proceedings: geological society 237 



planes. Their positions are believed to have been fixed by the ground 

 water level and the movement of water along the crossbedding planes. 



REGULAR PROGRAM 



Some fossil waters: Chase Palmer. No abstract. This paper is 

 to be published in Economic Geology. 



The stratigraphic break below the Jurassic sandstone in southwestern 

 Colorado: Whitalin Cross and Esper S. Larsen. (Communicated 

 b}' Whit]vl\n Cross.) The representation of the Hayden geological 

 map of Colorado by which the Dakota sandstone (Cretaceous) is shown 

 to overlap Jurassic strata in Gunnison Canyon and extend for 35 miles 

 up the canyon to the vicintiy of the town of Gunnison in contact with 

 pre-Cambrian rocks has been found to be erroneous. Likewise, the 

 representation of the Hayden map that Colorado shales (Cretaceous) 

 rest on the pre-Caml)rian on either side of Tomichi Creek above Gunni- 

 son, is incorrect. The error as regards the Dakota sandstone is not 

 one of observation but is due to the fact that A. C. Peale, who mapped 

 the zone of the Gunnison Canyon, included in the Dakota 400-500 feet 

 of beds which are now knoA\ai to belong to the Jurassic and are assigned 

 to the Gunnison group, embracing the McElmo formation and the La 

 Plata sandstone. 



The representation of an area of Colorado shales resting directly on 

 crystalline rocks is due to F. M. Endlich and was a result of incomplete 

 observations, for while some shales occur east of Cochetopa Creek, tihey 

 are underlain by the Dakota sandstone and the Gunnison sandstones 

 and shales in the full section normal for the region. 



Between the Gunnison Canyon and the Uncompahgre Valley below 

 Oura}', the Jurassic strata must overlap in succession the Triassic and 

 all Paleozoic formations of the region, but there are no exposures to 

 illustrate this transgression in the branches of Cimarron Creek, north 

 of fncompahgre Peak, as indicated on the Haj'den map. The volcanic 

 rocks descend to the stream beds at the places where the uncomformity 

 is represented. 



From Uncompahgre Valley around the w^estern slopes of the San 

 Juan Mountains and eastward on the south side to the ridge between 

 Pine and Piedra Rivers, the Jurassic La Plata sandstone rests on a 

 variable thickness of Triassic strata, of the Dolores formation. While 

 the relation of the two formations is in many localities that of apparent 

 physical conformity, actual erosional unconformity is shown in other 

 places, and there is clearly a constant hiatus below the La Plata, repre- 

 senting an unknowm amount of Triassic beds. These relations have 

 been fully described in the Ouray, Telluride, Rico, La Plata, and Engi- 

 neer Alountain folios of the Geological Survey. 



In the valley of Piedra River we have found evidence that the La 

 Plata sandstone overlaps the entire section from the Triassic to the pre- 

 Cambrian. The actual angular unconformity is beautifully exposed 

 in the Piedra Canyon in the .stretch between two of its western tribu- 



