any one time their full significance. We now know, however, that the shapes of the pebbles and the scratches upon them indicate glacial action, and that where we stand amid a landscape clothed in the verdure of summer, but a short time ago, geologically speaking, winter, centuries if not millenniums long, reigned and wrought. So just as the pearly shell held to the ear repeats the story of its ocean home, these faceted pebbles could tell us if we listened of the march of a vast sheet of ice by which they were caught up and swept onward, and in which they were held and rubbed, now against their fellows and now against the rocky floor beneath, until they acquired the shape which they now possess. We know this because we know that it is just what glaciers do to the rocks they carry nowadays, and no other force can quite imitate their effects. Having thus once become aware of the former existence of a glacier in the place where we are standing, the interpretation of every feature around us will be influenced by that fact. If along the roadside we find a place where the blanket of soil has been recently removed so as to expose the hard ledge beneath, the surface of the latter will be seen to be smoothed and leveled, and in occasional exposures, scratches or grooves somewhat like those on the pebbles just spoken of except that they are deeper, more regular and nearly all run in one direction, may be seen. These clearly mark the planing down by the glacier of the surface of the rock over which it passed and the direction of movement. The recording, according to the direction of the compass, of these striæ in several places in any given locality is always a work worthy to be performed, since it gives not only the general direction of movement of the glacier, but its local variations. Sometimes two sets of striæ can be seen running in different directions, one of them crossing and hence later than the other. These usually mark the movement of a later glacier, for we know that the ice which covered the country advanced and retreated no less than five times. The evidence of these successive stages can be sought to better advantage, however, in the topography of a region than in the markings on the rock surfaces. It is evident that the effect of the movement of such a vast mass of ice and rock over a region must have been to plane down its prominences and smooth and round over the whole area. This smoothing and rounding effect is well shown in the mountain prominences seen in the accompanying cut. Here the glaciation has been so recent that vegetation and decay have not come in and obscured the surface as is the case over most of the area of the northeastern United States. Contrast this with the high bluffs along the Mississippi river in Wisconsin, which are in a non glaciated area, or with pinnacles such as Western States, the result of long erosion there are many of in Colorado and other untouched by glacial action and the effect of glacial movement can readily be seen. If now one thinks for a moment what 1672 avoidance of this area by the glacier: one that the bed of Lake Superior lying north swallowed so much of the ice that there was little to be passed onward and the other that the dryness of the region caused the act of ve and snow which it received to be sta Consolering now again the deposition of the rock and so by the melting of the glaDer be seen that two agencies would efect its scrbie-the is gravity, the econd win. The melting of the glacier will be marked by streams of water in very car, whe wood float and dismice such of the material as they could cambo the large boulders would evibently remain very nearly where they lay The the art movement of the glacier ceased. The should expect, therefore, beans of large boulcers and pebbles in some rinces and others broad, fu accumulatives of fine soil, especially if a clayey nature, scs is usualy found in cacers formed of the fine patices the wear of the 708 I ad ther. Of CER. I single bill or TALIT THETICS Tariations Sine course material v roadi- be frend show70s the force of vice with prevailed as formacion bet one Tepet i general EstriS SIC as I have indi I would be expected Sturms best the dig 1 10 the glacier's me has wel be at the ter vher the ere be sk as Som the term the accumination of racer. Di may 1674 the swift rushing waters are dropped as EACH ROUNDED BACKWARD CUTTING BY STREAM. of the Middle West. One at Lemont, Illi- It is evident that the flowing waters from sea. THE PEBBLES OF A BROOK BED ARE SORTED AC- |