480 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



them as a whole, and they are all included, so far as they may be present, in the 

 "Red Beds," in the dark greenish-blue areas of the map (9). 



Cross has recently divided the "Red Beds" of the plateau province in south- 

 western Colorado into the Cutler formation (Permian!) and Dolores formation 

 (Triassic). Below the Cutler occur the Rico and Hermosa formations (Pennsyl- 

 vanian), whose approximate equivalents farther south, in the Rio Grande region, 

 are "red beds" as described by Lee.^^^* 



The Cutler formation is thus characterized by Cross and Howe : ^^^ 



The Cutler formation embraces somewhat more than the lower half of the Red Beds section 

 of southwestern Colorado. Its strata are invariably red in color and include sandstone, arkose 

 grit, conglomerate, shale, and limestone. The maximum observed thickness is about 1,600 feet. 



The formation seems conformable with the underlying Pennsylvanian beds, but above it 

 occurs a stratigraphic break with at least local unconformity. The base of the formation is 

 indicated by the Pennsylvanian fossils of the Hermosa or Rico formations and in a broad way 

 by the color line. No fossils have been found in the Cutler beds. 



Great variability in lithologic constitution, both vertical and lateral, is one of the most 

 striking features of the Cutler formation. The sandstones are sometimes fine grained and 

 massive, but bedding is ordinarily distinct and few homogeneous beds exceed 10 or 15 feet in 

 thickness. All strata are calcareous, and the finer-grained sandstones grade into calcareous 

 shales and impure marls or into sandy limestones. These rocks are naturally more or less friable 

 and crumbling. 



The finer-grained strata are of the strongest red color, which is due to a ferritic pigment, 

 and they are also commonly characterized by abundant bronze or rusty mica, which renders 

 them fissile. Clay beds are rare, as is massive limestone. Commonly the more calcareous 

 strata are nodular or gnarly and grade into calcareous sandstones. Greenish and grayish tints 

 are locally found in the nodular limestones and a mottling with red is common. Some of the 

 nodular limestones appear to be intraformational conglomerates. 



The sandstones frequently grade into arkose grits and these into conglomerate. With 

 increasing coarseness of grain the red changes to pink, and locally beds of coarse grit are gray 

 or almost white. In other cases the finer matrix of grits and conglomerates is dark red. The 

 cement of the strata is calcite, and most of the conglomerate and arkose beds are comparatively 

 resistant to weathering and form prominent ledge outcrops on all steep slopes. 



The grit beds often reach 35 feet in thickness. They are variably massive, being in some 

 places almost homogeneous from top to bottom, whUe more frequently divided by several thin 

 shale or sandstone layers. Cross-bedding is almost universal. Sporadic pebbles are present in 

 all grits, and with their increase the stratum becomes conglomerate. 



The sandstones are mainly quartzo§e, the grits contain much feldspar, mica, and small 

 pebbles like the larger ones of the conglomerates. The latter contain pebbles of granite, gneiss, 

 and various schists, of quartzite and limestone, of greenstone and porphyry, and many of red, 

 pink, smoky, or white quartz, part of which may come from veins. 



The pebbles are in general larger near the San Juan Mountains. Bowlders a foot in diameter 

 are occasionally present, but most pebbles are only a few inches in diameter. The relative 

 abundance of different rocks among the pebbles varies according to locality. The greenstone 

 schists, metadiorite, and green porphjnry are most abundant in the Uncompahgre Valley expo- 

 sures; granites and quartzites are always prominent. 



Taking the formation as a whole, the grits and conglomerates comprise about one-third or 

 less of its total thickness in the quadrangles surveyed, and they are distributed throughout the 

 section. It may be assumed that as distance from the source of the pebbles increases, the for- 

 mation becomes more and more a series of fine-grained sandstones and shales, with subordinate 

 grits and conglomerates. 



