LOWEK CRETACEOUS. 603 



from the diagonal basement beds. * * * These changes in thickness were very gradual 

 and can only be noted by comparing widely separated local sections. Hence the rocks of this 

 division, as a whole, notwithstanding the variations to be noted, may be considered as presenting 

 a remarkable example of uniformity of thickness and composition. In its entirety this division 

 is composed of more calcareous rocks than the other divisions of the Comanche series. Although 

 its interior margins ultimately pass into the Basement sands, this change mostly takes place 

 beyond the borders of the east-central province, within which it is practically a great limestone 

 formation (Edwards limestone) , initiated by beds of clay (Walnut formation) at its base. 



* * * The limestones of the Fredericksburg division thin toward the Rocky Mountains 

 and the Ouachita uplifts to the north, and thicken southward or toward the Rio Grande, being 

 in the neighborhood of 700 feet in the latter region and less than 30 along the southern foot of 

 the Ouachita Mountains of Indian Territory. They ultimately pass into clays and sands 

 around the western ends of the Ouachitas in southern Kansas and west of Texas in eastern 

 New Mexico. * * * 



In the area of its most typical if not greatest development, in the Grand Prairie region 

 between the Brazos and the Colorado, as will be more fully set forth later, the limestone is further 

 differentiable into two well-marked mappable subdivisions. In this region aU sections of the 

 Fredericksburg division present the following sequence: 



1. At the base there are a few feet of calcareous clays intercalated with brecciated lime- 

 stones, passing up into chalky layers, which become more numerous toward the top. These 

 are the Walnut beds. 



2. The chalky bands which begin to appear in the top of the Walnut clays are the beginning 

 of a succeeding limestone formation (Comanche Peak limestone), which in this region is sepa- 

 rated into two well-defined stratigraphic units, (a) The lowest of these is the Comanche Peak 

 formation. This formation is composed of compact white rocks which on weathering shatter 

 into numerous angular or conchoidal flakes, seldom weathering into ledges, and contain great 

 numbers of casts of mollusks and echinoids. (b) Without break or apparent change in compo- 

 sition the rocks above the Comanche Peak begin to become harder, weathering into ledges often 

 semicrystalline in character, and containing fhnt nodules and peculiar fossils of the types known 

 as Requienia and Rudistes. These rocks form the Edwards limestone. The Comanche Peak 

 beds are chiefly distinguishable from the succeeding Edwards limestone by the absence of flints 

 and certain very peculiar fossils o^f the overlying beds, and by the fact that they weather into 

 slopes instead of bluffs. 



Stratigraphically there is no break between the various formations, and they pass upward 

 or downward into one another by gradual transition. The marly lime of the upper portions of 

 the Walnut formation grades into the chalky limestones at the base of the Edwards formation 

 without demarcation. 



The combined Comanche Peak and Edwards limestone thins out north of the Brazos, 

 where it is no longer separable into individual beds [and is called the Goodland limestone]. On 

 the other hand, it thickens south of the Colorado, where several distinct subdivisions could be 



made. 



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WASHITA DIVISION. 



The Washita division has been defined as the uppermost of the three stratigraphic groups 

 of the Comanche series. It is composed essentially of sediments laid down in a shallowing sea 

 accompanying a regional uplift which followed the subsidence of earlier Comanche time. The 

 Fredericksburg division ceases everywhere throughout its extent with the purer limestones of 

 the Edwards formation. These limestones represent the deposition of the deepest waters of the 

 Comanche epoch and the culmination of the subsidence which had been progressing since the 

 beginning of Cretaceous time. The succeeding sediments of the Washita division within the 

 east-central province are all of a less purely calcareous, [more] argillaceous, or arenaceous nature. 

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