If the foregoing is true, one should be able to determine the approximate time of year fish died by the degree of disarticulation. One might assume that blooms of blue-green algae and hence precipitation of CaCO₃ would take place sometime during August and/or September. Thus we should expect the most fish and those most perfectly preserved to have died during this time period (group I). Those fish that died in October and/or November should be the most completely disarticulated (group VI), and those that died in June or July should show only a slight degree of disarticulation (group II). The distribution of the stages of disarticulation seems to fit almost exactly the pattern that one would expect if this interpretation is correct.

Gunter (1947) has shown that annual periods of excess salinity in Texas lagoons cause an annual increase in the death of fish and occasionally a catastrophic mass mortality. Because Lake Gosiute is known to have been saline, it might be assumed that somewhat similar chemical conditions prevailed in the Fossil Lake. It is not known definitely that the two lakes were ever connected but if they were, it was most probably a narrow connection near the southern end of Fossil Lake and probably a rather temporary connection.

That a rather long period of aridity occurred in the general region is demonstrated by various depositional features, primary structures, and salt deposition in Gosiute Lake in a part of the Green River Formation. These deposits appear to have settled down during Lostcabinian (late to early Eocene) times. Because the Wasatch Formation immediately underlying the Fossil Butte Member of the Green River Formation in the Fossil Syncline Basin is Lysitean (mid to early Eocene), it is probable that the fish deposits are of Lostcabinian age. Thus it may well be that the long period of aridity occurred during the deposition of the fish beds. While the Fossil Lake probably never reached the high degree of salinity present in Lake Gosiute, it was probably sufficiently saline that periods of excessive evaporation could increase the salinity enough to contribute to the mortality of fish and occasionally cause a catastrophic mass mortality such as those described by Gunter. The presence of aragonite and dolomite in the shales suggests that at least some of the carbonate deposition might well have been because of excessive evaporation and high concentrations of carbonates (Smith 1974, pers. comm.).

At the University of Wyoming, an attempt is being made to interpret scales of fish from the quarries. In a number of specimens annuli can be observed and circuli counted. Although removal of scales for study is extremely difficult, a number has been removed and photographed with a scanning electron microscope. In Lake George, Florida, black crappie develop annuli during January, February, March, and April (Huish 1954). Climatic conditions in north central Florida may be similar to those that existed in western Wyoming during the early Eocene. Thus annuli may have developed at the same time of year. By counting the number of circuli between the last annulus and the edge of the scale, it should be possible to determine the approximate time of death of a fossil fish. If this should correlate with the degree of disarticulation, a check on this interpretation should be possible. Sufficient data are not yet available, however, for the results to be conclusive.

It is not intended to suggest that the conditions outlined above could account for all of the fish concentrations in the Green River Shales. In certain shales in the Green River Basin, for example, the concentrations of Knightia in nonvarved shales appear to have been deposited in quite shallow water. These fish are extremely well preserved but were obviously laid down under conditions quite different from those at Fossil Butte. Much study of these occurrences will be necessary before interpretations are possible.

The story, as told here, should make it clear that the earth is ever-changing. What was once a beautiful, deep lake in an area of lush tropical forests is now a dry, sagebrush desert. What was once an area in which sediments were accumulating is now an area of erosion, occasioned by a broad uplift of the region near the end of Tertiary time.

It is hoped that this brief story of the geologic history of Fossil Butte National Monument will give the reader some appreciation of the geologic complexity not only of the monument itself but of the surrounding area as well. Certainly such knowledge will add to the enjoyment of a visit to the monument.

It may seem that geologists have all of the answers. This, however, is not so. Interpretations are made on the basis of available evidence. Each time a geologist studies an area more geologic data and more evidence become available and our interpretations become a little more accurate. It will be many years before all details of this fascinating history will be known.

GLOSSARY

ANAEROBIC. Usually in reference to organisms that can live without oxygen. ANGULAR UNCONFORMITY. Two rock layers which are not parallel; the underlying older layer dips at a different angle (usually steeper) than the younger top strata. ANNULI. Marks on fish scales produced by periods (usually winter) of nongrowth. ANTICLINE. A fold in stratified rock with the strata sloping downward in opposite directions from the fold crest. ARAGONITE. A carbonate mineral with specific characteristics. AUTHIGENIC. A mineral (such as quartz or feldspar) which is formed after the deposition of a sedimentary layer in which it occurs. BENTONITE. A light-colored, soft, porous rock formed from the minute clay crystals of eroded volcanic ash. It has the characteristic of swelling when wet (water absorption) and contracting when dry. BRACKISH. A condition in a body of water in which the salinity (salt level) is below that of sea water, but higher than that of fresh water. CARBONACEOUS. Rock or sediment which contains carbon or altered organic material such as coal. CHERTY. Containing chert: a dull-colored, flint-like quartz often found in limestone. CIRCULI. Ridges on fish scales produced during growth of the scales. CLAST. Rock fragments which are the result of weathering of a larger rock mass. CLASTIC. Rocks that consist of particles derived from pre-existing rocks or minerals. CLAYSTONE. An indurated clay without the lamination or fissility of shale. CONGLOMERATE. A coarse-grained sedimentary rock composed of fragments larger than 2 mm in diameter in a fine-grained matrix. CROSS-BEDDING. An internal structure in sedimentary rock in which the upper sedimentary layer runs across the grain of the main bed; it is caused by changing currents depositing sediment across the grain of the original deposits. DIAMICTITE. A sedimentary rock containing a wide range of particle sizes. DIP. The downward inclination of a rock layer; the vertical angle is determined by its relationship to a horizontal plane. DOLOMITIC. Containing a measurable amount of the mineral dolomite; a mineral consisting mainly of magnesium carbonate and calcium carbonate. FACIES. Lateral variations in the appearance or composition of a rock layer. The variations can be lithologic or paleontologic. FAULT. A fracture in the earth’s crust along which displacement (movement) has occurred. FLUVIAL. Pertaining to a river or rivers. Fluvial sediments are those transported and deposited by stream action. FORMATION. A rock layer that is mappable; has a distinctive lithology or series of lithologies. A mappable sequence of uniform or uniformly varying rocks. GASTROPODAL. A rock containing an abundance of gastropods. HOGBACK. A long, narrow, sharp-crested ridge formed by the outcropping edges of steeply inclined resistant rocks. IGNEOUS ROCKS. A rock or mineral that has solidified from molten or partly molten material. INTRUSIVE. Igneous rock formed by the forcing of molten material into a pre-existing rock. IRONSTONE. A rock composed of various iron minerals that accumulated during or shortly after deposition of the enclosing sediments. LACUSTRINE. Pertaining to, produced by, or formed in a lake or lakes. LATERITIC. Containing laterite: a red, porous material usually developed in a tropical to temperate climate. It is a residual or end-product of weathering. LIGNITE. A brownish-black coal that is intermediate in coalification between peat and subbituminous coal. LITHOLOGY. The scientific study of rocks: composition, texture, color, origin, etc. MAGNETITE. A black, opaque mineral that is strongly magnetic. MARLSTONE. An impure limestone. METAMORPHIC. Rocks whose structure has been changed by pressure, heat, chemical reaction, etc., such as limestone into marble. MUDSTONE. An indurated mud without the lamination or fissility of shale. OPERCULAR OPENING. The gill opening of fish. OSTRACODAL. A rock, usually a limestone, that contains an abundance of the small crustacean, ostracods. OVERTHRUST. A low-angle thrust fault of large scale, usually measured in miles. PALEOLIMNOLOGY. The study of ancient lakes. PAPER SHALE. A form of finely laminated shale that weathers into extremely thin, curled flakes. PHOSPHATIC. Containing phosphates. PHYTOPLANKTON. Floating microscopic plant life that occurs abundantly in lakes and oceans. PLATY. Rocks (sandstone or limestone) which separate into small slabs. PORCELLANITE. A dense cherty rock resembling porcelain. PUDDINGSTONE. A conglomerate consisting of well-rounded pebbles and cobbles sparsely packed in a fine-grained matrix. SILTSTONE. An indurated silt without the lamination or fissility of shale. STRATA. Rock layers of distinct composition and origin. SYNCLINE. A fold in stratified rock in which the strata slope up from the axis of the fold forming a v opposed to anticline. TAPHONOMY. The branch of paleoecology which deals with the change from living animals to fossils. TECTONIC. The forces which result in structural changes in the earth’s crust. THRUST FAULT. A fault in which an upper segment of rock (hanging wall) moves upward at a low angle (less than 45°) relative to a lower segment (footwall). TONGUE. A rock unit that wedges into, but disappears within, another rock unit. TUFFACEOUS. Sediment that contains up to 50% volcanic ash or dust. UNCONFORMITY. A substantial break or gap in the geologic or stratigraphic record. UNGULATES. Hooved mammals. VARVE. A set of rock laminae in which different types of sediment were deposited in the winter and in the summer. Thus a couplet of each sediment type would represent the deposition of one year. WELL SORTED. A rock in which nearly all of the sediment particles are of one grain size. ZOOPLANKTON. Floating microscopic animal life that occurs abundantly in lakes and oceans.