56 



ANNALS NEW YORE ACADEMY OF SCIENCES 



The lense-shaped cross-section of one of the sandstone members indi- 

 cates that it was deposited by ai. stream in a relatively narrow basin, which 

 had been eroded in the "imderlying clays. The fre- 

 quent occurrence of cross-bedding in connection 

 with sandstones which have sharp contacts at the 

 top and bottom appear to indicate deposition by 

 streams or wind ; the structure of the cross-bedding 

 sometimes being that usually assigned to stream 

 deposition, and in other cases being of the type 

 usually assigned to deposition by wind. The fre- 

 quent sharp contacts in the formation point to the 

 period of deposition of the formation in this region 

 as a period of alternating deposition and erosion, 

 deposition being the dominant process in the long 

 run.* The sharp contact at the top of the forma- 

 tion also indicates a period of erosion before the 

 deposition of the basal sandstones of the overlying 

 formation. This erosion interval may have been of 

 long or short duration, so far as direct evidence 

 from the contact is concerned. 



CANYONS OF SOUTHEASTERN COLORADO 



Lee (1901, 7) has described the Morrison for- 

 mation in some of the river canyons in southeast- 

 ern Colorado. He reports it to vary considerably 

 in thickness from place to place, being 85 feet thick 

 at the mouth of Plum Canyon, 132 feet in Eed 

 Eocks Canyon and 175 feet in Chaquaqua Canyon. 

 The formation is made up largely of variegated 

 clay-shales or joint-clays. Sandstone occurs in 

 subordinate amounts, varying much in its position 

 in the column, from one point to another. In Eed 

 Eocks Canyon there is a prominent sandstone at 

 the base; there is none at all in Plum Canyon; in 

 Chaquaqua Canyon, four miles from the mouth of 

 Plum Creek, there is a coarse, cross-bedded sand- 

 stone 50 feet or so from the top of the formation. 

 Across the canyon from this point, perhaps two 

 miles away, 30 feet of limestone is found at the 

 same horizon. Some of the sand is very pure and 

 is used as a flux in assaying. The limestones also 



Fig. 15. — Lower part of 

 the Garden Park sec- 

 tion. (Fig. 11.) 



Scale, 12 Va feet to 1 

 inch. 



* See discussion of structures, p. 118. 



