INTERCALATED SANDSTONES 



739 



erosion plane of the dolomite. Where conformable it may contain 

 lower Beekmantown fossils. Sometimes the lower beds are slightly 

 disturbed, while the upper ones do not partake of such disturbance. 

 This indicates an interval of time after the deposition of the lower 

 beds, during which they were disturbed, and after which the higher 

 strata were deposited upon them. Southward, higher members of 

 the Beekmantown series come in beneath the Saint Peter sandstone 

 and lower members of the Chazyan above it. In the Arbuckle 

 Mountains the upper bed (here known as the Simpson formation) 

 is about 2,000 feet thick, while the Beekmantown below the Saint 

 Peter is of similar thickness. This sandstone thus has all the char- 

 acteristics of a regressive-transgressive sandstone intercalated be- 

 tween limestones, and may be regarded as a typical example. (Gra- 

 bau-3; 4.) (Fig. 152.) 



II — )| 1,10 c/< f>,,.er i- ||M P II' h- 



/ Pre-Cam/r, t O/a-ZarxT 



.-//^/y-^yy^^f^m 



Fig. 152. Diagram representing the Cambric, Lower Ordovicic (Beekman- 

 town), and Middle Ordovicic (Chazyan) formations, and the 

 compound transgressive-regressive-transgressive overlaps between 

 Oklahoma and Minnesota. The intercalated sandstone is the 

 Saint Peter. 



The Dakota Sandstone. This sandstone of Mid-Cretacic age has 

 essentially the same relation to the enclosing calcareous strata as has 

 the Saint Peter. It contains, besides, an abundant flora of terres- 

 trial plants, so that its value as a record of complete emergence be- 

 comes even greater. In southern Kansas this sandstone lies at the 

 top of the Fredericksburg division of the Comanchic, while in Texas 

 it lies on top of the Washita or Upper Comanchic. In either case 

 there is commonly a gradation from the limestones and shales to 

 the sandstones. The top of this sandstone passes upward into the 

 marine Eagle Ford formation in Texas, while further north, in 

 Colorado and the Dakotas, it grades into the much later Benton 

 beds. 



B. NONMARINE PROGRESSIVE OVERLAP. This term 

 is applied to the large structure normally produced during the 

 formation of a great fan or subacrial delta, from the wash carried 

 by the streams from the mountains and deposited upon the plains 

 adjoining. Such a subaerial fan will of course grow in extent year 



