60 STANTON: UPPER CRETACEOUS STRATIGRAPHY 



a short distance to the north it is said to be 2000 feet thick and 

 is described as ' ' gray sandy shale with calcareous bands and sand- 

 stones." Eastward, however, for nearly 100 miles thru the La 

 Plata and Durango quadrangles and on to Monero, New Mexico, 

 the work of Schrader* and Gardner^ has shown that the Mancos 

 shale maintains about the same thickness and character as at 

 the type locality. From Monero southward along the eastern 

 border of the great San Juan area, past El Vado to Gallina and 

 beyond, a distance of about 50 miles, it continues to be essentially 

 a shale with no conspicuous sandstone intercalations. On this 

 point the observations of Lee and Stanton on a recent recon- 

 naissance from Albuquerque to Monero confirm the descriptions 

 of Gardner who mentions a sandstone 30 feet thick, 275 feet 

 below the top of the Mancos, 10 miles north of Gallina and states 

 that farther south the upper part of the formation consists of 

 300 feet of argillaceous sandstone and sandy shale grading up into 

 the Mesaverde formation. The important point to be remem- 

 bered is that so far the lower three-fourths of the Mancos con- 

 tains no sandstones except a band usually about 50 feet thick, 

 of yellowish thin-bedded, shaly, somewhat calcareous sandstone 

 and sandy shale, characterized by Ostrea lugubris, Scaphites 

 warreni, and Prionocyclus wyomingensis, representing one of the 

 most useful, widespread and persistent faunal zones in the Upper 

 Cretaceous of the Rocky Mountain region. At Mancos this 

 horizon is about 400 feet above the Dakota but the distance from 

 the Dakota varies considerably, doubtless on account of varia- 

 tion in the rate of sedimentation. 



South of GalHna along the west base of Naciemento Moun- 

 tains structural complications and an overlap of the Tertiary 

 prevent continuous observation of the Mancos for about 20 miles 

 but the overlying Mesaverde is readily recognized when it 

 reappears and is continuously exposed to the neighborhood of 

 Cabezon where it swings west with a low northerly dip. From 

 that point southward the underlying rocks, including the Man- 

 cos, are splendidly exposed along the Rio Puerco for 35 miles to 



^U. S. Geol. Survey, Bull. 285, pp. 241-258. 



5 U. S. Geol. Survey, Bull. 341, pp. 335-351. Jour, of Geology, 18: 702-741. 



