GLACIATION OF CACHE PEAK AND MOUNT INDEPENDENCE, CASSIA COUNTY, IDAHO

NICKLAUS WELTY The College of Wooster Sponsor: Robert J. Varga, The College of Wooster

INTRODUCTION (Coney, 1987). Four metamorphic core complexes constitute the Albion Mountains: With a summit 3151 m (10339 ft) above sea Moulton, City of Rocks, Independence Lakes, level, Cache Peak is the highest point in both and Big Bertha. As Cache Peak was formed the Albion Mountains and in southern Idaho. from the Independence Lakes metamorphic Northwest along a ridge from Cache Peak’s core complex, we are primarily interested in summit is Mount Independence, 3033 m (9950 this its structure. Cache Peak is comprised ft). Together, Cache Peak and Mount mainly of the Green Creek Complex Independence form a cirque approximately 1.5 (Armstrong, 1968). On Cache Peak, the km long and 0.8 km wide on the north side of Green Creek Complex is a porphyroblastic Cache Peak and the east and south side of biotite-quartz-plagioclase-microcline gneiss Mount Independence. This cirque contains the (Armstrong, 1968). The Green Creek Independence Lakes, five tarn lakes that Complex formed in the Precambrian and was fluctuate seasonally. In addition to the main buried in the Cambrian (Armstrong, 1968). cirque, Cache Peak possesses a cirque (960 m During the Paleozoic and Mesozoic, the wide by 570 m long) on its east face and two section was buried even deeper until erosion in smaller cirques (350 m wide by 350 m long the Jurassic began to wear the overlying and 300 m wide by 360 m long) on the west sediment away (Armstrong, 1968). Finally, face. A small rock glacier, informally referred rapid, post-Oligocene uplift exposed the to here as the Independence Rock Glacier, is complex fully (Armstrong, 1968). present in the smaller cirque on the west face, and was studied in detail. This study analyzes Glacial History of Region a variety of data collected on or around lateral The morphology of the Cache Peak’s glacial and terminal moraines in order to map the last landforms demonstrates that the region has 140 Ka of glacial activity on Cache Peak experienced more than one recent glaciation (Figure 1). (Anderson, 1931). The most recent glaciation GEOLOGIC SETTING the region experienced is likely the Pinedale Glaciation, which correlates with most of the The Albion Mountains Wisconsin glacial advance (Pierce et al., Covering northern Utah and southern Idaho, 1976). Multiple researchers have developed a glacial chronology for the Pinedale period. the Albion Mountains are a 36 mile long, 10 northeast-trending range (Anderson, 1931). Using Be dating in the Wind River Range, Physiographically, the Albion Mountains are Wyoming, Gosse et al. (1995) estimate the considered part of the Basin and Range Pinedale glacial maximum occurred Province, which extends from Canada to approximately 22.4-21 Ka years ago, and northern Mexico, approximately 3000 km, and lasted 5900 years. has a maximum width of about 1000 km Whereas evidence of an earlier glaciation is longer and should have gentler slopes relative present on the peak, it is more difficult to to younger moraines. accurately gauge the age. Despite this, the Degree of soil maturity was also used to earlier glaciation likely corresponds to the estimate moraine ages. Older soils will have Bull Lake glaciation, correlating with the late experienced a greater buildup of loess, Illinoian glacial period (Pierce et al., 1976). indicating more time has elapsed since the Bull Lake glaciation reached a maximum formation of the moraine. roughly 140 Ka (Pierce et al., 1976). RESULTS METHODS Locations where moraines were tested were Pinedale Glacial Extent recorded in a Trimble GeoExplorer. This data Pinedale-age glaciation evidence, manifested was then transferred into a digital raster as relatively shallow weathering pits on graphic of a 7.5-minute USGS map of the boulders, high boulder frequencies, younger study region soil and steeper moraine slopes, is not as extensive as evidence of previous glaciations On each set of suspect moraines, several on Cache Peak (Figure 1, lightest shading). techniques were employed to estimate the age On the large cirque on the north side of Cache of the landform. First, surface boulder Peak, evidence for Pinedale glaciation frequency was utilized. A boulder was taken disappeared around 1800 m from the cirque to begin at 25cm (~10in). In this technique, 2 headwall. Additionally, evidence on south 25 m is squared off on a moraine. After this, side ends roughly 1060 m from the cirque the relative quantities of quartizite and granite headwall. No evidence for Pinedale-age boulders are recorded. Older moraines will glaciation was uncovered in the east cirque; have a lower frequency of granite boulders to only moraines with deeper weathering pits, quartzite boulders as the weak granite low surface boulder frequencies, and mature weathers at a faster rate than the more resistant soils containing large quantities of loess. quartzite. Bull Lake Glacial Extent Using a ruler, depths of weathering pits on granite boulders were measured to estimate Evidence for Bull Lake-age glaciation is more age. Here, one expects the older moraines to spatially expansive relative to Pinedale-age contain granite boulders with deeper glaciation (Figure 1, medium shading). weathering pits relative to recent moraines. Deeper weathering pits, lower boulder frequencies, mature soil, and gentler moraine The third technique used for moraine age slopes are indicative of Bull Lake glaciation. estimates is a sonic-based procedure intended Bull Lake-age moraines are present up to 4800 to provide a sense of the percent of fresh clasts m from the cirque headwall on the north face present. This technique involves striking the of Cache Peak. On the south face, evidence boulders in the area with a rock hammer and for Bull Lake-age glaciation ends around 1600 determining the resulting sonic frequency. A m from the cirque headwall. Furthermore, high pitched “ring” indicates the outer layer of there is a large bedrock tower approximately the boulder has not weathered significantly, 500 meters from the point Bull Lake evidence while a lower pitched “thud” indicates the ends. This tower would have been demolished outer layer of the boulder has experienced by any advancing glacier, and thus provides a significant weathering. Thus, a “ring” maximum limit of Bull Lake-age glaciation. indicates a younger boulder and a “thud” Finally, on the east side of Cache Peak, Bull indicates an older boulder. Lake-age evidence is found up to 3000 m from The angles formed by moraine slopes were the cirque headwall. recorded to provide further information on ages. Older moraines have been exposed Figure 1: Map displaying the maximum glacial extent of the Pinedale-age, Bull Lake-age, and possible pre-Bull Lake-age glacial extent.

Pre-Bull Lake Glaciation? there is a lateral moraine with deeply weathered granite boulders. This moraine There is preliminary evidence for pre-Bull yielded boulders with much deeper weathering Lake glaciation of Cache Peak. This pits than nearby Bull Lake-age moraines suggestion is based on evidence uncovered (Table 1). It is not possible to definitively west of the Bull Lake limit on the north side of state here whether this landform is a result of a Cache Peak (Figure 1, darkest shading). Here, Table 1:Summary of Relative Moraine Age Data Age of Mean Total Mean Mean Slope Mean Soil Glaciation Weathering Boulder surface Angles Ring/Thud Components Pit Depth Ratio boulder Ratio (Inner/Outer (cm) count per 25 (granite/qua ) m2 per rtz) moraine Pinedale 1.9 57/9 30 17°/23° N/a Low loess levels Bull Lake 2.9 60/88 20 19°/16° 3/5 Higher loess levels Pre-Bull 5.9 N/a N/a N/a N/a N/a Lake

pre-Illinoian glacial advance, the features At the minimum, Cache Peak has experienced present are consistent with a stage of two separate glaciations: the Pinedale advance glaciation prior to the Illinoian. However, the around 22 Ka (Gosse et al., 1995) and the Bull features could also have resulted from an Lake glaciation around 140 Ka (Pierce et al., initial surge of Bull Lake glaciation, and the 1976). Bull Lake-age glaciation was much other Bull Lake features examined are a result more expansive than Pinedale-age glaciation; of late Bull Lake glaciation. evidence of Bull Lake-age glaciation extends up to 3 km further than evidence of Pinedale- Rock Glacier age glaciation on the north side of Cache Peak. A relict rock glacier, informally called Cache Peak may have been glaciated prior to Independence Rock Glacier, is present on the 140 Ka, as is evidenced by severe weathering south-facing side of Cache Peak. Based on on a lateral moraine out of the direct path of medium-sized trees growing on the feature, the Bull Lake glacial advance. Finally, to the Independence Rock Glacier has not been fully comprehend the glacial history of Cache active for at least 100 years. As Hamilton and Peak, it would be useful to perform Whalley (1995) point out, classification of cosmogenic dating of boulders on Pinedale rock glaciers is fraught with disparities. and Bull Lake-age moraines. Despite this, Hamilton and Whalley (1995) propose the term ‘rock glacier’ be applied to REFERENCES CITED landforms existing on valley floors composed Anderson, A.L., 1931, Geology and Mineral Resources of angular rock debris generally possessing of Eastern Cassia County, Idaho, Ph.D. Thesis, steep front and lateral slopes with lengths University of Chicago, 169 p. greater than widths. Morphologically, this Armstrong, R.L., 1968, Mantled gneiss domes in the describes the Independence Rock Glacier well. Albion range, southern Idaho, GSA Bulletin, v. 79, Rock debris in the feature was quite angular, no. 10, p. 1295-1314. and the largest boulder present was roughly 33 Gosse, J. C., Evenson, E. B., Klein, J., Lawn, B., m3. Slopes measured on the Independence Middleton, R, 1995, Beryllium-10 dating of the duration and retreat of the last Pinedale glacial Rock Glacier were around 32°; steep, but less sequence, Science, v. 268, no. 5215, p. 1329-1333. than the angle of repose for the constituent At the top, Independence Rock Hamilton, S.J., and Whalley, W.B., 1995, Rock glacier sediment. nomenclature: A re-assessment, Geomorphology, v. Glacier gradually grades into a talus. The 14, no. 1, p. 73-80. maximum length of Independence Rock Pierce, K.L., Friedman, I., Obradovich, J.D., 1976, Glacier is approximately 260 m, while the Obsidian hydration dating and correlation of Bull maximum width is approximately 80 m. Lake and Pinedale glaciations near West Yellowstone, Montana, GSA Bulletin, v. 87, no. 5, CONCLUSION p. 703-710.