THE PLIOCENE PERIOD 829 



East of the Rocky Mountains, on the western border of the 

 plains, deposits of this class are wide-spread. Some of them are 

 Pleistocene, and some are probably older. In many places these 

 gravels show by their constitution that they came from the moun- 

 tains, and in some situations they have been shifted repeatedly 

 (p. 181), always farther from the mountains and to lower levels 

 (Fig. 555), with the result that they now constitute a series of 

 deposits of somewhat different ages, rather than a single formation 

 assignable to a definite epoch. 



In the Mississippi basin, far from all mountains, there are 

 patches of gravel on various hills and ridges, which are interpreted 

 as the remnants of a once more or less continuous mantle of river 

 detritus. The definite correlation of these gravels is not now 

 possible, and they may not all be of the same age. They are not 

 older than Cretaceous, and are older than the glacial drift. Their 

 similarity to the Lafayette (Pliocene) gravels farther south sug- 

 gests their correlation with that formation. The material of these 

 gravels, almost wholly quartz, quartzite, and chert, is partly local, 

 and partly from the north. The leading topographic features of 

 the Mississippi basin have been developed since their deposition, 

 for their remnants are on the crests of the highest lands within the 

 area where they occur. 



About the Atlantic and Gulf coasts similar deposition gave rise 

 to the Lafayette (Orange Sand) formation. This formation has 

 been so variously interpreted that it merits special consideration, 

 and the interpretation here given it is not unchallenged. 



The Lafayette Formation 1 



This formation has an extensive distribution, (1) between the 

 Piedmont plateau and the Atlantic, (2) on the inland part of the 

 Coastal Plain of the Gulf of Mexico, and (3) in the southern part of 



1 The fullest sketch of this formation as a whole is that of McGee in the 

 Twelfth Annual Report of the U. S. Geological Survey. A few references to 

 other accounts of the formation in special localities, often under other names, 

 are as follows: Safford, Am. Jour. Sci., Vol. XXXVII, 1864; Hilgard, Agric. 

 and Geol. of Miss., 1860, and Am. Jour. Sci, Vol. XLI, 1866, and Vol. IV, 

 1872; Salisbury, Geol. Surv. of Ark., Report on Crowley's Ridge, 1889; 

 Durable, Jour. Geol., Vol., II, 1894, p. 560; Smith, E. A., and Johnson, L. C., 

 Geol. Surv. of Ala., 1894. 



