By the beginning of Late Cretaceous time the eroded surface of the
Monument was part of a low plain near sea level, and the sea was
gradually encroaching from the east or northeast. Gravel and sand
carried in by streams combined with the white kaolinite on the surface
to form the 40-foot basal conglomerate of the Dakota Sandstone (fig.
24).
As the land gradually subsided nearer to sea level, swamps which were
formed along the coast supported considerable vegetation. As the trees
and plants died and were covered by silt and mud, they gradually changed
to peat which finally became compacted into coal and brown or black
coaly shale containing plant remains. You can dig out some of this coal
and perhaps find some plant remains near the top of the west canyon wall
just below the highest sandstone bed in figure 24, which is outside the
Monument.
For awhile the coast alternately sank slightly below and rose slightly
above sea level. Beach sand covered the swamp deposits, then more swamp
deposits covered the sand. Some of the sand contains seashells, such as
oysters and clams.
Except on Black Ridge, the Dakota has been entirely eroded from the
Monument, but it crops out with the underlying Burro Canyon in a series
of low hills south of the Colorado River. The Dakota Sandstone is about
200 feet thick.