762 J, BAERELL MEASURExMENTS OF GEOLOGIC TIME 



shows a degree of completion of this oldest erosion cycle which is repre- 

 sented by a i)eneplain. At any one place the evidence of three successive 

 levels is usually all tliat can be oliserved and discriminated: often but. two 

 may be detected. 'I'lic relations are liest seen wlicre hard rocks rise to 

 moderate heights, as in plateaus not far ivom tlu' sea. TherCj as in the 

 Appalachians, the streams respond quickly to the new upwarps of the land, 

 or possibly what nm^ be equally concerned, n.ew sinking of the sealevel. 

 The hard nx-ks, because of thcii' retiistance, sur\ mac for a greater t;ime and 

 record a griiater nuiid)er of pulses of diastrophism. The prohle of Hgure 2 

 brings out the significance of slopes in -a. diagrammatic form. 



-In parts of the Appahtchians, particularly in southern New England, 

 where broad terranes of metan4or]>hic rocks occur, successive baselevets are 

 recorded by means of penej)lai]is or valleys -;xt successive ele^■ations which 

 average about 200 feet ai>art. Suc^-essively older ones appear in passing 



V/^TTTTA 



PrGTRT;; 'J.-'-Diaf/rammatic Section of Valley 

 Showing composite slope resuUiug from pulsatorj^ uplifts in quickening periods 



into the mountains. There they become more obscure, and it becomes 

 jnore difficult to discriminate bet^^■een them or detect accurately their 

 original differences in elevation. The writer sought the approximate 

 dates of these liaselevels Ijy tracing the erosion planes into New Jersey, 

 Pennsylvania, and Maryland, (jpposite to where the formations of the 

 coastal plain attain a wide exposure. By projecting from opposite direc- 

 tions the slopes of these baselevels recorded by erosion and the slopes of 

 the formations recorded by deposition where they approach nearest, the 

 two series were interrelated ; but the formations have their dates fixed 

 by fossils, and this leads in turn to the dating of the erosion forms. 

 Those below 500 feet in.elevation are the Ooluinbia group of Pleistocene 

 age. Those beginning witb the Lafayette or Appomattox and rising 

 higher are- Pliocene'. Back of the Pliocene series are moie^-disma'nth^ ! 

 'forms which belong to the older Tertiary and even to the upper Cre- 

 taceous.^" Figure '3 shows the type of valley profiles '\^bi(•l1 pertain to tbc 



>" .Tpsep.h Barrel! : Piedmont terraces of tlie- northern Appalachians and their. mode of 

 "oi-igin." rost-Jurassic history of the northern Appalachians. Bull. Geol. Soc. Am., vol. 

 44j'iS1.8, pp. 688-696, ■ The burden of other work has prevented up to tho prespiit time 

 the olaboration of this subject beyond the first hrief publication. 



