The sediments that make up the Wasatch Formation in the Fossil Basin
were deposited mainly by streams flowing into the basin from the
surrounding uplands. Rock types are variable and for the most part
individual types cannot be traced over large areas, a condition typical
of fluvial environments. The mudstones and siltstones probably were
deposited along flood plains, while the lenticular sands and
conglomerates were laid down in stream channels. Where streams entered
the lake tongues of deltaic deposits, sands or mudstones wedge into the
Green River Lake sediments.
The reason for the red color of the Wasatch Formation is not fully
known. According to Van Houten (1948), the red is most likely the result
of tropical, red lateritic soils forming in the uplands and being
deposited on flood plains. The oxidation of organic material in a humid,
subtropical environment causes the red color. The bands of purple, gray,
orange, and other colors are due to various stages in the reduction of
the iron oxides in the rocks.
The presence of palms in the lake sediments and of bones of primates and
crocodiles in both the lacustrine and fluvial sediments indicate a
heavily forested, tropical environment.