"Rock of the Month # 54, posted for December 2005" ---

Granite boulders, and their pleasing contribution to a striking upland landscape, make a fitting end to the "Rocks of 2005". The local granite is notable for its rounded, scored, "organic" outcrops and boulder piles. Hummocks, blocks and larger rock walls and towers, frequently coated with brightly coloured lichen, assume fanciful shapes in warm late-afternoon light, as if lifeforms in their own right. The photographs, taken 25 / 28 May 1998, were a difficult choice, so scenic was the area and so pleasant the weather. They are not the most spectacular, but were chosen to emphasize the rock itself, and the lichen's intimate association with its substrate. A return visit the following year confirmed the first impressions, that this is a beautiful hiking area imbued with alluring scenery and an interesting, dry upland ecosystem on top of that massive, pink granite.

The Sherman Mountains and Vedauwoo Glen are located in southeastern corner of Wyoming, east of Laramie, transected by the I-80 highway that climbs east from Laramie (elevation circa 7,200 feet / 2,195 m) before dropping down to the state capital, Cheyenne. The upland meadows and rugged, forested terrain includes the Sherman Range, Vedauwoo Rocks, the Devil's Playground, Pole Mountain (9,053 feet / 2,759 m) and Tie City (Forest Service, 1973).

The most prominent, landscape-shaping local rock is the pink, massive, mid-Proterozoic Sherman granite (Blackstone, 1988, pp.114-116). Mid-Proterozoic granitoids of various ages are abundant in parts of the Rocky Mountains, including the circa 1080 Ma Pikes Peak batholith (Smith et al., 1999), famed for its pegmatites and unusual minerals (such as green amazonite feldspar and assorted micas) and the Silver Plume and Cripple Creek granites, circa 1400 Ma (Tweto, 1987). The Sherman granite, which intruded a series of older (1700 Ma) metamorphic rocks has been sculpted into dramatic forms by protracted weathering and erosion.

The local granites are frequently quite coarse-grained, rough surfaces displaying smoky quartz and larger alkali feldspar crystals 1-2 cm wide, with minor, smaller, darker hornblende and lesser biotite mica. Some granite displays a flow texture with aligned feldspar phenocrysts up to 3 cm long, and parallel orientation of medium-grained, aphyric cognate xenoliths of granite which may exceed 50 cm in length.

The colours of certain lichens may be dependent on trace metal contents, and distribution of some species may both serve as environmental indicators and also reflect natural metal enrichments in local rocks (Easton, 1994). The untutored eye can easily find at least four species of lichen on local rock surfaces.

References

BLACKSTONE,DL (1988) Traveler's Guide to the Geology of Wyoming. Geol.Surv.Wyoming Bull. 67, 2nd edition, 130pp.

EASTON,RM (1994) Lichens and rocks: a review. Geoscience Canada 21, 59-76.

FOREST SERVICE (1973) The Medicine Bow National Forest in Wyoming. Forest Service, U.S. Department of Agriculture, 1:126,720 scale map.

SMITH,DR et al. (1999) Petrology and geochemistry of late-stage intrusions of the A-type, mid-Proterozoic Pikes Peak batholith (central Colorado, USA): implications for petrogenetic models. Precambrian Research 98, 271-305.

TWETO,O (1987) Rock Units of the Precambrian Basement in Colorado. USGS Prof.Pap. 1321-A, 54pp.