Mesozoic Rocks of Colorado. — Stevenson. 395 
Upper Dakota.750 ft. 
Middle Dakota.600 “ 
Lower Dakota. 350 “ 
Total 1,700 ft. 
The increase in thickness southward from southern Color¬ 
ado is as remarkable as the increase in thickness of the Fox 
Hills northward from southern Colorado. 
The Upper Dakota consists of little aside from sandstones 
in the Canadian river region ; it forms bold hills on the trib¬ 
utaries of Mora river and its upper beds are the surface rocks 
of much of the plain on both sides of the Mora and Canadian 
canons. It forms the upper part of the walls in those canons, 
where the thickness is not far from 800 feet. The rock is hard, 
gray to yellowish gray and usually fine grained, so that it re¬ 
sists erosion admirably. This upper division of the Dakota 
shows some shales and limestone southward from Mora 
canon in several of the gaps cut through it by streams trib¬ 
utary to the Pecos river. 
The Middle Dakota is shown on the tributaries to the Mora 
river as well as in the deep canons of the Mora and Canadian 
rivers, on Galisteo creek near the old stage-road and at three 
miles east from Galisteo. If consists of shales and irregular 
limestones with, in the Galisteo region, occasional beds of 
gypsum. Films of gypsum are not rare north from the Pecos 
river, but the quantity appears to be more important on the 
waters of the Galisteo. Some fairly good exposures on Mora 
creek show this group to be made up of dull-gray, flaggy and 
soft incoherent sandstones, with variegated shales and some 
excessively hard limestone. The section of this division 
shows a good deal of variation. 
The Lower Dakota is represented everywhere by gray to 
yellowish, gray sandstones, with occasional bands of white. 
In the many gaps in which these rocks are exposed, one can 
find hardly any marked characteristic by which to distinguish 
them from those of the Upper Dakota. The Lower Dakota is 
distinct first in Moreno valley at somewhat more than 35 miles 
north of the locality at which the Triassic rocks re-appear; 
but continuous exposures begin only on the waters of the 
Mora river, whence the rock can be followed to Galisteo 
creek, without break other than those made by streams cutting 
across it. It forms the top of the bluff at whose foot the Atchi- 
