_Time involved._--Though it cannot be reduced to numerical terms, the time involved in the deposition of these several formations of the Paleozoic must have been very long. It is probably to be reckoned in millions of years, rather than in denominations of a lower order. _Climatic conditions._--Little is known concerning the climate of this long period of sedimentation. Theoretical considerations have usually been thought to lead to the conclusion that the climate during this part of the earth's history was uniform, moist, and warm; but the conclusion seems not to be so well founded as to command great confidence. _The uplift._--After sedimentation had proceeded to some such extent as indicated, the sea again retired from central Wisconsin. This may have been because the sea bottom of this region rose, or because the sea bottom in other places was depressed, thus drawing off the water. The topography of this new land, like the topography of those portions of the sea bottom which are similarly situated, must have been for the most part level. Low swells and broad undulations may have existed, but no considerable prominences, and no sudden change of slope. The surface was probably so flat that it would have been regarded as a level surface had it been seen. The height to which the uplift carried the new land surface at the outset must ever remain a matter of conjecture. Some estimate may be made of the amount of uplift which the region has suffered since the beginning of this uplift, but it is unknown how much took place at this time, and how much in later periods of geological history. The new land surface at once became the site of new activities. All processes of land erosion at once attacked the new surface, in the effort to carry its materials back to the sea. The sculpturing of this plain, which, with some interruption, has continued to the present day, has given the region the chief elements of its present topography. But before considering the special history of erosion in this region, it may be well to consider briefly the general principles and processes of land degradation. CHAPTER III. GENERAL OUTLINE OF RAIN AND RIVER EROSION. _Elements of erosion._--The general process of subaerial erosion is divisible into the several sub-processes of weathering, transportation, and corrasion.[5] [5] There is an admirable exposition of this subject in Gilbert's "Henry Mountains." _Weathering_ is the term applied to all those processes which disintegrate and disrupt exposed surfaces of rock. It is accomplished chiefly by solution, changes in temperature, the wedge-work of ice and roots, the borings of animals, and such chemical changes as surface water and air effect. The products of weathering are transported by the direct action of gravity, by glaciers, by winds, and by running water. Of these the last is the most important.