374 THE JOURNAL OF GEOLOGY. 



at present produced this plateau. Evidently it could be pro- 

 duced by denudation only at or near baselevel, for the effect of 

 erosion upon a mass high above baselevel is to accentuate its 

 topographic relief, not to reduce it. We naturally ask ourselves,. 

 "At what stage in geologic history did this denudation occur ?"^ 



Date of the peneplain. The erosion which accomplished this 

 great work must have commenced after the formation and dis- 

 location of the Triassic beds, for the even crest line of the trap 

 ridges, a part of which — perhaps all — were contemporaneous 

 with the sandstones, is a part of the dissected peneplain ; but to 

 fix the date of the completion of the peneplain, we must turn to 

 evidence presented in New Jersey.^ There we learn that by the 

 close of Cretaceous times, the country was eroded nearly to base- 

 level, and we may therefore speak of the relative position of the 

 land and sea, to which the land was at this time reduced, as the 

 Cretaceous baselevel, and this land surface as the Cretaceous 

 peneplain. 



Elevatioti of the peneplain. In post-Cretaceous, presumably 

 early Tertiary^ times, the land was elevated to nearly its pres- 

 ent height and remained at that altitude, so far as topographic 

 evidence shows, during Tertiary times. The proofs of this ele- 

 vation are the valleys which the streams have sunk below the 

 general level. That this was not a simple uplift, but was accom- 

 panied with tilting and warping, is clear from the following con- 

 siderations. The depth to which a stream can cut its valley 

 depends directly upon its height above baselevel. If the pres- 

 ent surface were a peneplain uniformly elevated, the head waters 

 and middle courses of a river would not be cut so deep in the 

 surrounding plain as its lower course. But the reverse is true 

 of the rivers of Connecticut. The depth of the valley increases 

 inland, being greater in those regions where the peneplain was 

 raised the highest. A comparison of the upper and lower val- 

 leys of the Housatonic, Naugatuck, Quinnebaug, and of the 



' Bulletin of Geol. Soc. of Amer., vol. ii, p. 554. 



^It is not desired to affirm that these periods of erosion and elevation began and 

 ended promptly with the beginning or end of a period. The time statements must 

 be considered as only approximate. 



