This tongue of the Wasatch Formation can be seen in the northwestern
part of Fossil Butte National Monument and over most of the northern
part of Fossil Basin. In the area of its distribution, the Mudstone
Tongue separates the underlying Fossil Butte Member of the Green River
Formation from the overlying Angelo Member of the Green River Formation.
The tongue thins and disappears to the south, that is, basinward. As
with the Sandstone Tongue, the edges of the Mudstone Tongue where it
pinches out represent an ancient shoreline of Fossil Lake. To the north
and west the Mudstone Tongue merges with the Tunp Member of the Wasatch
Formation.
The Mudstone Tongue is a composite of dark-red mudstone which becomes
lighter in color basinward, changing to light red, pink, or
greenish-gray claystone. The unit is a mixture of silt and clay derived
from the north and west, and was deposited as a large delta in Fossil
Lake.
An interesting aspect of the Mudstone Tongue is the presence of algal
logs. These are cylinders of limestone that apparently formed as an
encrustation of calcium carbonate around logs and branches that fell
into the edge of the lake. The calcium carbonate resulted from the
action of algae which grew around the log. Successive growths of algae
resulted in successive layers of calcium carbonate being deposited.