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Histories:  Trempealeau Co. Historical Accounts:

"History of Trempealeau County Wisconsin, 1917":

Chapter 2

Successive Stages of Formation

(By George H. Squier)

 -As transcribed from pages 19 - 22

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 submerged, had received deposits of sediments of great thickness, and had also been intruded by enormous masses of eruptive rocks, its later history consisted, first, in the folding and faulting of the strata so that they formed mountain ranges comparable, perhaps, to the largest of our present mountains, 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 northward. 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 

Some time during the Tertiary there was again an elevation, and the streams resumed their down cutting. 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 PreCambrian, 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 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 wore more carefully studied it became evident that there had been more than one invasion, several, indeed, separated by periods of relative warmth, loamingly 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 anq 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 Illnciers 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 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 Trempealeau 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.


Resources for the above information:

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.




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