636 PRINCIPLES OF STRATIGRAPHY 



to the northwest by streams from an old Appalachian continent, 

 these deposits sometimes replacing marine sediments westward, 

 at others building out upon a dry land or, in one case at least, a 

 desert area. (Grabau-27). 



A remarkable feature of these deposits is the repeated succes- 

 sion upon light colored pebble deposits, with evidence of torrential 

 origin, of finer red sediments with characters suggesting flood plain 

 and eolian origin. Such are in the Ordovicic the light Bald Eagle 

 conglomerate followed by the red Juniata shales, in the Siluric the 

 Shawangunk conglomerate followed by the red Longwood shales, 

 in the Devonic the Hamilton and Oneonta sands followed by the 

 Catskill red beds, in the Mississippic the Pocono conglomerate and 

 sandstone followed by the red Mauch Chunk shale. This succes- 

 sion seems to indicate conditions which permitted easterly winds 

 to sweep across a more or less elevated land mass (Appalachia), 

 where they were deprived of much of their moisture, thus creating 

 semi-arid conditions on the west of this land mass. Moderate 

 aridity, with periodic torrential rainfalls and swollen streams 



Fig. 128. Hypothetical section of Appalachia in- Pal?eozoic time to show the 

 possible arrangement of the winds, and the corresponding de- 

 posits. 



forming alluvial fans of pebbtes, seems to have existed repeatedly, 

 but in each case was followed by drier conditions such as would 

 be produced by a renewed elevation of the land, and the consequent 

 deposition in the lee of the land mass of highly oxidized sands and 

 dust as river flood plain and eolian formations, which are now seen 

 in the red beds. The conditions favoring such deposition are illus- 

 trated in the preceding diagram (Fig. 128). 



Deposits of a similar character are found in the Old Red sand- 

 stone of western Europe, and the Siluric deposits of the north 

 of England and the south of Scotland also show much evidence of 

 deltaic origin. 



One of the most striking examples of a seashore delta of 

 Upper Devonic age seems to be represented in the black shale of 

 Ohio, Alichigan, and western New York. To be sure this has also 

 been interpreted as a deep sea deposit, but its peculiarities all point 



