572 W. M. DAVIS— DATES OF TOPOGRAPHIC FORMS. 



quehaniia Rivers. — In New Jersey and Pennsylvania the same lesson is re- 

 peated, but always with the innumerable variations on the main theme that 

 render the study of physical geography so delightful. The main streams are 

 the Delaware and the Susquehanna. They both rise in the plateau belt of 

 the Appalachians, and here their valleys are deep and narrow. The head- 

 waters of the West branch of the Susquehanna particularly are notable in 

 this respect. The North branch of the same river, above Wilkes-Barre, pos- 

 sesses a system of strong meanders, such as rivers often acquire when close 

 to baselevel, but not characteristic of rivers having a relatively rapid de- 

 scent ; it may be that these meanders are inherited from the previous cycle. 

 The case seems analogous to that of the Seine, in whose lower course the 

 meanders are extraor^dinarily well developed, although the plateau in which 

 they are sunk is but little dissected by the side streams.* 



Passing southeast of the plateau, the two Pennsylvanian rivers traverse 

 the Alleghany or folded division of the Appalachians, this being narrow in 

 the Delaware basin but broadly developed about the Susquehanna. The 

 weaker Siluro-Devonian beds are generally reduced to a lowland farming 

 country, except where the Oriskany sandstone or a Chemung conglomerate 

 of more resistance than the adjacent beds rises in ridges of moderate height. 

 The hard Medina and the Carboniferous sandstones hold their crests close 

 to the Cretaceous peneplain. During the passage of the rivers across this 

 division of the country they cross few of the higher ridges, but on emerging 

 from the great valley they perforce must cut their w^ay through the Kitta- 

 tinny. North or Blue mountain, which runs continuously along its northern 

 side. Here, therefore, the problem of a broad lowdand back country drained 

 through a narrow gap in a high ridge again confronts us ; and although the 

 supposition of lakes and fractures is often met with, it does not seem to have 

 any support. It is true that at the Delaware water gap there is a fracture 

 in the formation through which the gap is cut, but this fracture is vastly 

 older than the gap, vastly older than the present lowland form of the lowlands ; 

 it probably served to locate the river during the Cretaceous or some earlier 

 cycle ; but the gap as we see it is practically the work of the stream plus the 

 wasting of the slopes, since the Tertiary cycle was ushered in by the uplift 

 of the Cretaceous lowland. As the stream cut down, the back country was 

 reduced in level ; the rock-sill of the gap was the local baselevel of the upper 

 basin. 



Although this has been a generally accepted geological principle, since it 

 was so clearly enunciated by Jukes in 1861, it appears necessary to state ex- 

 pli^tly the process that it involves in order to counteract the implication 

 that one sometimes meets in the accounts of our river histories ; for example, 

 in the Physical Geography of New Jersey, as presented in the first volume 



* See the Etnt major map of France, 1 : 80,000; sheets 19, 20, 30, 31. 



