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altitude of 1,690 feet, on the middle of the south line of sec. 9, T. 107 N., R. 63 W. The lowest points are along James River, the surface of which has an altitude of about 1,200 feet in the southern and 1,230 feet in the northern portion of the region.

GEOGRAPHY.

The whole surface was originally prairie with the exception of a few small groves along James River. One of these which attracted much attention in the early settlement of the country lies in a bend of James River near Forestburg. The whole surface is underlain by a clayey subsoil with the exception of some sandy areas in the vicinities of Letcher, Forestburg, and Huron. North and northwest of Forestburg the sand is so extensive that sand hills 15 to 30 feet in height have been formed. One of these opposite the mouth of Redstone Creek is

known as Belchers Mound.

The area is drained mainly by James River and its tributaries. None of the tributaries contain flowing water throughout the year except Sand Creek for 7 or 8 miles above its mouth and Pearl Creek for half that distance. James River flows in a trough about half a mile wide and from 50 to 80 feet deep. It is a sluggish stream, having a width of 80 to 100 feet in ordinary stages and a depth of 3 to 10 feet. Its bottom land is fertile, but much of it is subject to occasional

overflow.

The arrangement of the streams has been mainly determined by the movements of the ice sheet during the Glacial epoch, as will be explained in a subsequent section.

GEOLOGY.

The geology of this area is simple. Nearly the whole surface is covered with the glacial clays and stream deposits of the Quaternary. The rocks lie nearly horizontal and there are no traces of recent igneous action, exposures of older rocks occurring only in the southern quarter.

The following rocks occur in this area: The Algonkian, the Dakota, Benton, Niobrara, and Pierre formations, and the glacial deposits of the Quaternary. (See Pl. III.)

ALGONKIAN.

The crystalline rocks are represented by two formations, which were possibly formed at widely separated times. The first is the granite, which is nowhere exposed but has been struck in drilling wells at several points, and the other is the Sioux quartzite, which outcrops east of Mitchell and is often found in drilling. There are two theories concerning the relation of the granite to the quartzite. According to the first it is a much older formation, corresponding in age to the granite extensively exposed along Minnesota River below Bigstone Lake, which is believed to antedate the schists and quartzites of Minnesota and the Black Hills. According to the other view this granite

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INDEX MAP OF EASTERN SOUTH DAKOTA, SHOWING AREA UNDER

CONSIDERATION.

is an eruptive rock, which has been intruded into the quartzite in the form of dikes or sheets. If this is the case, it corresponds to an igneous rock forming a dike in the quartzite north of Corson, S. Dak. In favor of the latter view may be urged the evident unevenness of its surface. For example, it has been struck at the depth of 500 feet, or 850 feet above the sea, in sec. 25, T. 103 N., R. 61 W., while at Mitchell, less than 5 miles away, at a depth of 710 feet, or 590 feet above the sea, the quartzite, which according to the first view must lie above it, had not been penetrated; or, in other words, the granite had not been reached.

Besides the instance already mentioned, the granite has been struck in two or three wells about 5 miles north of Farmer, and a little beyond the north line of this area near Hitchcock. In the latter case it seemed to be overlain by several feet of quartzite.

The granite from the Budlong and Motley wells north of Hitchcock and from wells north of Farmer, in Hanson County, is a fine-grained, light-gray rock abounding in transparent feldspar, while that from the wells southwest of Mitchell is darker and coarser.

From wells at Huron and near Esmond it appears that below the water-bearing rock there are several feet of secondary deposits from granite, such as arkose, impure kaolin strata, and the like, then weathered granite before the sound rock is reached.

The deepest well at Huron, city well No. 4, opposite the college, according to the report of Mr. F. H. Holton, who drilled it and submitted specimens of the lower strata, shows the following record · Record of deep well at Huron, S. Dak.

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Coarse sand with rounded grains about the size of No. 6
shot, largely reddish-brown concretionary grains with
some white quartz-like material struck at corresponding
levels in the Wilcox well and citywell No. 3. The brown
grains are apparently siderite, or carbonate of iron
Gray sandy rock with green and brown specks and whitish
opaque grains like weathered feldspar (arkose), and
lumps of coarse red granite.

[blocks in formation]

Soft, fine-grained, kaolin-like clay.

2

1,138

Hard rock, probably granite

1,139

The Sioux quartzite, which is exposed in the southeastern portio of the area under consideration, is commonly a dense and firmly silic fied rock. It is usually fine grained and thick bedded, but in som cases a few feet of it contains numerous pebbles, while not infre quently the rock is so thin bedded as to be worthless for building purposes. Moreover, at some points it is so imperfectly consolidated that it may be dug with a pick. A case of this sort occurs southwes of Bridgewater, in the valley of Wolf Creek. This soft rock, how. ever, is very limited in extent, the normal hard quartzite occurring within a few feet of it. Associated with the quartzite and interstratified with it are occasional thin beds of a red, hardened clay, called "pipestone." When this has been exposed to the weather it becomes chalk-white. Examples of the latter condition are found at the Wolf Creek locality.

area.

The peculiar interest of the quartzite in connection with the subject under consideration is the fact that it is the bed rock over all the The water-bearing strata rest upon it and its presence marks the lowest horizon at which a flow may be obtained. Hence it is of importance to be able to recognize it and to have a general knowledge of its depth below the surface.

In drilling, the quartzite may be distinguished from pyrite, which is of about the same hardness, by its thickness, the latter being rarely more than a few inches thick. Greater thickness also serves to distinguish it from some of the hard layers of the Dakota formation, which are rarely over 4 or 5 feet thick. Moreover, the Dakota rocks are usually cemented by carbonate of lime, which effervesces with acids, or by iron oxide or carbonate of iron, which is of a dark or rusty color, while the quartzite is uniformly of a light pinkish shade. However, in some cases a compound microscope is necessary to detect the difference. When so examined quartzite is recognized by clusters of sand grains cemented together so firmly by glassy silica that usually the original grains divide along the fractures as easily as they separate where cemented. Similar examination suffices to show the presence of granite, which, as has already been stated, has been struck at several points in this area.

BED-ROCK CONTOUR.

In general the quartzite underlies the whole area under consideration and has the configuration shown by the contour lines in Pl. III. The most conspicuous feature of the bed-rock surface is a high ridge, having a breadth of about 12 miles, which enters the east side of the area, affording surface exposures in the vicinity of Spencer and Bridgewater. Its outcrop area narrows rapidly toward the west, so that the westernmost appearance of the rock on the surface is on Enemy Creek, in the western part of T. 102 N., R. 59 W. In the triangle between the limits indicated it lies next underneath the drift and shows frequently in the bottom of the valleys of the larger

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