394 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



gin of the strata beyond contravention/ occurs where a certain facies — 

 for example, the Catskill facies of the Upper Devonian — advances pro- 

 gressively farther from the direction of supply, the higher beds gradually 

 displacing the Chemung facies from Xew York and central Pennsyl- 

 vania. But the Catskill facies began in fact in the Oneonta formation, 

 and in the upper part of this it can not be said that overlap away from 

 the source of supply was exhibited. The Oneonta is separated from the 

 overlying Catskill by a landward retreat of the non-marine beds, an 

 accompanying incursion of the marine beds, the basal Chemung. Sub- 

 sidence for a time gained on sedimentation, and the direction of overlap 

 is seen to be due to the relative dominance of opposing factors. These 

 must be more fully discussed under the topic of the delta cycle. 



Mud and sand may be worked by waves and currents to indefinite dis- 

 tances from shore, the only condition being that the bottom must be 

 sufficiently shallow to be afi'ected by waves. In the case of epicontinental 

 seas this is commonly true for much of or even the whole area. Mud 

 settles from suspension for some distance, also, beyond these limits, and 

 at the present time gives rise to the blue muds which mantle the slopes of 

 the ocean basins to distances of from 100 to 200 miles beyond the limits 

 of the wave-worked bottom. 



In the Arabian Sea such muds in fact are carried from ?00 to 800 

 miles from land, owing to the character of the ocean currents. Further, 

 it has been noted that in the direction of the prevailing winds desert 

 dust is carried from the Sahara and from Australia for hundreds of 

 miles from land and to such an extent as to visibly affect the air and the 

 color of the water. Applying these observations to the past, it is seen 

 that where uplift of an old land takes j^lace without a shallowing of the 

 sea, marine sands and clays will be spread over areas where previoiisly 

 there was a development of limestones. This does not carry any impli- 

 cation in regard to a movement of the shoreline and, provided the water 

 is fairly deep and the waste is fine, deltas may not be built seaward in 

 marked degree. If the sea was originally so deep that its bottom was 

 but little affected by waves, the shallowing of the sea by uplift or sedi- 

 mentation may cause a progressive advance of marine clastic sediments 

 away from the source of supply. This is to be noted, for example, in the 

 Upper Ordoviciaii, where the deposition of argillaceous sediment grad- 

 ually displaced that of limestone toward the west.' Grabau cites the 

 Pottsville of the Appalachians as a group of formations whose conti- 



« Op. cit., p. 636. 



■^ E. O. Ulrich : Revision of tlie Paleozoic systems. Bull. Geo!. See. America, vol. 22, 

 ;9X1, p. 296. 



