fame, etc. Trebling the size of the present state park is mandatory if we are to preserve a representative sample of the variety of glacial features the area has to offer. This must be done nownot a few years from nowto avoid their loss. The recommended area is absolutely minimal; anything less will be much "too little too late."
In connection with each area, representative similar features else where in the state are cited where appropriate or known. At the close of this book other features or areas of marked interest are mentioned in hope that someday they too will become a part of the Reserve or at least that their owners will treat them in the best tradition of conservation and not exploit them to the loss of mankind.
Last Updated: 1-Apr-2005