A river or creek draining an area that has exposures of fossiliferous rocks is bound to have a few durable fossils mixed with the gravel in its bed. While rivers and streams do not produce quantities of good fossils, they may provide clues to productive areas in the rocks.
Any detailed map, particularly any topographic map, clearly shows all rivers. Rivers in any part of the United States known to have sedimentary rocks of Paleozoic or younger age are potential fossil sites. (See state maps in Appendix.)
The best time to collect is during low water, usually in late summer. A pair of tennis shoes and a swimsuit make wading a pleasure, though canoes may be necessary on larger rivers to go from gravel bar to gravel bar. Some fossils will be found loose in gravel bars and beaches along the river edge. These will be worn to some degree and may have been transported a long distance by the river or by a glacier, making identification difficult.
The best collecting will be from shale and limestone exposures cut through by the river. Water softens the rock and it also expands while freezing, which helps loosen fossils. Early spring thaws wash down quantities of rock from the riverside exposures. Some riverbank collecting areas of a century ago still produce fossils, although the bank may have moved back hundreds of yards during that time. Flood waters remove loose material from the banks and cuts and expose fresh material for weathering. Collecting is a matter of walking along the river and sampling the exposures in the banks and the river bottom.
Rivers near Cedar Rapids, Iowa, cut through coral-bearing horizons of the Devonian, and some of them are carpeted with worn pieces of coral weighing up to fifty pounds; Kentucky rivers have done the same thing with Devonian and Ordovician corals. The Falls of the Ohio is a site famous for more than 150 years for the quantity, variety, size, and preservation of the corals exposed in the river during low water.
New York rivers in the area west of Buffalo cut through the seemingly bottomless Devonian shales and are all good sites to collect brachiopods, corals, and occasional trilobites. Texas creeks, when they have water in them, flow through Cretaceous rocks, particularly in an area from Dallas south for many miles. The flash floods in these stream beds tear loose and later deposit large echinoids, ammonites, and clams. Maryland and Virginia rivers cut into Miocene and Eocene fossil beds, releasing their modern-appearing shells and redepositing them along the edge of the river.
The first Pennsylvanian plant concretions in the Mazon Creek area of northern Illinois were collected in the bed of the crqek. The river is rarely hunted now, because nearby strip mines have uncovered the same concretions, but modern-day hunters still find fossils where collectors hunted in the 1860s. Much of the hunting is done by groping in the muddy water for the rounded concretions.
Western rivers flow through thick beds of basalt that contain petrified
wood, particularly in Oregon, California, Washington, and Idaho. In the John Day basin of Oregon, deep-cutting rivers expose Tertiary beds with rare fossils of nuts and fruits. South Dakota streams in the Badlands wash loose mammal teeth and bones, and in other areas uncover giant concretions containing well-preserved ammonites.
The list could go on and on. Only a few states in the Northeast cannot be expected to produce fossils from at least some riverbeds. Perhaps the greatest exposure of fossil beds in the world is the Grand Canyon, where sedimentary fossil-bearing beds of many geologic periods are exposed by the Colorado River. No collecting is allowed, and even if it were, the 5,000-foot-high banks would be difficult to scale.
Dredging of some Florida rivers produced great quantities of bones and teeth, mostly of Pleistocene animals. Scuba divers have now explored some of these rivers and found underwater caves and small areas loaded with accumulations of bones, which must have been there since the animals died in the cave or in the river many thousands of years ago. Use of scuba gear is a new technique in fossil collecting.
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