The ice-sheet was also the probable cause of the erosion of the horizontal bedded stones, yet it appears that the ice did not greatly change the contour of the ground; for it is well known that glaciers do move over lands that are not frozen to the ice without causing much disturbance, especially where the gradient is small, and this was the probable condition of the Western plains during the ice age. Thus it seems that whatever disturbance this region has undergone could be partly attributed to ice-sheets without the presence of bowlder drift, because the temperature and texture of the ground in the limestone region were unfavorable for such accumulations; yet it may be owing to the action of ice that minerals once diffused are now found collected in fissures. The deep valleys through which the large rivers now pass on their way toward the sea were once filled with glaciers which flowed into them from their tributaries. Thus the deep trenches of the plains are largely the work of glaciers. It is generally supposed that the driftless region of Wisconsin was free from ice during the frigid period. But it seems impossible for this region to have escaped being covered by ice and snow, with the great lakes filled with glaciers, and the regions on all sides of the driftless area covered with ice. The reason why this territory escaped the drift from the north was on account of the hindrance which the drift-bearing ice-sheet encountered in the deep basin of Lake Superior. In this great depression the ice-sheet from the north was relieved of bowlders and other glacial drift, as well as obstructed in its southern movement. Therefore, the snow and ice which gathered on the driftless region had little movement in any direction, while the temperature and consistency of the ground under the ice were not favorable for the production of bowlder drift; and, when we consider that the Mississippi valley was deprived of great sources of warmth during the culmination of a glacial period, we are forced to the conclusion that its wide lands were also covered with snow and ice. The tropical waters of the North Atlantic were so much chilled by the floating icebergs of North-eastern America, Greenland, Iceland, and Northern Europe that the Caribbean Sea, its warmest reservoir, was reduced to a temperature so low that the easterly winds which blew over its waters were unable to prevent ice-sheets from gathering on Eastern Nicaragua.