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Histories:  Trempealeau Co. Historical Accounts:

"History of Trempealeau County Wisconsin, 1917":

Chapter 2

Geologic Divisions

(By George H. Squier)

 -As transcribed from pages 17 - 19

In order to understand the significance of the statement that our rocks belong near the top of the Cambrian and base of the Ordivician, it is necessary to have some knowledge of the geological time scale. The scale here given is the one commonly accepted as the standard:

Pleistocene.
Tertiary.
Cretaceous.
Jurassic.
Triassic. Permian.
Pennsylvanian.
Mississippian.
Devonian.
Silurian.
                 /   Upper
Ordovician      Middle.
                 \   Lower 
                               \
                    
              - Our local rocks
                               /
                 /   
Upper 
Cambrian       Middle
                 \   Lower

Pre-Cambrian

All of the periods are subdivided into numerous "formations," but in this list only the subdivisions are indicated that apply to the Cambrian and Ordovician, and only the larger subdivisions even for these. The range of our local rocks is also duly indicated. Since the older rocks are at the bottom, it will be seen that the Potsdam Sandstone (Cambrian) and the Lower Magnesian Limestone (Ordovician) are very ancient. The Lower and Middle Cambrian are not present in this region, consequently the Upper Cambrian rests directly on the Pre-Cambrian.

It is to be understood that the Pre-Cambrian is not a period comparable to the others in the table. It is, indeed, properly not a name at all, but merely a convenient designation for all of the immense series of rocks antedating the Cambrian, and includes a time, perhaps, as long as all succeeding time. The rocks have been so extensively folded and faulted and so generally metamorphosed and intruded by eruptives as to constitute a very complex problem, and while it is evident that the long series is capable of subdivision into periods comparable with those given above, the subdivisions proposed have not been accepted with the same approach to unanimity as these.

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 characterized 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.




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