Bioturbated
Sediment churned and stirred by organisms. The most bioturbated sediments of Fossil Lake indicate lake margins.

Butte
A small, isolated, flat-topped hill resulting from the erosion of near-horizontal strata. The sediments of Fossil Butte actually dip eastward a few degrees below the horizontal.

Calcium Carbonate (Calcite)
Fundamental component of limestone. Produced by a reaction of calcium and bicarbonate ions in water. In a lake these ions are provided primarily by the inflow of streams and rivers.

Chert (Flint)
A hard, dense, mostly quartz sedimentary rock formed by organic or inorganic precipitation or replacement. Chert nodules in Fossil Lake sediments indicate saline, alkaline conditions at the time of their formation.

Dolomicrite
Mud-size dolomite crystals in a carbonate rock. The presence of dolomicrites indicates a saline to hypersaline environment.

Dolomite
A carbonate (limestone) rock distinguished by a significant concentration of magnesium. Indicative of saline-alkaline conditions. Some ostracode fossils are found in these sediments.

Formation
Fundamental body of rock which is characterized by similarities such as rock type, chemical composition and structures, textures or fossils distinct from adjacent rock bodies. The sediments of Fossil Lake are found within the Green River Formation.

Fossil
Any evidence of past life. Bones, teeth, shells and plant material are the most obvious kinds of fossils, but burrows, trackways, footprints, molds and impressions are also considered fossils.

Kerogen
Mineralized organic material, similar to fossil fuels like coal and oil. Formed by the decay of the aquatic plants of Fossil Lake. This dark material gives the “oil shales” of Fossil Basin their distinctive color and odor.

Lamina
A layer of sediment or sedimentary rock less than 1 cm thick. Alternating couplets of lighter limestone and darker kerogen laminae are sometimes incorrectly called varves.

Limestone
A sedimentary rock consisting largely or entirely of calcium carbonate. The laminated limestones of Fossil Lake are characteristic of chemically-stratified lakes, and are the result of changes in temperature/pH or photosynthetic activity causing the calcium carbonate to precipitate out of the water.

Member
A specially developed part of a varied formation, distinguished by color, hardness, composition or fossil content. The Green River Formation has been subdivided into the Fossil Butte and Angelo Members in Fossil Basin. The middle unit of the Fossil Butte Member is the most fossiliferous in this area.

Micrite
Microcrystalline carbonate rock. Described in the Fossil Lake region as kerogen-rich laminated micrite (KRLM) or kerogen-poor laminated micrite (KPLM) it is primarily in this rock that the majority of Fossil Lake fossils are found.

Oil Shale
Dark gray or black shale containing kerogen. Some of the oil shales in this area are so rich in kerogen that early fossil hunters used to burn the rock in their campfires for fuel.

Ostracode
A small crustacean (other members of this superclass include crabs, lobsters, shrimp and barnacles) that is a good indicator of salinity and fluctuations in the positions of shorelines. The presence and concentration of ostracode fossils in the sediments of Fossil Lake distinguish near-shore from deeper water environments.

Paleontology
Paleo = “ancient” + onta = “existing things” + logy = “science”. The study of fossil plants and animals. Careful examination of the diverse fossils in Fossil Basin helps establish this ancient environment.

Saline
Salty. Fossil Lake’s salinity varied with the lake’s size, depth, and amount of material washed in by streams.

Shale
Fine grained, easily-split (fissile) sedimentary rock composed primarily of clay minerals and quartz. Early discoverers described the fossils of Fossil Lake as being in shale instead of limestone because shale is often used as a generic term to describe any fine-grained fissile sedimentary rock rather than to distinguish its mineralogy.

Siliciclastics
Sediment which comprises particles composed of silicate minerals and rock fragments, such as the claystones, siltstones and sandstones of Fossil Basin.

Soft Sediment Deformation
Sedimentary beds, cross-beds, or laminae that were disrupted, convoluted or contorted prior to lithification. Much of the soft sediment deformation found in Fossil Basin may be soils or turbidites.

Tuff
Compacted (lithified) volcanic ash. There are a number of tuff layers in Fossil Lake sediments, indicating the region underwent a certain amount of volcanism during the life of the lake. These layers are important in dating the lake since it is from these sediments samples can be obtained for radiometric dating. Such volcanic material is considered to be “frozen in time” at the instant the minerals within them crystallized whereas sedimentary rocks are the products of the deterioration of preexisting material and are subject to weathering and erosion which corrupts the closed system required by absolute dating techniques. The K-spar tuff layers in Fossil Basin contain a high percentage of potassium (K) feldspar.

Turbidite
Sediment or rock deposited by a swiftly moving bottom-flowing current (turbidity current). The turbidites of Fossil Basin may reflect periods of rapid expansion of Fossil Lake.

Varve
A banded layer of silt and sand deposited annually in lakes especially near ice sheets. The sand layer represents summer deposition; silt represents winter deposits—hence a pair of these deposits signifies a one-year period in the life of the lake. Once used to determine the age of certain stages of Fossil Lake, the varve concept is no longer an appropriate model for Fossil Lake due to variations in laminae thickness and the mineralogical character of Fossil Lake sediments.