GEOLOGIC HISTORY 325 



was upon a comparatively level and a deeply eroded crystalline floor 

 that the materials of the Trias were laid down (see plate 64). 



There is independent proof that the main lines of drainage flowed 

 westward during pre-Cretaceous time, and the later sediments of the 

 Paleozoic series must, in some measure, represent the material denuded 

 from this belt of high land. 



During Triassic time the portion of the plateau not submerged beneath 

 the Triassic estuary, which occupied the central portion of the district, 

 drained northwestward into this estuary and furnished material to the 

 sediments accumulating there. 



Post-Triassic elevation inaugurated a period of erosion which must 

 have continued during Jurassic and into early Cretaceous time. It was 

 during this period that a nearly featureless plain sloping seaward was 

 formed. The fact of peneplanation may be observed in the level sky- 

 lines of the ridges and hills, and it explains the discordance between sur- 

 face configuration and underground structure. The time of peneplana- 

 tion is fixed by the age of the deposits borne on its surface. The oldest 

 deposits on this peneplain are the Patapsco and the Raritan, belonging 

 to Lower Cretaceous time. . The peneplain also carries deposits of Upper 

 Cretaceous, Tertiary, and Quaternary time. These deposits indicate that 

 the time of the peneplanation was Jurassic, and that it was terminated 

 by submergence of the land. This submergence was not due to a single 

 crustal movement, but to a complex interrupted series of movements. 



First there was subsidence with tilting, which perhaps produced estu- 

 aries bordering the sea and extending inland roughly parallel to the 

 present sea border. In these estuaries were deposited the clays and 

 gravels of the Patapsco formation. An elevation followed which brought 

 the Piedmont district above the sea, and the erosion, which succeeded 

 this elevation, removed the Patapsco deposits except from the deepest 

 portions of the inland estuary. 



Following the erosion interval was a second subsidence and the re- 

 newal of estuaries, in which Raritan deposits were laid down. The 

 uplift that followed was more marked than the preceding, and the pla- 

 teau remained above water during a long period of time, in which a 

 great thickness of Raritan was removed. The third post-Triassic subsi- 

 dence, which followed this erosion interval, lasted during all Upper 

 Cretaceous time, was more extended than any of the preceding, and was 

 oscillatory in character. The adjoining land must have been low, so 

 that the streams furnished only fine sand deposits. Following on Cre- 

 taceous time was an elevation and an erosion interval of long duration, 

 during which portions of the Coastal plain was intermittently beneath 

 and above water. The plateau remained above water continuously 



