Rock Glaciers of Two Ages in the Capitan Mountains, Lincoln County
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that ranges from a roof zone of gra- nophyric granite to an intermediate zone of aplite to a core of porphyritic granite. The intrusion shows a progressive devel- opment from west to east of a coarse and porphyritic texture and an increase in mafic minerals. The textural and miner- alogical variations appear to be gradation- al with no evidence for multiple intru- sions. The granophyre zone is fine grained (0.5–2 mm) and has a micrographic tex- ture. It encompasses an area from the west contact zone to just east of Capitan Gap where it grades into an equigranular tex- tured aplite (Fig. 3). The aplite zone extends eastward to the saddle at the head of Hinchley Canyon and also crops out at Capitan Gap where deeper parts of the pluton are exposed. The porphyry zone FIGURE 2—Location of the study area in the Capitan Mountains. Base map is the U.S. Geological stretches from the saddle at the head of Survey’s topographic map of Roswell quadrangle 1:250,000 scale series. Hinchley Canyon to the east end of the primary factors in the generation of rock lish the age of the rock glaciers and to pre- New Mexiiico glaciers (Morris, 1981). Frost wedging sent a chronology based upon activity, and supplies the debris for the development of (4) to attempt to relate the distribution of GEOLOGY rock glaciers and occurs under periglacial the older and younger forms to climatic climates that are characterized by fre- fluctuation and to the lithology and struc- Science and Service quently recurring freeze and thaw (Flint, ture of the pluton. • 1971, pp. 271–274; Washburn, 1980, pp. ISSN 0196-948X Volume 21, No. 3, May 73–79). This process is promoted by snow Geographic and geologic setting cover of short duration and enough mois- 1999 ture in the ground for cryofraction to The Capitan Mountains are located in occur (Corté, 1987). Climatic elements Lincoln County, New Mexico, approxi- Editors: Jane C. Love, Nancy S. Gilson, Jeanne Deardorff conducive to the development of intersti- mately 80 km northwest of Roswell and 40 Cartographer: Kathryn Glesener tial ice in rock glaciers are (1) sufficient km northeast of Ruidoso in south-central EDITORIAL BOARD precipitation for the formation of the ice New Mexico (Fig. 1). The range extends in and (2) temperatures that are low enough Steve M. Cather, NMBMMR, Chairman an east-west direction for approximately Thomas Giordano, NMSU for the maintenance and creep of the ice 32 km and is approximately 7 km wide. Laurel B. Goodwin, NMIMT (Wahrhaftig and Cox, 1959). Interstitial ice West Mountain is a nearly isolated upland Published quarterly by (permafrost) can exist in rock glaciers if with an altitude of 2,272 m and is separat- New Mexico Bureau of Mines and the mean annual temperature at ground Mineral Resources ed from the main part of the mountains by a division of New Mexico Institute of level is 0– -1°C especially if the snow cover Capitan Gap (Fig. 2). The summit east of Mining and Technology is thin and of short duration (Péwé, 1983). Capitan Gap is 2,600–3,050 m in altitude Rock-glacier debris is derived primarily and rises approximately 900 m above the BOARD OF REGENTS Ex-Officio from cliffs undergoing the blocky weath- surrounding alluvial-covered lowlands. Gary Johnson, Governor of New Mexico ering that occurs in massive igneous, The head of Hinchley Canyon and Pierce Michael S. Davis, Superintendent of Public Instruction metamorphic, and sedimentary rocks Canyon Pass are well-defined saddles that Appointed where two or three structural trends of break the relatively flat crest (Fig. 3). The Robert E. Taylor, Pres., 1997–2003, Silver City Randall E. Horn, Sec./Treas., 1997–2003, Albuquerque joints are present (Evin, 1987). The process saddle at the head of Hinchley Canyon Ann Murphy Daily, 1999–2004, Santa Fe is particularly prevalent if two joint trends divides the range into western and eastern Sidney M. Gutierrez, 1997–2001, Albuquerque are parallel and orthogonal to each other sections, which have distinctive geologic Kathryn E. Wavrik, Student Mem., 1999–2000, Socorro and the other set is oblique. Well-devel- and physiographic attributes noted below. New Mexico Institute of Mining and Technology President . Daniel H. López oped bedding planes in sedimentary rocks Summit Peak and Capitan Peak are promi- New Mexico Bureau of Mines and Mineral Resources are also an important control. Frost wedg- nent landmarks above 3,050 m in the east- Director and State Geologist . Charles E. Chapin ing is effective in dislodging blocks and ern part of the range. The north and south Subscriptions: Issued quarterly, February, May, slabs from cliffs with blocky structure flanks of the mountains are cut by numer- August, November; subscription price $10.00/cal- because freeze-thaw action pries apart endar year. ous steep-walled canyons with maximum Editorial Matter: Articles submitted for publication rock along joints, fractures, and bedding depths of approximately 330 m. Many should be in the editor’s hands a minimum of five planes. small canyons and ravines are tributaries (5) months before date of publication (February, May, August, or November) and should be no The purpose of this paper is: (1) to pre- to the larger canyons. longer than 20 typewritten, double-spaced pages. sent criteria for distinguishing rock glaci- The Capitan Mountains are formed by All scientific papers will be reviewed by at least two people in the appropriate field of study. Address ers of two ages in the Capitan Mountains an intrusion of mid-Tertiary age com- inquiries to Jane C. Love, Editor of New Mexico using physiographic preservation and soil posed of alkali feldspar granite with con- Geology, New Mexico Bureau of Mines and Mineral development, (2) to note the geographic tact zones that are poorly exposed except Resources, Socorro, New Mexico 87801-4796. Published as public domain, therefore reproducible without distribution of the rock glaciers in rela- at the east and west ends (Allen and permission. Source credit requested. tionship to the textural and mineralogical McLemore, 1991). It is a single but textu- Circulation: 1,000 zones of the Capitan pluton, (3) to estab- rally and compositionally zoned pluton Printer: University of New Mexico Printing Services 58 August 1999 New Mexico Geology FIGURE 3—East-west cross section along the south side of the Capitan Mountains showing major topographic features, distribution of the textural zones, and the areal extent of older and younger rock glaciers. Location of the textural zones is from Allen and McLemore (1991, p. 118, fig. 7). 1, average alti- tude of rock-glacier heads; 2, average altitude of rock-glacier fronts. Vertical exaggeration approximately x 2.6. Summit Peak Hinchley Canyon FIGURE 4—South side of the Capitan Mountains showing distribution of talus and frost rubble (light areas) east and west of Hinchley Canyon. The deposits are more extensive on slopes eroded in aplite west of the canyon than on slopes eroded in porphyry to the east. Photograph by Francis G. Varney. mountains and contains as much as 20% for south-central New Mexico (Van described the forms in the western part of phenocrysts of anorthoclase and, rarely, Devender et al., 1984) suggest the follow- the range in detail. The average altitude of plagioclase. The distribution of vein and ing approximations of current mean annu- the heads is approximately 2,574 m, and breccia deposits in the range indicates that al temperatures for the Capitan Moun- the average altitude of the fronts is the granophyre and aplite zones are more tains: 5–8°C for the slopes (2,300–2,750 m) 2,474 m (Table 1). The lengths are between intensely fractured than the porphyry and 3°C for the crest (3,000 m). 56 and 1,565 m (Table 1), and the widths zone (McLemore and Allen, 1991). Vegetation zonation in the Capitan average approximately 70 m. The rock Rock-fall talus accumulates below cliffs Mountains ranges from desert grassland glaciers in ravines and tributary canyons and steep slopes, and frost rubble forms near the base to the spruce-fir association are the extensions of talus at the drainage extensive sheets on the upper slopes of the on the crest (Martin, 1964). The pinyon- heads. Those on the floors of major mountains; both obscure the bedrock in juniper association extends up to an aver- canyons are composed of talus that accu- many areas (Fig. 4). Some rubble sheets age altitude of approximately 2,200 m on mulated as sheets along the base of side have undulating surfaces formed by many the lower slopes. The transition associa- walls and head walls. Frost rubble small ridges and furrows. The ridges and tion is between approximately 2,200 and encroaches upon the heads and flanks of furrows are parallel to the contours indi- 2,700 m, and the spruce-fir association is many rock glaciers on canyon floors and cating movement downslope because of above 2,750 m on the upper slopes and suggests downslope movement of debris solifluction or frost creep. Talus and frost crest. Aerial photographs reveal that the after the rock glaciers became inactive rubble, nearly devoid of soil and vascular slopes and crest of the mountains are more (Fig. 6). plants, cover approximately 30–60% of heavily forested in the eastern part of the The rock glaciers have steep fronts 5–60 slopes eroded in granophyre and aplite range in porphyry terrane as compared to m high that slope approximately 30° (Fig. compared to approximately 20–30% for the slopes and crest in the west in gra- 7). Heads merge with talus on the steep slopes eroded in porphyry.