Description of the Fort Benton Quadrangle

DESCRIPTION OF THE FORT BENTON QUADRANGLE. INTRODUCTION. steeply inclined sides. In the center of the range of the throat of a volcano, the dikes which radiate heat and cold, varying greatly, of course, with the the limestones and other stratified rocks are hori from these centers of activity, and the loose mate altitude. The annual rainfall is from 13 to 20 General relations. The Fort Benton quad zontal, or nearly so, while on the flanks structure of rials and lava flows of which the cones Structure_. , of. inches, being greatest in the mountains; June and rangle extends in longitude from 110° to 111° of the mountains they are sharply BheuLMoun- were built. The sedimentary strata October are the rainy months. The snowfall is and in latitude from 47° to 48°. It is inclined and dip away from the moun through which the volcanoes broke up heavy and the mountain tracts are well watered. 69.25 miles Ions; from north to south. extent°ofan tains to the lower plains country. The horizontal and upon which their materials accumulated are, Agriculture is therefore confined to the foothills . quadrangle. 47.36 miles wide, and contains 3272.7 attitude is well shown in the rocks of Belt Park, over most of the area, nearly horizontal or inclined and stream bottoms, except where irrigation is square miles. It includes part of Choteau, the whereas farther north the mountain masses near gently to the north. These rocks belong entirely possible. northwest corner of Fergus, and the eastern part Monarch are formed of blocks of tilted limestones to the Cretaceous system, and denudation has Culture. Fort Benton, the town from which of Cascade counties, in central Montana. The dipping northward. This structure is also seen exposed them over considerable areas within the the quadrangle is named, is the oldest settlement. Little Belt Mountains quadrangle adjoins it on the in the walls of Belt Creek, which expose a com mountain tract. They are generally unaltered It was formerly important as the head of naviga south, and the Great Falls quadrangle on the west. plete section of the beds from the nearly level except at the immediate contacts with the dikes tion upon the Missouri River, and is now a dis The greater part of the quadrangle is a nearly flat coal seams of Belt and Armington, with dip and in rings or contact zones about the cores of tributing center for a large area of sparsely settled region which forms the western border of the increasing southward, to the core of gneisses and massive rock. In these places they show indura country. The Great Northern Railway (Montana Great Plains. The southwest corner is part of schists south of Monarch. tion and baking, which about the volcanic cores Central) crosses the plains in the northwestern cor the Little Belt Range, one of the front ranges of The simple structure of the uplift is, in this at South Peak and Shonkin Creek have completely ner of the quadrangle, to the north of the Missouri the Rocky Mountains. In the center of the quad area, much modified by local intrusions of igneous metamorphosed the sediments to dark, flinty horn- River, and a branch line of the railroad, running rangle an isolated group of peaks known as the rock. Where these intrusions form laccoliths the stones, to pink, green, and lavender-colored adi- from Great Falls to Neihart, is the outlet for the High wood Mountains rises above the general level stratified rocks are uplifted in local domes, like noles resembling porcelain, or to vitreous quartz- coal mines at Belt and Armington, and for the of the plains. those of Tiger Butte and Barker Mountain. In ites. In these altered forms the original bedding silver mines of the Little Belt Mountains. Stage Description of the plains. Three-fourths of the this way the symmetrical distribution of the for planes are recognizable only in the horizontal lines to the Judith region traverse the plains, quadrangle is an open, treeless but grass-covered mations is disturbed. Local areas of younger color banding, as the rocks are shattered by a passing through a number of minor settlements. tract that appears almost level, though possessing formations are left as inliers in synclinal basins, close system of prismatic vertical joints, which The greater part of the plains country is devoted a slight general northward inclination. Between as may be seen at the head of Dry Arrow Creek, results in their breaking into dicy debris on weath to stock raising. the High wood Mountains and the Missouri River while on the outer flanks of the range the ering. In no case have the sediments been altered Scenery. The great limestone cliffs and the ridges and hummocks of glacial sand and gravel laccolithic domes bring up the older rocks above to schists. The sedimentary strata formed an "sluice-box" canyon of Belt Creek (between modify the surface. South of Arrow Creek a nearly the general level, as is the case at Wolf Butte. eroded hilly country when the first volcanic erup Logging Creek and Riceville), the fantastic sculp featureless plain extends to the base of Level tree= When eroded these domes show areas of the older tions of the district broke out, as shown at South turing of Square Butte and the adjacent bad lands the Little Belt Range. North of the i piin." rock as outliers surrounded by younger forma Peak and elsewhere. That volcanic activity was of Arrow Creek, and the weird monuments of Missouri River the open plain stretches in unva tions. About these domes the dips are steep and intermittent and continued through a long period erosion along the canyon of the Missouri River ried continuity to the Canadian line. The region variable, but about the mountain borders, where of time is shown by the relation of the latest near Eagle Creek are the most remarkable scenic is devoid of marked elevations above its general not disturbed by igneous intrusions, as, for breccias to the general level of the plains about features of the region. surface, but is deeply etched by the streams which instance, north of Monarch, the strata dip at the mountains, for the earlier andesitic eruptions Literature. Previous publications upon the traverse it. All the larger streams flow in can angles of 6° to 10° away from the axis of the were succeeded by a period of extensive erosion geology of the region are few in number, are yons, the largest being that of the Missouri River, range, the dip becoming gradually less toward the before the basaltic lavas were poured out, as is based upon reconnaissance trips, and treat only of which is 400 to 600 feet deep. As a rule, these open plains country. Along the foothill tract shown by the occurrence of the latter rocks fill particular parts of the quadrangle. The most gorges are narrow, with steep walls, and so sharply from Riceville to Woodhurst this lessening dip is ing deep ravines cut in the earlier rocks, and important papers upon the Little Belt Mountains cut that the break in the plain is recognizable only most apparent, since the hard strata cap hilltops resting upon the core rocks of earlier dissected are the following: Relations of the coal of Mon within a short distance of the canyon brink. whose slope often corresponds to the dip of the centers of activity. tana to the older rocks, by W. M. Davis; with The general aspect of the plains area would be beds. These marginal tracts are areas of plana- Nothing now remains to show the original out appendixes on Petrography, by W. Lindgren, monotonous if it were not for the neighboring tion. Where protected by a mantle of stream lines of the volcanoes. It is evident that the dif and on Fossils, by R. P. Whitfield: Tenth Census, mountain ranges. The scenery along the stream drift or " wash," the surface over large areas is a ferent vents were not active at the same time, but Vol. XV, pp. 696-737. Notes, by J. S. Newberry: courses is, however, sufficiently rugged to be very uniform inclined plain. Ravines expose represent a succession of volcanoes following one Annals New York Acad. Sci., Vol. Ill, No. 8, picturesque, and along the Missouri River it is bluffs in which the plain is seen to be cut across another, the ashes and lava flows of later vents 1884. A detailed description is given by the especially so. The Great Falls of the Missouri the edges of the upturned strata of sandstones partially covering those of earlier eruptions. writer in Twentieth Ann. Rept. U. S. Geol. Sur River are a few miles west of the limit of the and shales, or even across folds, leveled off or Drainage. The Missouri River crosses the vey, Part III, 1900, pp. 271 et seq., in which the quadrangle. Many lateral ravines which incise truncated by the ever-varying water courses. In northern part of the plains and drains the entire petrography is treated by Prof. L. V. Pirsson. the canyon walls cut the plateau into the peculiar other areas the strata have a nearly constant dip quadrangle. It is throughout a flowing The rjver A brief sketch of the Highwood Mountains and topography so commonly called "bad lands." away from the mountains, their outcropping edges stream, unaffected by the aridity of this systems- their remarkable rocks is given by W. Lindgren Pyramidal and rounded terraced buttes and forming broad bands, and as the softer shales region. Belt Creek is the largest of the streams in the report above cited, and by Weed and Pirsson ridges alternate with deep ravines, while more wear more rapidly than the sandstones, the sur heading in the mountain tracts.

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