Early Pleistocene Glaciation in Idaho

EARLY PLEISTOCENE GLACIATION IN IDAHO By CLYDE P. Ross ABSTRACT This ridge and much of the Custer quadrangle, in which it lies, were visited during the summer of 1928 On and near Railroad Ridge, Custer County, Idaho, there are deposits of unconsolidated material, believed to be of glacial by the ~riter and his assistants, Robert R. LeClercq origin, which arc out of harmony with the present topography. and Willianl D.11ark, in .the course of a geologic study The cvidence suggests that this material is the product of glaci of this part of Idaho. ation in cal'ly Pleistocene, possibly Nebraskan timc and indi Railroad Ridge was so nalned because of the fancied cu.tes that at that time the country was far less rugged than at resemblance of the ridge, viewed from a distance, to a· prcsent. Valley cutting of the order of 1,400 feet took place in the interval between this early glaciation and Wisconsin railroad grade. Plate 17 shows the general appearance timc, when the glacial cirques and U-shaped vaUeys that arc of the ridge, and, by way of contrast, the cirque cliff prominent features of the present topography were formed. Imo\vn as the Chinese Wall, a short distance to the This deep cutting is in marked contrast with the erosion of 100 west. As can be seen fronl Plate 16 the ridge trends. feet or less that has occurred in Recent time. nearly east, and its highest and smoothest part is 2 Similar though less complete evidence of ancient glaciation followed by deep el'Osion is known on Loon Creek, farther miles long and has an average width of somewhat, north, and on the Little Wood River, farther south. All three less than half a nllie. This portion slopes from an localities are close to the highest peaks in Idaho. Evidence altitude of 10,400 feet above sea level near its west· of similar conditions in the Pleistocene epoch has been found end to 9,400 feet at its east end. It therefore has an by other investigators in MOl1tana, Wyoming, and perhaps average grade of 10 per cent, which appears flat when Wnshington. Although two widely separated periods of glaciation are recorded in northern Idaho, neither appears to contrasted with the cliffs and steep slopes on every be ns old as that here described. side. From the high west end of 'the ridge there is· an abrupt descent of over 1,400 feet to the lake at· FOR~WORD the head of Livingston Creek, and on the south It has long been known that the mountains of cen there is a comparably sharp drop into Jim Creek. tral and northern Idaho bear nluch evidence of former On the east and north the contrast with the surround-· glaciation. The glacial phenonlena comnlOnly ob ing grea is less striking, as nluch of the country here is· served are confined to existing valleys and were doubt a dissected plateau whose surface lies more than 9,500 less produced late in the Pleistocene epoch. Recent feet above sea level, as shown on Plate 16. (See also work indicates that there was in addition a much pl. 18, A.) The streanls, however, are rapidly cutting earlier glaciation, which preceded the development into the plateau, and two of thenl, Lake and Silver of the present topography. The best evidence for Rule Creeks, are already cutting into Railroad Ridge. this now known is fOWld on and near Railroad Ridge, Plate 18, A, shows the extent to which Silver Rule in unsurveyed Tps. 9 and 10 N., R. 16 E., south of Creek is incised below the plateau level. Much of the Salmon River, in the southern part of Custer the cutting of these strealns took place in Pleistocene COtlnty, but deposits that lllay have been produced time, for the valleys of all the larger streams, including during the ancient glaciation have been noted on those just mentioned, have been glaciated in their Loon Creek, farther north, and on the Little Wood upper reaches, as shown in Plate 19. Hiver, in Blaine COtmty, farther south. It may be The top of Railroad Ridge is only ab<:>ut 400 feet significant that all these localities, the only known below the highest peak in its imnlediate vicinity places where there are traces of the ancient glaciation, and about 1,400 feet below the summi~ of Castle are near the highest swnmits in Idaho. Peak, 7 lniles to the south. Castle Peak is the highest. peak in the Custer quadrangle and one of the highest RAILROAD RIDGE in Idaho, being exceeded in altitude only by Mount DESCRIPTION Hyndman, 12,078 feet, which is 30 miles southeast of it, and by a few peaks of intermediate height along The smooth, almost level top of Railroad Ridge the range between these two. l contrasts so sharply with the ragged pinnacles around I Since this was written Lee Morrison, of the U. S. Geological Survey, has found it that it has long been an -object of local interest. that one of the peaks in the Lost River Range is nearly 600 feet higher than this. 123 124: SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1929 Railroad Ridge is remarkable not only for the topo where in the general region, already presented in out graphic characteristics and relations sketched above line 2 and to be further discussed in later papers, that' but also for its geologic composition. Plate 16 shows a mass of coarse detritus so situated can not be older the geology of· this ridge and its vicinity. The than late Pljocene, but its lack of accord with modern boundaries on this map at a distance from the ridge drainage lines shows that it is older than the present are subject to revision in detail, but the essential erosion cycle. As the upper reaches of all the larger Jeatures are correctly shown. A large area on Railroad stream valleys in the high mountains show abundant Ridge and smaller areas both north and south of it evidence of late Pleistocene (Wisconsin) glaciation it are mapped as glacial detritus. As will be seen below, is evident that detritus formed earlier than these theeproof of the glacial origin of this material is not valleys must be older than that formed in Wisconsin absolute, but it is regarded as highly probable. The time. Direct evidence that the material is older than boundaries of these areas were traced with care, but the Wisconsin glaciation is furnished by the fact that it is possible that there are additional small areas. it forms the upper part of the glacial cirque wall near .Numerous erratic boulders on Slate and Livingston the head of Silver Rule Creek, as can be seen in Plate Creeks, composed mainly of granitic rock, suggest 19, A. Similarly the detrital mass is truncated by the the former presence of morainal deposits at high upper part of the valley of Jim Creek, which has a levels. Boulder Creek and the region south of it were cirque at its head and has the typical U-shaped cross .notreached during thepresent study, but in thesummer section of a glaciated valley, as is evident from Plate of 1929 Thomas H. Hite found extensive deposits of 19,B. similar detritus on the ridges near the mouth of Boulder ORIGIN Creek and south of it. Plates 17, A, 18, B, and 19, A, show that the deposit There is no positive way of distinguishing between on Railroad Ridge is coarse, poorly sorted, and poorly many glacial deposits and the deposits of a torrential :rounded. N early all of it is composed of granitic rock stream from the appearance of the material alone. The' like that at the head of Boulder Creek mapped on hypothesis of glacial origin fits the general relations of Plate 1. 'Near the base fragments of the rocks under-' this material, however, far better than any other. lying this great mass of debris have been incorporated The former presence in this location of a stream of in it, but even here they·are subordinate in size and adequate size and power to lay down such a deposit is amount to the granitic. rock. The deposit is suffi almost inconceivable. Furthermore, the shape of the ciently compacted to form slopes steeper than would surface on which the material lies, the apparent lack be maintained in loose gravel but contains little or no of space for a drainage basin of adequate size at alti cementing material. The granitic rock composing tudes above the present position of the .material, and most of the fragments has been sufficiently weathered the strictly localized source of most of it, all argue to roughen the surface, but most of the boulders are against a fluviatile origin. firm and fairly fresh. The material composing the The detritus lies on a gently inclined surface that smaller detached masses is essentially similar to that shows no evidence of having been a part of a stream in the main body described above. In all exposures valley. Along the north side of Jim Creek and at the noted there is little variation either in average coarse west end of Railroad Ridge the deposit of detritus is ness or in degree of rounding of the material.· thin. Here it is evident that the surface on which this material was laid down is irregular in detail and Railroad Ridge was referred to by Umpleby 18 as one of t.he remnants of a postulated peneplain which he contains small shallow undrained hollows. This sur considered to be of Eocene age.

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