SCIENTIFIC EXPEDITION TO NORTHWESTERN COLORADO 



!°5 



The route passed first up a broad valley which separates the escarp- 

 ment of Mesa Verde sandstones on the one hand from the Dakota sand- 

 stone on the other (Fig. 3). The surface debris of the valley conceals 

 the greater part of the Mancos formation, though the lower portion 

 outcrops on the north side (Fig. 8) and the upper part on the south 

 side (Fig. 7) . The valley itself results from the erosion of these yielding 

 shales, the bounding ridges being the resistant sandstones of the Mesa 

 Verde and Dakota formations. A very much generalized cross-section 



Little. Book Clijfs 



Grand Hogback 



Fig. 3. — Generalized cross-section showing relation of formation to valleys and 

 ridges between Newcastle and Meeker. 



of the route to Meeker is shown in Fig. 3. The strata from Newcastle 

 to Rifle Gap dip in a southeasterly direction, giving the ridges a 

 northwesterly strike. Beyond the Gap the dip changes to westerly 

 and consequently the strike of the ridges and bluff lines becomes 

 northerly, thus changing the direction of our route. Throughout the 

 region the directions taken by the valleys and ridges, and hence the 

 roads, are controlled by the direction of the dips of strata. 



The passage through Rifle Gap (Fig. 9) carried us out into the 

 Wasatch (Tertiary) formation, with the Little Book Cliffs towering 

 majestically on our left and the Grand Hogback (Mesa Verde formation) 

 on our right (Fig. 3). At Meeker the dip again changes, this time to 

 northwesterly, with corresponding change of strike of bluffs and ridges 



