University of Washington Publications in Geology The Geomorphology and Volcanic Sequence of Steens Mountain in Southeastern Oregon

THE EASTERN SCARP OF STEENS MOUNTAIN
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EROSION

DRAINAGE

At the southern end of High Steens a rugged crest separates the previously mentioned Wildhorse Canyon from the headwaters of the streams of the eastern scarp. In a similar manner, towards the northern end of the mountain Kieger Canyon parallels the scarp for several miles until it gradually swings westward as a tributary to Donner und Blitzen River. The cirques at the head of these two valleys, which are on the same north-south line, lie within six miles of each other. This alignment probably has been caused by a minor structural displacement parallel to the scarp, although the actual fault plane was not observed by the writer.

Most of the creeks on the eastern scarp have been developed by consequent drainage. Structural control due to branching faults was observed only in the north fork of Mann Creek and in the adjoining valley to the north. The differential resistance of the rock has resulted merely in small irregularities. The minor forks of most of the streams are insequent, but the large forks of two valleys appear to have originated as independent consequent streams.

Erosional depressions near the base of the scarp suggest that both forks of the south fork of Alvord Creek originally flowed directly westward. The gathering ground is too small to attribute the re-adjustment to normal capture. They each were probably diverted to the deeper valley on the north by their own valley filling. There is similar evidence in the valley of Willow Creek. The south fork of the latter appears to have originally formed the headwaters of Little Willow Creek.

Many of the names (see sketch map, fig. 23) used for different creeks on the eastern scarp are well established by local usage, but, in some cases, the name is more indefinite and varies with different inhabitants. Most of the larger creeks have a smaller adjacent valley to which the diminutive is applied. Owing to the fact that two small parallel valleys between Dry Creek and Mann Creek are apparently unnamed, the author, following this precedent, has called them Little Dry and Little Mann. The valley designated as Toughey Creek is also locally known as Little Indian.

GLACIATION

I. C. Russell⁶ and Waring⁷ considered the broad U-shaped valley of Kieger Canyon (fig. 21) to be due to stream action subsequently modified by glacial erosion. Smith⁸ later observed the well-marked evidence of glaciation in Wildhorse Canyon. In addition, however, the presence of board shallow cirques roches moutonnees, glacial lakes and moraines on the top of the mountain indicate that it suffered considerable erosion from an extensive snow field, reaching five or six miles from the crest.

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⁶I. C. Russell, "Hanging Valleys," Geol. Soc. America Bull., vol. XVI, pp. 83-87, 1905.

⁷Gerald A. Waring, "Geology and Water Resources of the Harney Basin Region," U.S. Geol. Survey, Water-Supply Paper 231, p. 28, 1909.

⁸Op. cit., p. 424.

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Fig. 21. Looking northward down the U shaped glacial valley of Kieger Canyon from the top of its cirque directly west of the head of Mosquito Creek.

All the larger valleys on the eastern scarp of High Steens also show marked indications of glacial erosion in their upper parts. At about two miles from the foot of the scarp, they usually end in a broad cirque some 2,500 feet above the desert (fig. 24). Here two or three small tributaries as a rule descend precipitously from well-defined glacial valleys about 1,500 feet above. These shallower valleys extending locally almost a mile farther to the west, end in small higher cirques in which favorably situated snow banks usually survive the summer's heat.

It is probable that this glaciation was superimposed on a previously well developed drainage system. Russell⁹ commented on the absence of marked morainal material in Kieger Canyon. In like manner there is little suggestion of glacial debris in the valleys of the eastern scarp, but it is possible that it may have been removed by subsequent rigorous erosion. As they approach the scarp the valleys lose their glacial characteristics.

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⁹I. C. Russell, "Hanging Valleys," Geol. Soc. America Bull., vol. XVI, p. 84, 1903.

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DIFFERENTIAL EROSION

There are many irregularities in the erosion of the eastern scarp. Some of these are controlled by minor faults. A few of the features low on the scarp have been modified by landslides. The major factor, however, is the differential erosion of the various volcanic rocks. The distribution of these rocks, their many intrusive phases, and their variations both in jointing and in resistance to erosion are all factors which have contributed to the rugged topography of the eastern scarp.

The approximate concordance of elevation of some of the shoulders (fig. 6) on the scarp suggests a periodic uplift, permitting a great erosional break. Although they are locally bounded by faults, these shoulders all appear to be the direct physiographic expression of a change in rock type.

WIND EROSION

At the southern end of High Steens, the scarp presents a broad shoulder over a mile in width. Here, irregular prominences project abruptly from a gently rolling terraine. The surface rock is formed by a thick flow of dacite, which welled from a local vent. Steeply inclined flow structure and differential alteration account for the topographic irregularities. The predominant resistance of this lava explains the preservation of the relatively level surface, but not the extensive removal of the basaltic series, which is about 3,000 feet in thickness.

The surface of the dacite shows indications of extreme wind erosion. In the highest portion of the shoulder, the rock has been completely stripped of soil even in relatively level areas. The rock, thus exposed, shows marked abrasion. Locally it exhibits shallow pot holes containing angular blocks that have been faceted by the wind (fig. 22). The best defined angle is predominantly pointed slightly south of west. The abrasive agent would have been formed by the disintegration of the dacite, for a minutely hackly jointing causes the latter to break into a fine gravel.

Fig. 22. A pot hole on the surface of the dacite between Toughey Creek and Indian Creek viewed from the north. The angular boulders pointing southward testify to wind erosion from that direction. The orientation of the boulder at the base of the hammer was disturbed.

The distortion of the juniper trees on this shoulder testifies to a high westerly wind, which results in the dust storms frequently encountered in the region. This particular locality is especially exposed for it is situated at the southern end of High Steens and only slightly to the north of a well-defined gap in the eastern wall of Wildhorse Canyon.

With these points in mind the writer attributes the formation of the shoulder to wind erosion. Unlike typical basalt, the series, which has been removed, is characterized by a peculiar porous texture which renders it extremely susceptible to mechanical disintegration. Its own coarse particles would have formed a fairly effective abrasive agent.

Fig. 23. Sketch map of the drainage on the eastern scarp of High Steens. (click on image for an enlargement in a new window)

LANDSLIDES

Landslides have occurred at several localities on the lower part of the eastern scarp. They appear invariably to have been associated with incompetent tuffaceous beds. The characteristic reverse rotation causes the development of a humocky topography, which locally has permitted the formation of small ponds. The slides are best defined on the shoulder north of Alvord Creek, to the south in the broad valley formed by its southern fork, and on the northern side of the valleys of both Pike and Little Alvord Creeks.