DEBRIS-COVERED MESAS OF BOUEDER, COL. 



505 



The mesas are bordered by rather sharp declivities. Between 

 the foot of these declivities and the flood plains of the creeks is 

 an inclined surface which may be called the mesa-terrace. It is 

 a wide shelf-like surface extending from the base of the mesas 



Fig. 1. — The geological formations in order from the left (west) are : (1) Crys- 

 tallines of the Mountains ; (2) Lower Red Beds ; (3) Upper Red Beds ; (4) Como 

 (Atlantosaurus Beds); (5) Dakota; (6) Benton; (7) Niobrara; (8) Ft. Pierre 

 and Fox Hills; (9) Laramie. 



proper to the border of the present flood plains, where in some 

 cases it is terminated by a bluff, while in others it descends in 

 a gentle transition slope not always separable from the mesa- 

 terrace, making one continuous incline from the base of the mesas 

 to the border of the flood plain, into which it passes by imper- 

 ceptible gradations. Where this transition slope has been 



