42 



ANNALS NEW YORK AC AD E 31 Y OF SCIENCES 



Physiogeaphic Occukrence and Distribution of the Morrison 



Formation 



The Morrison formatioii is generally exposed at the surface in two 

 characteristic waj'-s. The usual occurrences are in the steep faces of hog- 

 backs and in the walls of river canyons. The hog-backs are long, even- 

 topped ridges running practically parallel with the axes of mountain 

 uplifts, with a comparatively steep slope on either side, but one side often 



Fig. 1. — Generalized section of Rocky Mountain hog-tack, shoioing the usual 

 physiographic position of Morrison outcrops. 



1. — Pre-Cambrian crystaUines. 4. — Sundance. 



2. — Palseozoic. 5. — Morrison. 



3. — Red Beds. 6. — Capping sandstone (usually Cloverly 



or Lakota). 



being steeper than the other. They are formed through uplift followed 

 by erosion of the tilted and raised sedimentary beds which formerly 

 covered or lapped against the mountains. The softer material is quickly 

 eroded away, leaving ridges protected by cappings of harder strata. The 

 Morrison formation is typically non-resistant, and so is usually found in 



Fig. 2. — Qeneralized section of a Rocky Mountain or Great Plains river canyon, 

 showing a common position of Morrison outcrops. 



1.- — Capping sandstone. 3. — Red Beds. 



2.- — Morrison. 



the erosion cliffs on the inner sides of the hog-backs. Exposures of this 

 kind occur in eastern Colorado, extending north into Wyoming and south 

 into ISTew Mexico, around the Black Hills, on both sides of the Laramie 

 Mountains; around the Bighorn and Owl Creek Mountains; around the 

 Wind Eiver Mountains ; south of the Uinta Mountains ; and in the Grand 



