The abundance of plant life in the Colorado area during the Cretaceous
period is indicated by the extent of coal deposits of this age. About
one-fourth the area of the state is underlain by coal seams varying in
thickness from a few inches to fifty feet or more, most of it being
Cretaceous. In the northern Colorado district the coal-bearing formation
is the Laramie. Near Denver there is some coal in the Arapahoe formation
which overlies the Laramie and is of later age.
Coal mines often produce excellent plant fossils, and occasionally other
evidence of prehistoric life. In a mine near Canon City, Colorado, a
series of natural casts of dinosaur feet was taken from the overlying
rock after the coal had been removed. One of these, in the Denver Museum
of Natural History, is seen to consist of sandstone inside a very thin
layer of dark clay. Flattened against the lower surface is the
carbonized stem of a Cretaceous plant which grew in the swamp where the
coal deposit was formed.
Since the shape of dinosaur feet is unmistakable we can only assume that
a large reptile of this type walked over the surface of swampy ground in
which a great thickness of decaying vegetation had accumulated. A layer
of mud settled over the top and became sufficiently firm to retain the
mold of the feet as the animal moved along. Any plant material either on
the mud or included in it was pushed to the bottom of the impressions
and flattened out by the weight of the huge creature. Then sand was
washed into the footprints from some nearby source during a heavy
rainstorm.
Following these events there was probably a subsidence of the area, and
a great thickness of rock-making sediments was built over the ancient
swamp. The buried vegetation gradually became converted into coal, the
sand consolidated into a firm sandstone, and the mud produced the shales
forming the roof of the present mine, which is now at an elevation of a
mile above sea level as a consequence of the general uplifting of the
Rocky Mountain region during late Cretaceous and subsequent time.
When the coal was removed, the hard sandstone casts separated readily
from the softer shales surrounding them. A small amount of the shale
adheres to the sandstone, and some of the flattened vegetation, now in
the condition of coal, still remains attached.