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c Physiographic interpretation. Rock floor contour is an ex- 

 pression of the differences in character and structure of the bed 

 rock formations themselves, brought about by ordinary surface 

 weathering and transporting agencies, varied in their action and 

 effects only by certain differences in elevation above the sea. It 

 is apparent therefore that it would be possible by careful observa- 

 tion of surface features to gather data sufficiently definite to fur- 

 nish a basis for suggestions about hidden and hitherto unknown 

 or undiscovered structural and stratigraphic characters. But the 

 application of it to practical engineering problems is a complicated 

 and difficult matter. And this difficulty is nowise simplified by 

 the occurrence of a drift soil that tends to obscure many of the 

 more delicate features. For example, the later narrow stream 

 gorges marking the stage of extreme regional elevation are com- 

 pletely buried. Only an occasional stream like the Hudson has 

 maintained its course unchanged and has begun excavating the 

 channel again. But even in this case, as will be shown under a 

 separate head, the work of reexcavation is only just begun and 

 the amount yet to be done and the corresponding original depth 

 of the gorge are wholly unknown. 



Certain surface features, however, are readable and, considered 

 with due regard for all possible causal factors, give very useful 

 suggestions. From them one obtains clews as to (i) the attitude 

 or relations of the hard and soft beds and the weak zones, (2) 

 the dip and strike of strata, (3) the persistence of a formation, 

 (4) the occurrence of faults, (5) the direction of the chief dis- 

 turbances, (6) the resistance and durability of local rock types — 

 in short the structural characters of all kinds because differences 

 in the distribution of these characters have given the different topo- 

 graphic forms and geographic areas. They have made the features 

 of the Highlands look different from those of the Catskills, and 

 those of Wallkill valley different from the Croton. Because of 

 the long train of conditions with which these surface features are 

 each involved and the structures that they indicate they become 

 easily the chief factors in preliminary judgment of comparative 

 practicability of rival locations, and are the most reliable guide to 

 direction and character and extent of exploratory investigation for 

 many engineering enterprises. 



d Physiographic zones. In summarizing the physiographic 

 data it appears that the following belts or zones may be regarded 

 as fairly distinct units: 



