852 PRINCIPLES OF STRATIGRAPHY 



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were raised excessively and so became subject to pronounced ero- 

 sion, or wbere otber portions were proportionally more depressed 

 and so escaped great erosion, or where other causes were active. 



The end of the period of peneplanation is commonly marked by 

 the age of the strata overlying the peneplain surface. Here, how- 

 ever, it must be borne in mind that slow subsidence of a peneplain 

 surface produces a gradual deposition of formations which suc- 

 cessively overlap each other, each later one in turn coming to rest 

 upon the old peneplain surface beyond the edge of the preceding 

 one. Thus, the pre-Cambric peneplain of North America is over- 

 lain by Lower or Middle Cambric strata in the southern United 

 States, by Upper Cambric strata in the L'pper Mississippi Valley 

 and northeastern New York, by Lower or Middle Ordovicic in 

 northwestern New York, by Middle, and later Ordovicic, in por- 

 tions of Canada, and by later formations in other parts. In each 

 case the age of the peneplain terminates with the age of the over- 

 lying bed, while the part still aljove water continues to be subject to 

 erosion. Thus these higher portions continued to be peneplained, 

 though at an exceedingly slow rate, long after the southern end 

 of the peneplain was buried under thousands of feet of strata. 



High-Level Plains of Arid Regions. In arid regions, where 

 the rainfall is insufficient, and where a large part of the erosive 

 work is done by wind, high-level plains of erosion comparable to 

 peneplains, but having no definite relation to sea-level, may come 

 into existence. Under the influence of arid erosive forces, the 

 initial relief of even a rugged region will gradually become extinct, 

 partly by erosion and partly by filling of the desert basins with 

 waste. The process has been fully described by Davis (12) and 

 enlarged upon by others. A few quotations from Davis will serve 

 to point the essentials of the process and its results : Under the 

 conditions cited "the most perfect maturity would be reached when 

 the drainage of all the arid region becomes integrated with respect 

 to a single aggraded basin-base-level, so that the slopes should 

 lead from all parts of the surface to a single area for the deposition 

 of the waste. The lowest basin area which thus comes to have a 

 monopoly of deposition may receive so heavy a body of waste that 

 some of its ridges may be nearly or quite buried. Strong relief 

 might still remain in certain peripheral districts, but large plain 

 areas would by this time necessarily have been developed. In so 

 far as the plains are rock-floored, they would truncate the rocks 

 without regard to their structure." (12: j(?p.) 



"As the dissected highlands of maturity are worn down, the 

 rainfall decreases, and the running streams are weakened and ex- 



