ROCK FORMATIONS 361 



idly eastward, their maximum thickness in the eastern part of the field 

 being only 114 feet, while in some places not only were they completely 

 eroded away, but the underlying Trinidad sandstone also was deeply 

 eroded. The post-Laramie formation has a maximum measured thick- 

 ness of 1,200 feet. The lower half consists principally of sandstone that 

 is commonly conglomeratic at the base. The upper half has a greater 

 proportion of shale, but in some places near the top there are thick beds 

 of conglomerate, apparently belonging to a younger formation, the Poison 

 canyon, but not separated from the underlying beds by any sharp line of 

 demarkation. 



Evidence of Unconformity 



In order to appreciate the significance of some of the facts relating to 

 the unconformity here described, it is necessary to consider certain con- 

 ditions existing previous to the uplift and erosion that produced it. 



(a) The absence of arenaceous material from the marine Cretaceous 

 shale in the Raton field warrants the inference that the land areas which 

 furnished the sediments either were located at great distances from this 

 field or were so- nearly baseleveled that the streams draining them could 

 transport only clay and fine silt, and apparently proves fallacious any 

 postulate that would account for the conglomerate at the base of the post- 

 Laramie formation without renewed uplift in the mountain region from 

 which the conglomerate was derived and the removal from the uplifted 

 portions of considerable thicknesses of sediment. 



(b) The change in the character of the sediments from the fine text- 

 ured shale of the Pierre to the coarse sand of the Trinidad sandstone may 

 be due to uplift in neighboring areas, but this postulate is not necessary 

 to account for the presence of the sand, for this sandstone represents the 

 last stage of the vanishing Cretaceous sea and the sand may have been 

 transported along shore from great distances. 



(c) The Cretaceous coal measures lie conformably on the Trinidad 

 sandstone and were probably accumulated on broad, low-lying flats that 

 were subject to slight oscillations. There are no fragments in them larger 

 than grains of sand and the bedding is comparatively regular, differing 

 notably in this respect from the post-Laramie beds, which are lenticular 

 in many places. Few organic remains were found that would indicate 

 the conditions under which the sediments were laid down. In one local- 

 ity the supposed seaweed, Halymenites major, was found in rocks above 

 the lowest coal bed, but fossil leaves indicative of fresh-water conditions 

 were found in several other places. Judging from the regularity in the 

 bedding of the sediments, no great difference in the original thickness of 



