DESCRIPTIVE details: RIO PUERCO FIELD 623 



Feet 

 Shale 50 



Shale with limestone concretions (Cephalopod zone) containing Pin)ia 

 petrina White? ; Pecten sp. ; Trigonarca sp. ; Isocardia sp. ; Veniella ? 

 sp. ; Cardium sp. ; Turritella sp. ; Volutodcrma sp. ; undetermined 

 gastropods; Mctoicoccras sp. related to M. swallovi (Shumard) ; 

 Mctaicoccras sp. (U. S. G. S. locality No. 3520) 10±: 



Shale. In the lower part were found Ostrea sp. ; Avicula gastrodes 

 Meek ; Pinna petrina White ; Veniella sp. ; Pholadomya sp. ; Turri- 

 tella sp. related to T. tchitei Stanton; Metoicoceras sp. (U. S. G. S. 

 locality No. 7191). The fossils were collected at some distance from 

 the line of measurement of the section and may possibly belong to 

 the Cephaloped zone 600± 



Sandstone, shaly, with impure limestone in lenses and concretions. 

 (This sandstone and the limestone concretions above and below it 

 seem to constitute the Concretion (Septaria) zone of Herrick and 

 Johnson.) It contains Ostrea sp. ; Exogyra columhella Meek; 

 Aijomia sp. ; Avicula gastrodes Meek; Cardium sp. ; Legumen sp. ; 

 Pecten sp. ; Pinna petrina White; Isocardia sp. ; Liopistha (Psilo- 

 mya) sp. ; AncTiura sp. ; Prionotropis sp. ; Acanthoceras 1 sp. (U. S. 

 G. S. locality Nos. 7192 and 3513) 50X 



Shale alternating with layers of yellow sandstone 78 



Shale, carbonaceous, dark-colored ; has general aspect of the shale 

 associated with the coal beds of the Mesaverde in this field 2 



Sandstone, coarse-grained above, shaly below ; weathers to irregular, 

 rounded masses ; contains Halymenites similar to H. major Lesq., 

 worm borings, and a variety of indefinite markings 25 



Shale, dark-colored (Gastropod zone), containing Ostrea sp. ; Exogyra 

 .columl)ella Meek; Camptonectcs symmetrica Herrick and Johnson; 

 Plicatula sp. ; Gcrvilliopsis 1 sp. ; Pinna petrina White; Pinna sp. ; 

 Liopistha (Psilomya) sp. ; Ai-ca sp. ; Trigonarca sp. ; Cardium sp. ; 

 Lucina sp. ; Turritella sp. ; Ancliura fusiformis White non Meek ; 

 AncJiura sp. ; Cinulial sp. ; Serpula sp. (U. S. G. S. locality Nos. 

 7205, 3517, and 3518) 35 



Sandstone, soft, friable ; contains small pebbles, principally of quartz 

 and chert 5 



Sandstone, yellow 8 



(Abrupt change in lithology.) 



Sandstone, coarse-grained, gray to pink ; varies greatly from place to 

 place in thickness, composition, and color. This sandstone has the 

 stratigraphic position of the Dakota, but its color and variable char- 

 acter are suggestive of Morrison 100 



Sandstone and shale, variegated (Morrison). 



1,290 



Two collections of fossils were made east of the Rio Puerco about 3 

 miles north of San Francisco. The first is from a zone of limestone con- 

 cretions that seems to occupy a horizon about 50 feet above the top of the 



