There are natural exposures of rock in mountain cliffs and hillsides, especially in the West, that have never been explored by a fossil hunter. Some, such as sedimentary rock caps in the Rocky Mountains at 1 0,000 and 12,000 feet, are nearly inaccessible. Topographic maps give clues to such potential fossil sites: look for steep hillsides and cliffs, which are evident on the maps.
Couple viewing fossils protected from elements by plastic shield, at a station along a fossil trail in Badlands National Monument, South Dakota. (Photo South Dakota Department of Highways)
Any location where loose rock is exposed is a potential fossil site, so long as the rock is sedimentary and of proper age. Fossils may erode from the rock and be carried by rains into depressions and ditches. The surface rock on an exposure which is not too steep will probably be black from extreme weathering, and all fossils will have been obliterated. One of these slabs, when turned over, will show a much cleaner side. Turning slabs also exposes scorpions and snakes and should be done with a longhandled pick.
It is often difficult to locate the source of a fossil found near the bottom of a mountain slope or hillside. Soft layers slump over other layers hiding them completely. Pieces roll down the hill and are moved by animals. It is relatively safe to assume that the original location of a fossil was on the slope above the spot where it was found, but perhaps not directly above. A fossil hunter narrows his search as the gold prospector does when he finds a trace of color in a river bed; the prospector follows it upstream until he reaches a point where there is no more gold. The source must lie below that point. Similarly the fossil hunter traces a fossil up the slope.
C. D. Walcott uncovered a slab of shale containing an extraordinary, soft-bodied Cambrian worm while collecting in the mountains of British
Shale formed of volcanic ash entombed these leaves. Oligocene; Florissant, Colorado.
Columbia; he searched for several years before he found the source. This pocket ultimately produced hundreds of slabs of intricately preserved, soft-bodied animals, most of which were new to science.
The folding and severe faulting of western and eastern mountains makes prediction of the location of any rock layer nearly impossible. But it also makes any area potentially worthwhile, even though the predominant rocks look unpromising.
Not far from Walcott's quarry in the Canadian Rockies is a location high up on Mount Stephen where about fifty acres produce an abundance of Cambrian trilobites. The spot is surrounded by unfossiliferous rocks so old that fossils would never be expected anywhere in the area. Geologists believe the block of fossil-bearing shale was torn loose from an unknown location and fell on the side of the mountain during some extensive folding and faulting.
The Cambrian trilobites of Utah are found on the sides of small mountains in the central part of the state. Shark Tooth Hill, near Bakersfield, California, is slowly being dug away for its dental fossils. The Badlands of South Dakota are ideal places to collect vertebrate fossils and some marine fossils. So are the chalk hills of Kansas, which have fossils of marine dinosaurs and giant fish. Hills near Kemmerer, Wyoming, formed of chalky white shale deposited in Tertiary lake beds, contain fossil fish. The foothills of the Rockies around Florissant, Colorado, have numerous areas of soft shale, derived from volcanic ash deposited in freshwater lakes. As the dust settled through the water it buried leaves and insects that are now beautifully preserved in the Oligocene shales.
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