144 The American Geologist. M^rch, i905 
through which this stream runs at this point is a very stony 
drift and it is certainly from this deposit that the boulders 
lying down in the gorge were derived as residuary matter. 
This same relation was observed on one of the tributaries 
of the Tongue river 15 miles farther south. 
About half way across the shelf however conditions 
change. Original Cretaceous beds are replaced gradually 
by granular assorted and roughly bedded deposits that are 
no doubt of delta origin. On the shelf itself along this 
outer marginal half there is more porosity and sandiness 
of subsoil as noted in the farm lands and the wells are very 
deep compared with those situated on the inner half of the 
mountain. One half of the total area is therefore no doubt 
a delta deposit. 
The accompanying figure representing a section of the 
two mountains and crossing the transverse portion of the 
Little Pembina, is in accord with the above conclusions. 
S e<an^ IfTo u n t A i t 
Tir»t 
!WountV\n 
Fig. 1. Section across the Pembina Delta, showing the subdivision of 
the Cretaceous shales and the extent of the delta deposit. 
Some portions of the delta deposit and accompanying 
drift liave economic value. There are excellent gravels and 
in some places the assorted granular clays are workable. 
The Cretaceous shales themselves however are so near by, 
and are so much more uniform, and are of so much greater 
range of use that the delta deposits have little interest from 
that standpoint. 
Cretaceous Shales. — The main body of the Pembina 
mountains is made up of Cretaceous shales. They underlie 
the delta deposit of "first mountain" and the Red river 
valley, in part, and they continue westward across the 
whole state of North Dakota. 
In color the shales vary from cream-white through yel- 
low and blue and green to brown and even black. Cream- 
white, bluish gray, greenish gray, and black colors are 
