412 J. BARBELL REC0GKlTJ02s OF AKCIENT ])ELTA DEPOSITS 



geos3aicliiie^ but spreading to some extent beyond it over the continental 

 interior. 



By the late Mesozoic the conditions had become reversed. The old 

 land of Appalachia now became a region of deposit, and a fraction of the 

 sediment was transferred back to it which ages before had been swept 

 westward from its uplands and mountain ranges. But not only were the 

 regions of erosion and sedimentation reversed ; the physiographic condi- 

 tions were also different. In the late Mesozoic a gentle and limited sub- 

 sidence due to tilting of the continental margin permitted fluviatile out- 

 building, but the subaerial delta plain faced the j^owerful waves of a 

 deep and wide ocean. In the Paleozoic by contrast it was the sinking 

 bottom of the geosyncline which gave the rivers tasks of infilling which 

 they were usually unable to accomplish; but the action of the interior, 

 sea was relatively weak, for its waters were shallow, shifting, and at 

 times drained away by slight continental movements. 



Thus in the late Mesozoic the outbuilding of the rivers was o23posed 

 by powerful marine planation which at last gained the mastery. In the 

 Paleozoic they contested rather against a zone of dovvnsinking which 

 crossed their paths. The character of the sedimentary formations of the 

 two eras was controlled by these different geologic conditions. In the 

 Mesozoic there developed a wide but thin mantle of detrital deposits, of 

 which a broad outer part must always have been marine. In the Paleo- 

 zoic the clastic sediments formed deposits more remarkable for great and 

 variable thickness than for width, and the proportion of marine beds was 

 determined by the varying rates of subsidence and erosion. The condi- 

 tions for river work during the Paleozoic were more highly variable and 

 gave rise to greater extremes than during the following era. AMien u])- 

 lift of Appalachia and resulting erosion were the dominant features the 

 interior sea may have been largely or wholly displaced. At other times, 

 when Appalachia had been worn low and could not supply its rivers with 

 abundant land waste, slight regional subsidence brought the sea east- 

 ward over the geosyncline and against the eroded mountain base. 



Where rivers maintain a land surface against a sea, delta conditions 

 are implied. In the case of the Upper Jurassic and Lower Cretaceous 

 deposits the landward margin of the subaerial beds has been eroded and 

 the corresponding seaward or subaqueous beds are concealed to the south- 

 east. Erosion exposes at the present time only a certain belt of the 

 ancient deltas, and the conception of the whole has to be supplied by 

 deduction from the principles of sedimentation. In the case of the Ap- 

 palachian geosyncline the portions of the sediments which are preserved 

 instead of thinning out against the old land are seen to diminish in 



