THE MORRISON FORMATION 66 1 



The general features of the whole region traversed have been 

 well described by Lee 1 and the description need not be repeated 

 except to say that it is a portion of the Great Plains region through 

 which the principal streams have cut canyons several hundred feet 

 deep, thus exposing good sections of the nearly horizontal strata. 

 The Dakota sandstone always forms prominent cliffs near the top 

 of the canyon walls and in some part of the course of each large 

 stream the cutting extends as low as the Red Beds. The conditions 

 are thus especially favorable for studying the strata immediately 

 below the Dakota, as the Dakota itself, and many of the other hard 

 beds are often continuously exposed for many miles, and furnish 

 convenient, easily-recognized reference planes. 



Purgatoire River. — Our first examinations were made on Purga- 

 toire River at Higbee Plaza, about twenty miles south of La Junta, 

 Colorado, where some marine invertebrate fossils, seen in the talus 

 by Mr. Lee, gave us an important clue to the solution of our chief 

 stratigraphic problem. At the top of the canyon wall is a cliff- 

 forming gray and brown, mostly massive, cross-bedded sandstone, 

 here fifty feet thick, but the upper part has been removed by modern 

 erosion. Back from the river, where it passes under the Benton 

 shales, its total thickness is not far from one hundred feet. This 

 is unquestionably Dakota, as is attested by its stratigraphic position, 

 its lithologic character and its flora, of which a few specimens, col- 

 lected here and at other localities in the region, have been identified 

 by Dr. Knowlton. Separated from this upper sandstone by about 

 fifty feet of dark shales and thin-bedded sandstones, usually in large 

 part covered by talus, is another lithologically similar coarse gray 

 sandstone, varying in thickness from fifteen feet, or less, to sixty feet. 

 This has also been referred to the Dakota by Lee, and probably by 

 every geologist who has worked in southern Colorado, the inter- 

 mediate more shaly portion being correlated with the "fire-clay band" 

 of the earlier reports. 



It was soon found that this intermediate shaly portion of the 

 "Dakota" was the source of the fossils found in the talus below, 

 and the fossils themselves were recognized as belonging to the 



1 Journal of Geology, Vol. IX, pp. 343-52; Vol. X, pp. 36-58; Journal of Geog- 

 raphy, Vol. I, pp. 357-70; Vol. II, pp. 63-82. 



