LARGER RELATIONS OF DELTAS 413 



thickness away from it — tliat is, tlie iiiaximimi thicknesses of tlie Paleo- 

 zoic clastic formations are commonly seen in their easternmost outcrops 

 in the Great A^alley — those outcrops nearest the old land from which the 

 sediments were derived. This means that they are now eroded from the 

 whole eastern part of the original geosyncline. Instead of restoring the 

 boundaries of the Paleozoic uplands at the present limit of the pre-Paleo- 

 zoic rocks, this zone of the Appalachian Mountains and the Piedmont 

 Plateau are to be looked on rather as the eastern side of the ancient 

 geosyncline and bordering a broad land farther to the east. At times this 

 intermediate belt was overflowed by the sea; at times it was subject to 

 folding, uplift, and erosion; but at other times it was a Piedmont plain, 

 the seat of river aggradation. In the Paleozoic Appalachian geosyncline, 

 unlike the late Mesozoic coastal formations, it is the seaward and not the 

 landward half in which the section has been preserved. But exactly as 

 the late Mesozoic fluviatile deposits imply a seaward phase farther from 

 the land, so in the Paleozoic the dominantly marine character of thick 

 sand and mud formations deposited in shallow water imply the former 

 existence of the fluviatile beds of deltas on the landward side. At times 

 the delta conditions doubtless disappeared owing to low relief of the lands 

 and the widespread seas planing inland against its shores. At other 

 times the delta conditions disappeared because of the entire retreat of 

 the sea; but much of the time they were present, and in the Upper 

 Paleozoic, when clastic sedimentation became more dominant, the sub- 

 aerial beds extended into the western half of the geosyncline and their 

 outer portions are therefore still preserved, interfingering with marine 

 beds farther to the southwest. 



For a complete conception of ancient conditions the imagination must 

 restore to a sedimentary system those parts which are destroyed as well 

 as those which are buried, and not invest them merely with the charac- 

 [ers exhibited by the visible portion, hut rather ascribe those characters 

 which would pertain to them as the several members of an organic whole 



The borders of the Appalachians have been used as an exam])le of the 

 varying development of delta conditions. But other uplands bordered 

 by geosynclines or facing slightly submerged parts of the continental 

 platforms have supplied similar physiographic settings. To simply de- 

 termine in such regions some formations as fluviatile and others as ma- 

 rine does not complete the conceptions of the larger relations of each to 

 the other, which is given by the theory of the growth and retreat of 

 deltas. 



When such a conception is attained, the mind may look back at the 

 completed geosyncline and view it as made of beds laid down on the one 



