In a part of western Colorado and eastern Utah the contact between the Burro Canyon formation (Lower Cretaceous) and the Dakota sandstone (Upper Cretaceous) is a broadly undulating erosional surface. Angular and subangular pebbles, cobbles, and boulders of sandstone, chert, and quartzite derived from the Burro Canyon have been found in the basal part of the Dakota sandstone. These fragments and a thin quartzite bed at the top of the Burro Canyon indicate that the formation was lithified, probably under subaerial conditions, before its erosion and before deposition of the Dakota sandstone. On the basis of fossil plants R. W. Brown (1950) placed the boundary between the Lower and Upper Cretaceous at the top of the Burro Canyon. The field evidence described here supports this division and further suggests a hiatus between the deposition of Lower and Upper Cretaceous sediments in part of the Colorado Plateau.