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PHYSIOGRAPHY OF THE BOSTON MOUNTAINS,

ARKANSAS1

THE highlands of Arkansas lie in the northwestern part of the state, and comprise about half its area. physiographically, into a north and a south

They are divided, part by the valley

of the Arkansas River. They are also divided structurally into the same parts, the former being a region of horizontally-bedded rocks, somewhat disturbed by faulting and folding, while the latter, known as the Ouachita Mountains, is distinctly a folded region.

This northern division of the Arkansas highlands, with its westward extension into Indian Territory, constitutes the southern part of the Ozark region. In Arkansas, it is divided into a low and a high part, the former extending northward into Missouri, and passing, along its southern border, into the latter, by an irregular but bold escarpment from 500 to 1000 feet high.

It is this latter region that is known in Arkansas as the Boston Mountains. Including that part which lies in Indian Territory, its total length is about 215 miles, about 170 of which is in Arkansas. Its average width approximates 35 miles. On the south, it passes into the valley of the Arkansas River by steep slopes, though less precipitous than those on the north.

These mountains are by far the highest part of the Ozark region as well as the most picturesque. Their highest point, so far as determined, is some miles east of the town of Winslow, on the St. Louis and San Francisco railway, where the altitude is 2,250 feet.3 From this region of highest elevation, they

Read before Section E of the American Association for the Advancement of Science at the Denver meeting, August 1901.

2 The Ouachita Mountains have been included by some writers with the Ozarks; but because of the great structural and topographic differences in the two regions, to say nothing of the probable historic differences, this is manifestly wrong.

3 Topographic map United States Geological Survey, Winslow quadrangle.

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FIG. 1. Photo from Branner's relief map of Arkansas.

gradually fall off to the east, sinking below the Tertiary deposits just west of the St. Louis, Iron Mountain and Southern railway and south of White River; also from this region of highest elevation, they fall off westward to the Grand River in Indian Territory. It will be seen that the east-west line along the crest of these mountains forms a gentle arch in the middle. Structurally, in the western part of Arkansas, these mountains are a broad, flat anticline, the strike of which is east and west. According to the geologists of the Arkansas Geological Survey, it appears that the extreme eastern part of the region is monoclinal in structure, with the dip to the south.2

With the exception of the Illinois River in the western part of the state, the drainage of the region is northward and eastward into White River, and southward into the Arkansas. The direction of the streams has been determined by the slopes incident to the uplift, modified in some cases by faulting and flexuring. The effect of the latter upon Little Red River and neighboring streams has already been noted by Professors Newsom and Branner.3 The westward course of the Mulberry River has been determined by a fault. Detailed work of the region would doubtless disclose numerous other similar examples.

The drainage of the region is that intermediate between youth and maturity. The streams are vigorous, and have completely dissected the plateau by the formation of gorges from 500 to 1000 feet deep, thus producing a very rugged topography over the whole region. Between these gorges the slopes often meet, forming more or less rounded hills; but more frequently the intervening area is occupied by flat-topped, sandstone capped hills of limited extent.

The tributaries of both the Arkansas and the White rivers have worked their way back to, and in many cases, far beyond

DR. N. F. DRAKE, in Proc. of the Am. Phil. Soc., Vol. XXXVI, No. 156, p. 332. 2 NEWSOM and BRANNER, "The Red River and Clinton monoclines, Arkansas," Am. Geologist, Vol. XX, July 1897, pp. 1-13.

R. A. F. PENROSE, JR., Ark. Geolog. Surv., Vol. I, 1890; section with pocket map. 3 Loc. cit.

the original water divide of the plateau, making the water divide as it now exists, a very zigzag line. In the western part of the state, the south-flowing streams are the stronger, and as a rule are robbing the White River basin of territory in this locality. Further east, in the middle portion of the region, the north-flowing streams are the stronger, and seem to be encroaching upon the drainage area of the Arkansas, while in the eastern part, the south-flowing streams head very near the north escarpment of the plateau.

The rocks of the region are mainly unmetamorphosed sandstones and shales, those at the base being of Lower Carboniferous age, and those at the top belonging to the Coal-measure series. These alternating hard and soft rocks have produced the terraces on the hill slopes, which are so characteristic of dissected regions of horizontal strata. As these terraces are often of considerable width, and are favorable horizons for springs, they are inviting to the farmer, and can be located miles away by the small farms on the mountain sides.

The low region to the north of the Boston Mountains is one of great denudation. From its northeastern part, all the rocks have been removed above the Ordovician, leaving those exposed at the surface. West and south of this is a region from which the Upper Carboniferous rocks have been removed, leaving those of Lower Carboniferous age at the surface. Standing up prominently on the latter are numerous hills of circumdenudation, composed of remnants of the horizontal strata of the Boston Mountains, and serving as living witnesses to their former extent. The height of these outliers very closely approximates that of the plateau of which they were formerly a part. This uniformity in height between the various parts of the dissected Boston plateau and its outliers suggests a peneplain, and herein. lies the physiographic problem of the region.

In a region of folded or inclined strata the determination of a peneplain becomes a question of comparative ease, for in those cases denudation will have reduced both hard and soft strata to practically the same level, the peneplain intersecting

strata of all degrees of hardness. But in the case of horizontal strata undergoing base-leveling, the conditions are quite different, for then the peneplain conforms to the hard stratum or strata that happen to be near sea level. If such a region be subsequently elevated, the streams are revived, the region dissected, and the former peneplain represented by the tops of the hills, which would still be capped by the hard strata that were conformable with the peneplain before the region was elevated. Now this is exactly the structural and topographic conditions of the Boston Mountains and their outliers. But it happens that these are also the structural and topographic conditions that would prevail in a region of horizontal strata that has been elevated from beneath the ocean and is undergoing the process of base-leveling for the first time. So the problem presents itself as to which condition prevails in the Boston Mountains, and unfortunately criteria for its solution are largely if not wholly wanting.

Ordinarily, for the determination of a peneplain we look to the streams. In such cases, as is well known, the streams are winding, and flow in more or less steep-sided, symmetrical valleys, which are themselves cut down in wider valleys. In the Boston Mountains there is no such evidence of a peneplain. The streams of the region are all young, with the characteristic steep-sided gorges of such streams. So far as the writer has been able to observe, there is nothing in the region indicating an uplift since the present streams came into existence. Their valleys are relatively wide at their mouths, and gradually decrease in width back to their sources, as would be expected of streams cutting into a plateau of horizontal strata. The slopes are undisturbed by terraces, excepting such as those mentioned above, which are due to structure. Along the southern base, the oldest of the streams have reached the temporary baselevel of the Arkansas River, and meander somewhat, but none of them to any great extent.

It follows that evidence of a former base-leveling, if there be such, must be looked for elsewhere than in the streams. A

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