PROBLEMS WITH A GLACIAL INTERPRETATION OF LANDFORMS AND SEDIMENTS IN THE SPRING MOUNTAINS, NEVADA

Whether or not there is a record of late Quaternary glaciation in the Spring Mountains of southern Nevada has been debated since the days of Eliot Blackwelder, and recently Orndorff et al., 2003 (Jr. of Arizona-Nevada Academy of Science 36:37-45) purport to have ended the debate with the discovery of tills east of Charleston Peak, and a claim that bowl-shaped valley heads in the Kyle Canyon drainage are degraded cirques. But tempering the glacial interpretation are the following:

1. Equivocal origin of “cirque” morphology. The bowl shapes and “U-shaped morphology” and “hanging valley” described from the head of Kyle Canyon are common in cliff-and-bench topography developed in horizontal strata in desert regions; there are numerous examples in the Grand Canyon. The hanging valley at Big Falls, and the alleged cirque floor above, are consequences of a thick resistant layer of limestone in the upper Monte Cristo Formation. There are no smooth headwalls or other evidence of glacial erosion in the Spring Mountains.

2. Lack of glacial depositional landforms. Nowhere in the range are there any of the well-preserved late Wisconsinan moraines so common in the glaciated desert ranges to the north, nor is there evidence of older or partly eroded moraines.

3. Lack of till. Two sediment bodies that were interpreted as till on the basis of striated pebbles are very probably colluvium. One of these, at the mouth of Big Falls wash, consists partly of poorly stratified diamict, dipping at ~30° NW and containing some openwork gravel. The diamict is likely debris flow deposits. Furthermore, the diamict is located in a stream valley where there is abundant evidence for recent mass wasting. The second body appears to be slope colluvium, partly clast-supported, with a complete spectrum of grain sizes up to one meter. Many stones in the two deposits are striated, and some striations could be glacial in origin, but many are short and deep for their length, and some occur on fresh, non-abraded surfaces and appear to be products of impact rather than grinding.

A definitive interpretation of the diamicts in question remains elusive, considering the mixes of angular and rounded clasts, and different types of striations. But it is at least as likely that rounding and striations were produced during various forms of mass movement, as it is that they are glacial in origin.