422 J. BARRELL RECOGNITION OP ANCIENT DELTA DEPOSITS 



distance from the margin of the original formation is shown by the 

 enormous thickness which the Catskill has here. Well defined mud 

 cracks and rainprints were found ranging through about 100 feet of beds 

 and lying about 1,000 to 1,200 feet above the base of the formation — 

 that is, in its lower part. The upper part shows a thick series of transi- 

 tion beds, largely sandstones with some red shales, grading into the 

 Pocono sandstone. In these red shales mud cracks were found ranging 

 through a thickness of 1,200 feet of beds and thereby showing character- 

 istic exposure of the clays to drying in the air. More doubtful evidences 

 of subaerial exposure were found in other jDarts of the section. The 

 Catskill formation may then be tentatively regarded as of truly fluviatile 

 and terrestrial origin. The climatic conditions were intermediate be- 

 tween the aridity of the Upper Mississippian period, which resulted in 

 salt and g}q3sum deposits, and the moister Pennsylvanian giving rise to 

 coal measures. Either extreme offers the conditions for a good record of 

 subaerial exposure ; in the first case by means of mud-cracked shales ; in 

 the second by means of vegetation j^reserved in situ. The intermediate 

 condition shown by the Catskill deposits prevents a good record of either 

 sort, but is marked by the variegation in colors — fully oxidized flood- 

 plain clays alternating with deoxidized sands. 



Such a relation of red shales and gray or green sandstones may then 

 be taken as presumptive evidence of subaerial river deposition. It should 

 not, however, be taken by itself as positive evidence, as the number of 

 cases studied on which the conclusion rests is still somewhat limited. 



Lateral and vertical variegations in clays. — The third case of varie- 

 gated beds consists of those in which a great variety of colors are found 

 and in which the colors are variable along the stratum. Such variations 

 and mottlings are especially developed in the late Mesozoic formations 

 of the Potomac group, discussed as an illustration of the delta cycle, and 

 are especially suggestive of the local variations of soil drainage which 

 are found on the terrestrial surface of certain deltas. "Wliere the ratio 

 of flowing water to transported sediment is large and the sediment is 

 carried chiefly in suspension the grade of the lower part of a river may 

 be very low, in many cases but a few inches per mile. The local inequali- 

 ties of the floodplain determine the presence of lakes, swamps, or dry 

 land. A forest cover in such a region supplies organic acids and favors 

 partial leaching and concentration of the iron and leads to development 

 of such high contrasts in coloration. The far greater uniformity which 

 prevails at the bottom of permanent water bodies does not favor this 

 effect. Other reasons were shown for regarding the Potomac group as 

 consisting of delta formations. Such variegated bods are tlien highly 



