USA > Wisconsin > Trempealeau County > History of Trempealeau County, Wisconsin > Part 5
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Geological history is the record of successive changes wrought by two sets of forces. The one, operating within the body of the earth, causes changes of level of the land surface in its relation to the water level, some being carried below, and some above that level. The other, the various agencies of disintegration, acting upon those surfaces raised above water level, tend to wear them down. This erosion of the land results in two complementary sets of phenomena: (a) the planing down of the land surface until, if sufficient time be allowed, even a mountainous region may be reduced to a nearly level plain but little elevated above the sea level, a "base plane"; and (b) the transference of the material thus eroded from the land surface, mainly by running water, but to some extent by wind, until it comes to rest in some body of water, or at least in some basin from which there is no outlet, were it accumulates and may come to form deposits thousands of feet thick.
In the process of transformation the material becomes more or less assorted, and is deposited, under varying conditions as coarse fragments- conglomerate, sand, or mud. In addition to the material thus removed from the land, the growing deposits include the remains of the sucessive generations of living creatures which made their home in the water in which the beds are accumulating, and, since there was a continuous change in the forms of life, we thus have furnished us a means of the greatest value in determining what position a particular deposit occupies in the world's time scale.
It will be realized that the geological time scale does not propose to place events with the same exactitude as when we speak of an event as having occurred in a certain year and century, A. D. or B. C. It corresponds more nearly to our custom of dividing human events into periods character- ized by some noteworthy set of conditions, as, for example, the time of the crusades or the period of the renaissance. Geologists have given much study to the problem of attaining approximate equality for their divisions.
19
HISTORY OF TREMPEALEAU COUNTY
Having thus considered the broad principles on which geological history is based, we may now address ourselves more specifically to the history of this particular region.
As already indicated, our Potsdam Sandstones, which include some shales and impure limestones, and constitute a part, but probably not all, of the Upper Cambrian, rest directly on the Pre-Cambrian.
While the area of the Pre-Cambrian had been more than once sub- merged, had received deposits of sediments of great thickness, and had also been intruded by enormous masses of eruptive rocks, its later history con- sisted, first, in the folding and faulting of the strata so that they formed mountain ranges comparable, perhaps, to the largest of our present moun- tains, and, second, a long period of erosion during which these were worn down until the region had become one of very slight relief, diversified only by hills of moderate elevation.
When again the region became depressed so as to be covered by a shallow sea, the beds of the Upper Cambrian were deposited. These deposits were made not only over the region in which they are now found, but also over the entire state, including the areas of crystaline rocks to the north- ward. Not alone the Cambrian, but also Ordivician rocks (Lower Magnesian Limestone, St. Peter Sandstone, Trenton Limestone) overspread all, or a considerable portion of the region. Other beds of the Ordovician and Silurian which now outcrop successively further south and east, undoubtedly extended much further northward and westward than at present, but we have no means of determining how far. We may be fairly confident that the lower Magnesian Limestone (that forming the tops of the bluffs along the Mississippi) overspread the entire country. Nor is there much doubt that the St. Peter Limestone (not now found in the county) did so also. There is considerable ground for the belief that the Trenton Limestone, of which only a few remnants are now found north of the Wisconsin River, in Vernon County, also overspread at least the southern part of the county.
While these processes were going on the region seems to have been affected by only slight changes of level, remaining quite near sea level throughout the Ordovician, Silurian, Devonian, Mississippian and most of the Pennsylvanian. But toward the end of the Pennsylvanian, or in the Permian, there was a period of elevation. In the eastern part of the United States, mountains (the Appalachians) were the result. But in Wisconsin there was only a moderate elevation, not sufficient to warp or disarrange the strata.
The necessary result followed. The region was brought under the influence of eroding agents. Streams began to cut their valleys. When they had cut as deep a they could at the then height of the land, they widened them, and as they had a long time in which to work-through the Permian, Triassic, Jurassic and Cretaceous-they cut away the entire surface, down to base level, leaving a great plain. Only a few hills-the Blue Mounds, Platte Mounds and others south of the Wisconsin River- which were composed of more than usually resistant rocks, remain to give us some idea as to the thickness of the rocks thus planed away.1
20
HISTORY OF TREMPEALEAU COUNTY
Some time during the Tertiary there was again an elevation, and the streams resumed their downcutting. Since the valleys which they then formed are those we now see, we are interested in knowing something of the plain as it was when they began to cut.
If we could reconstruct the Tertiary base plain as it was before the streams had cut deeply into it, we should find that near the Mississippi, it coincided closely with the present tops of the higher bluffs-those capped by the Lower Magnesian Limestone-but that it rose gradually to the northward, so that the hills in the northern part do not reach to within three or four hundred feet of the old plain surface. Going northward beyond the county, the plain would be above the present surface of the crystaline rocks over the greater part of the area of the state. This plain, we must realize, then lay so that the surface was nowhere more than three or four hundred feet above the sea level. The elevation during the Tertiary was in the nature of a tilting, as though a board was raised at one end, the other remaining on the surface, the amount of elevation increasing to the northward. It is to be further observed that the old Pre-Cambrian surface on which the Cambrian rests, is in itself a tilted base plain, having such a slope that if it were fully exposed, streams running over it would have swift courses and great erosive power.
We are to suppose the Tertiary base plain as floored with Cambrian or later rocks over the entire area of the state, except that included in Iron, Vilas, Oneida and adjoining counties, where it cut through to the Pre-Cambrian, also cutting some of that, making it an integral part of the plain and producing a surface which did not conform with the slopes of the surrounding Pre-Cambrian areas. The surface of these counties now has a nearly consistent level of about 1,600 feet, and as this surface was the level to which the Tertiary base plain was carried by its tilt, the amount of the tilt or elevation may thus be determined.
The greater part of the present area of the state, floored by Pre- Cambrian, has been stripped of its Cambrian and later rock covering, since that time. If we attempt to visualize the Tertiary base plain and consider the amount of material that has been removed, we shall realize that the aspect of the valleys has undergone constant though slow change.
It will be interesting here to picture the conditions just before the opening of the Pleistocene Period, when the valleys had reached their greatest depth. Of the various artesian wells from which we gain our knowledge of the position of the old rock bottom of the valleys, few, perhaps none, strike that bottom at the deepest part, but they indicate that the old channel of the Mississippi River was somewhere near two hundred feet below the present river level, or, say, three hundred feet below the present level of Trempealeau Prairie. That would indicate that our bluffs, which now rise about six hundred feet above the river, were then nearer eight hundred feet. The valleys were also considerably narrower and more canyon-like. Moreover, the thick deposits of clay that now mantle our lower hills and fill the coulies were then absent and only jagged ledges of rock, thinly covered with sandy soil, would meet the eye. The tributary valleys were also correspondingly deeper, and displayed the same characteristics
21
IHISTORY OF TREMPEALEAU COUNTY
in a less degree. It was a region, no doubt, of much scenic attraction, but rather inhospitable.
When, with the development of geological knowledge, scientists came to realize that the deposits which in the early days of geology were called diluvial, were really made by glaciers which had overspread great areas in many parts of the world, it was supposed that there had been but a single invasion, and it was called the Glacial Period. But as the phenomena were more carefully studied it became evident that there had been more than one invasion, several, indeed, separated by periods of relative warmth, seemingly even warmer than the present, and for this whole succession the term Pleistocene came to be applied.
These various invasions did not cover the same area, and the older ones seem to have been more severe; at least they extended much further south than the later. One, west of the Mississippi, advanced as far as northeastern Kansas, and east of that stream one reached southern Illinois. But there was an area, mostly in Wisconsin, and, broadly speaking, including the portion of the state lying between the Wisconsin and Mississippi rivers, and northward so as to embrace the larger share of Trempealeau and Jackson counties, which appears never to have been overspread by a glacier. The last-Wisconsin-glacier did not indeed advance nearly so far south as the limits named. There is some little doubt as yet as to the extreme southerly limits reached by the oldest glacier. The greater share of the region shows none of that modification of topography which is a distinctive characteristic of glacial action.
But though the glaciers did not overspread this region, they exercised a notable influence over the conditions within it. This was due (a) to the fact that some streams bearing glacial outwash traversed the region, (b) to the influence of the encircling glaciers on the climate, and (c) to the effect of the glaciers on the water level.
(a) Those streams, some portions of whose drainage basins were invaded by glaciers, received large amounts of glacial outwash-sand pebbles-and all such material capable of being transported by stream action could be carried far beyond the region of glaciation. Within the boundaries of Trempealeau County the Mississippi and Black rivers were the principal carriers of such material. It has been supposed that the Trempealeau Valley lay outside the glaciated region entirely. The writer was first to call attention to the deposits near Taylor and Blair. The Mississippi must have been the carrier of glacial outwash during most, if not all, of the glacial periods; but the Black only for some of the earlier.
(b) The climate of the driftless area-as the region not covered with glaciers is called-would have been subject to the chilling effect of the near-by glaciers. There is also reason to believe that the glaciers acted something like a mountain range in draining the air of moisture, rendering the region rather dry.
(c) There are two ways in which we may conceive of a glacier as affecting the water level. The first is by isostatic readjustment. This assumes that the crust of the earth has little stiffness and yields readily, either upward or downward in response to any change of weight near the
22
HISTORY OF TREMPEALEAU COUNTY
surface. As some of the glaciers attained a thickness of several thousand feet, they represented a great increase of weight over the surface, and as a consequence there was a downward warping of the crust. If, however, as some believe, the crust is much more resistant to such influences than the theory of "isostosy" supposes, the accumulation of such great masses of ice would, by increasing the gravitative energy of portions of the earth's surface relative to others, produce such a shifting of the center of gravity as to cause readjustment of the water level to compensate. One or the other of these agencies (not both, at least to the extent that the first agency was effective, the second was excluded) must, I think, be assumed to have been operative during each of the glacial periods. But other agencies not necessarily depending on the presence of the glaciers may have modified, increased or diminished, the results. It will be obvious that if a glacier enters a valley at some point below its head, leaving the upper portion free of ice, the result will be a dam, and the impounded water will form a lake. This also might operate in combination with the others, modifying the results. It is not possible in the present stage of the investigation to assign to these several agencies their proportionate share in bringing about the submergencies which we know from ample evidence to have affected the region of the upper Mississippi.
The stage of the submergence was quite variable; it stood, however, for a considerable time at a point between three and four hundred feet above the present river level, though there is much evidence of one actually overtopping the bluffs. The result of the submergence was the deposition of thick beds of lacustine material over the foothills and lower two-thirds of the bluffs. It is to this deposit that we owe the fact that the foothills furnish many of our finest farms. Without it they would be rocky ledges, or steep slopes, thinly covered with sandy soil.
Studied in detail, these deposits form an extremely complex series which could not even be described without filling many pages and using much illustrative material.
These periods of submergence did not, however, extend through the Pleistocene period; there were other long periods when the Mississippi Valley was occupied by a stream, either one comparable in size to the present stream, or one of vastly greater volume, carrying away the drainage from the glaciers and loaded with glacial outwash. These mostly flowed at a higher level than the present, a level marked by the deposits of Trem- pealeau Prairie. On the other hand, the warm interglacial periods were times of down cutting, during which the river often flowed at levels below the present. One such has been brought to our knowledge during the present summer (1917) through the sinking of the piers of the Burlington bridge at Trempealeau Bay, showing many feet of mud deposits loaded with shells and wood, also marginal peat bogs, and indicating river levels at from forty to sixty feet or more below the present. We can also trace lines of cliffs marking the shore lines for some of the river stages, though they have been partly obscured by more recent outwash from the bluffs. The interrelations of these various phases are still far from having been fully worked out.
44.06
N.
11
111
-W. _ 44.03
44.01
91.33
75%
91.31
08.16
1.2.16
87.16
9127
91.26
44.01
1
GEOLOGICAL PLATE, NO. I.
'The Mississippi River and its branch streams are represented in open hachures at the stage when they ran In gorge-like valleys, Itrely wide enough to carry the streams, while the outlines of the valleys as they now exist are shown in close hachures. A large amount of the space now included in the valley was then high land similar to the present bluffs. Ilow the divide between the Missis- slppl and the parallel branches would have been gradually cut away Is evident.
23
HISTORY OF TREMPEALEAU COUNTY
It remains before bringing this article to a close, to notice that feature, which, because it is so conspicuous and distinctive, has attracted the atten- tion of all who have entered the region, Indians apparently as well as whites, the Trempealeau bluffs.
It is, perhaps, generally recognized that these were at one time a part of the west (Minnesota) shore, but the. process through which they became separated is not well understood.
In one of the recent publications of our State Geological Survey, Mr. Martin, who, I understood, had not personally studied the situation, gives an explanation which is quite incorrect-impossible, indeed.2 His expla- nations and diagrams assume that the notch at Trempealeau Bay was the continuation of one of the valleys on the Minnesota side. But the valley in question is very much wider than the notch, and no explanation is offered of an adequate agency for the removal of the divide at the place where it is assumed to have been removed.
To correctly understand the process, it must be remembered that when the streams were "young," they were flowing in narrow, gorge-like valleys. and that in the case of the Mississippi, this was probably much nearer the Wisconsin than the Minnesota side of the present valley. On the Minnesota side several of the small streams united in one which partly paralleled the Mississippi, but which, in its meandering, approached it more closely for a stretch of its upper course than it did below. As the streams, having cut down to grade, proceeded to widen their valleys, the narrow divide between this parallel stream and the Mississippi was gradually cut away.
It must be borne in mind that so long as the streams were running on the rock bottoms, this divide might be wholly removed for some distance above our present Trempealeau bluffs without causing the diversion of the Mississippi into the smaller body, because, not only would the steeper grade of the smaller valley have carried its bottom above that of the larger stream, but the greater depth of the channel required by the larger stream would be sufficient to control its flow even though their surfaces had been at the same level. When, however, the conditions had changed so that the Mississippi did not keep its channel cleared out, but instead became gradually filled, its newer course was left unobstructed. Some other attendant circumstances, also, would have made that its most easy and natural course.
Naturally, when the large stream invaded the valley of the small one, there began a rapid process of erosion whereby the salient points and minor flexures were reduced into an adjustment to its own requirements.
The accompanying diagramatic map is supposed to show the conditions while the valleys were still narrower; the consequences of the widening of the valleys will be readily apparent.
The point where the Trempealeau chain of bluffs connected with the Minnesota shore is a matter of some interest. The projecting headland on the Minnesota shore which may be supposed to have marked the point of junction has, of course, been worn away, but it is believed that the long line of cliffs near Homer has resulted from such rapid wearing back
24
HISTORY OF TREMPEALEAU COUNTY
of the shore line and marks the probable line of junction, as it is also the point toward which the present trend of the Trempealeau bluffs points.
The conspicuous isolation and insular position of Trempealeau Moun- tain proper may call for a few remarks.
It is obvious that not only the larger streams, but the smaller ones, and the torrent courses were everywhere dissecting the region. Small valleys similar to those now extending into our bluffs would also have existed in the portions now wholly removed. One who is familiar with the present condition of our bluffs will realize how little erosion along their north side would serve to remove the low connecting ridges and leave, instead of a connected chain, three or four disconnected hills. The little valley between Trempealeau Mountain and Brady's Bluff had been cut so low that the flooded Mississippi was able to pass through and further rapid deepening was the result.
In reviewing briefly the facts of the preparation of Trempealeau County for the occupancy of man, a summary of the foregoing facts may prove of interest. At the end of the Pre-Cambrian period, Trempealeau County presented a sloping surface of bare rock, comparatively level, but containing some hills of moderate elevation. In the Cambrian period the region was depressed and covered with a shallow sea. During this and succeeding periods various layers of sandstone (pulverized rock) and limestone (pulverized shells) were deposited in the bed of this shallow sea. Just which of these layers were laid down in Trempealeau County is somewhat uncertain. The Pottsdam sandstone and the Lower Magnesian limestone still remains, the latter being seen in the tops of the Mississippi bluffs. The region remained submerged during the Ordovician, Silurian, Devonian, Mississippian and most of the Pennsylvanian period. But toward the close of the Pennsylvanian, or in the Permian period, the region was elevated above the sea level. Streams began to cut valleys. When they had cut as deep as they could they began to widen these valleys. This process continued during the Permian, Triassic, Jurassic and Cretaceous periods until the region was again a great sloping level plain. This plain was surfaced with the Lower Magnesian limestone and coincided with the present tops of the Mississippi bluffs. But it rose rapidly in elevation to the northward so that the present hills in the northern part of the country are three or four hundred feet below what was then the surface of the plain. In the Tertiary period streams began cutting through this plain. A vast amount of material was removed and the present valleys were formed. At the opening of the Pleistocene Period the rock foundation of Trempealeau County lay practically in its present form. The valleys, however, were much narrower and deeper and the sides much steeper. Except for thin deposits of sandy soil, all the county was a region of bare and jagged rocks. Then came the Pleistocene Period with its glacial periods, when glaciers formed and were melted again several times. A larger part of Trempealeau County is in what is called the Driftless Area, and was probably never covered with a glacier. But it was to the glaciers that we owe the present condition of the county. During the time of the glaciers the county received in the Mississippi, Black and, to some extent, the Trempealeau Valley,
25
HISTORY OF TREMPEALEAU COUNTY
sandy pebbles carried by the streams flowing away from the glaciers, and during the several times that the county was submerged during this era, the bare valleys and foothills, lying in the bed of the muddy lakes, formed by the melting glaciers, received the deposits which now constitute the foundation of our soil. At times during the Glacial Periods the Mississippi bed was higher than at present and at times lower. The original bed of the Mississippi was probably over the Trempealeau Prairie, and the Trem- pealeau Bluffs are probably really an extension of the Minnesota Bluffs, the belief being that in this region the Mississippi is now flowing in what was the bed of a nearly parallel tributary. In the rich deposits left by the glacial lakes vegetation began to grow, and the decomposing vegetation mingling with the deposits formed the soil as it was found by the early settlers.
There is little to be said as to the mineral resources of the county. Its wealth lies in its agricultural resources. It is among the possibilities . of the future that iron may be found in the underlying Pre-Cambrian rocks. And while it would be difficult, under present conditions, to mine it profitably, it would be possible that improved mining methods and the exhaustion of the more easily-mined 'deposits would sometimes make it possible.
Waterpowers have been developed at various points in the county, and the resulting mills have been an important factor in the economic develop- ment of the county.
The watercourses and many of the ridges are heavily wooded, thus furnishing the farmers with plenty of fuel and building material. Contrary to usual conditions where the coming of the white men has resulted in the denuding of the forests, there was little timber here when the settlers came but has been allowed to grow up in the past sixty years.
1-It is not to be understood that the history was quite as simple as the sketch indicates. Even a relatively stable portion of the earth's crust is rarely wholly so for prolonged periods. To record the minor oscillations, even if they were always determinable, would be quite unpractical in an article of this character.
2-Martin, Physical Geography of Wisconsin, 136-197.
CHAPTER III ARCHAEOLOGY.
(By George H. Squier)
It is so rarely the case that our present political divisions correspond closely with the outlines of any of the older tribal domains, or habitats, that when such happens to be the case, it is not only a matter of interest, but it furnishes a peculiarly satisfactory theme for the writer.
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