The wet climate of Late Jurassic time was followed by arid or semiarid
climate in the Early Cretaceous. Streams continued to deposit gravel,
sand, silt, and mud, but at a much slower rate. These deposits
eventually hardened into the conglomerate, sandstone, and green shale or
siltstone of the Burro Canyon Formation. This formation, together with
part of the overlying Dakota Sandstone, caps Black Ridge, the highest
part of the Monument (7,000 feet) about a mile west of the Coke Ovens.
Several airway beacons on this high ridge may be seen for many miles.
The Burro Canyon is best seen below the Monument on the west side of
Monument Road along the lower part of No Thoroughfare Canyon, where it
is about 60 feet thick (fig. 24).
[Illustration: BURRO CANYON FORMATION AND DAKOTA SANDSTONE, along
west side of No Thoroughfare Canyon, about 2½ miles northeast of the
Monument’s East Entrance. Basal sandstone above road and unexposed
green shale (brown in photograph) comprise the Burro Canyon, here 58
feet thick. White band two-thirds the way up the slope is 40-foot
basal conglomerate of the Dakota Sandstone, above which is 58 feet
of carbonaceous shale, a 14-foot bed of sandstone, and 17 feet of
sandy shale to the top of the hill. The top of the Dakota has been
eroded away. (Fig. 24)]
A few fossil plants and shells have come from the Burro Canyon
Formation, but the seeming absence of dinosaur bones suggests that
possibly these reptiles had to move to areas of greater precipitation,
where food was more abundant. Some dinosaurs may have lived in the area
at this time, but their bones either were not fossilized or they have
not yet been found.