Beaver Creek shows, south of Napoleon, a terrace not very distinctly formed, 100 feet above the stream. Near Beaver Lake it is well developed to the height of 65 feet. Ascending the south branch of Beaver Creek we find first a narrow valley between the moraine and the tableland west. This soon widens to 3 miles near Green Lake and to 10 or 12 miles near Ashley, and southeast from that point it seems to fill the whole space bounded by the Koto Hills and adjoining loops of the moraine. On the west side it extends up two or three tributaries into the table-land. This plain is more stony next to the moraine and a few feet higher, while its central portion is more covered with loam and free from stone. This plain is clearly a terrace about Green Lake, but south and southeast becomes a plain pitted with lakes. The general difference in elevation between the lakes and the upland is 40 to 50 feet. It may be that the whole region has been mostly filled to that depth. Such at least is shown to be true west of Ashley by artificial excavations. The higher levels between the lakes are frequently covered with loam, while lower surfaces and shoulders, which are about 30 to 35 feet above the lakes, are composed of stratified gravel 5 to 6 feet in thickness. The surface of the plain is considerably more excavated toward the northeast side, near the principal outlets of the moraine. At Lagrace a bowldery terrace is rather indistinctly shown about 240 to 250 feet above the Missouri and another much more distinctly shown at 100 feet. At Lebeau, a terrace 300 feet high lies in the angle between Swan Creek and the Missouri (Pl. XVIA). Around to the southwest and even extending as a channel through north, connecting the valleys of the streams, is a terrace 200 feet above the river. At the mouth of the Moreau, opposite this point, is a finely developed terrace estimated to be at the same level. This terrace is found developed along Swan Creek, south of Bangor at about 100 feet above the stream, and at Swan Lake the stream has risen nearly to its head. A view of Spring Lake outlet, through which much water poured into Swan Creek, is and shown (Pl. XVIB) and osars (Pl. XVII). Above the mouth of the Little Cheyenne is a fine exhibition of a bowldery terrace, which stands 250 to 270 feet above the river, and extends with considerable width most of the way from Forest City to Fairbank (Pl. XVIII). At Pierre, just north of the city, is a terrace having its higher points 325 feet above the river, although much of its surface, which is more than a mile in width, is about 270 feet (Pl. XIX). About Medicine Butte there is an accumulation of drift having the form of a terrace. The summit of the ridge east of Canning resembles a terrace (Pl. XX). Again, from Jones Landing, 15 miles south of Blunt, extending through to the Great Bend, is an unusually broad development of this terrace at the height of 270 to 300 feet. In the Great Bend a terrace rises nearly to the height of 235 feet, but its low development is probably the result of erosion. Chapelle Creek and the other streams in this region show no terraces higher than this one that has been traced along the Missouri River. Thus far we have spoken of streams entirely external to the First moraine, with the exception of Spring River. A few of these extend their tributaries, either recent or ancient, into the region between the First and Second moraines. The Second moraine is drained by Apple Creek, through Long Lake Valley, from near Crystal Springs, and through Lake Edwards and its valley, the principal outlets from this loop. Beaver Creek, through Blue Lake outlet, Green Lake, and Russian and Dutch outlets, also passes inside of the First moraine. Swan Creek does the same through Pembroke outlet by its south branch, while the north branch received water from the Second over the First about Bowdle. Furthermore, after the vacation of the First moraine, a new set of streams arose to drain the Second, viz, Spring River, Blue Blanket, Artichoke, Okobojo, and Medicine creeks. These lie for most of their course between the moraines. They do not differ greatly from the previous class of streams except that bowldery terraces are not so clearly marked. Their upper portions in the vicinity of the Second moraine are usually broad, shallow valleys occupied by low sand or gravel terraces, as in the case of Okobojo, near Copp, the Blue Blanket, in Campbell County, and Spring River, in McPherson County. Not only did these streams receive water from the ice sheet when occupying the Second moraine, but in a few cases the present drainage extends within the Second moraine. For instance, the western portion of the Blue Lake loop of the Second moraine drains westward into Lake Edwards; so also the southern portion of the Antelope Valley loop and the northwestern portion of the Ree Valley drain through the Second moraine. With these slight exceptions, the Second moraine forms the divide between the Missouri and James rivers. It will be seen from this discussion that in almost every case important streams outside the First moraine received waters from the Second; also that it is very difficult to decide from which moraine the terrace deposits were derived. It is not unlikely that both may have contributed to them. From the study of the region about the head of the south branch of Beaver Creek, it would appear that there was a period of rapid deposition followed by one of corrasion and excavation. The first suggestion would be that both marked different stages of the occupation of the First moraine, but the relation of channels to the Second moraine render it possible to suppose that the deposition may have been during the First moraine, while the excavation was principally during the second. We must conclude, however, that data have not been sufficiently collected to satisfactorily determine the question.1 Further discussion of this subject may be found in Proc. A. A. A. S., Vol. XXXVII, p. 203; also Bull. No. 1, South Dakota Geol. Survey, p. 123. |