578 W. M. DAVIS — DATES OP TOPOGRAPHIC FORMS. 



as yet little published material that can be used. The reports of the Gul/ 

 states, Alabama and Louisiana, are perhaps more complete than others in 

 this respect ; but as field investigations are now progressing there it is not 

 advisable to attempt to study out the development of the region from maps 

 and written descriptions, in Avhose preparation this aspect of its history was 

 hardly considered. 



Summary of Cretaceous and Tertiary Topography. — It has been found pos- 

 sible thus far to ascribe all the topographic forms of the Atlantic slope to 

 one or the other of a few simple categories. The uplands and the even crest- 

 lines of the ridges are parts of a Cretaceous peneplain of denudation, and are 

 for the greatest part of subaerial denudation ; occasional hills or submoun- 

 tainous elevations rise above this peneplain, these increasing to true moun- 

 tain heights in New Hampshire and North Carolina. Taking these two 

 elements together, and filling up the lowlands and valleys that have been 

 sunk to a lower level, and depressing the peneplain surface close to sea-level, 

 we have a restoration of the geography of our Atlantic border near the end of 

 the Cretaceous period. The shore-line stood farther inland then than at pres- 

 ent, while at the opening of Cretaceous time it had stood further out to sea. 

 The change from one stage to the other was accomplished by a gentle de- 

 pression, by which the Atlantic waters crept over the seaward border of the 

 lowland. During the stand of the sea at its highest level, perhaps near the 

 close of Cretaceous time, the larger rivers near their mouths appear to have 

 reduced their valleys to broad lowlands, on which flood- plain or estuarine 

 deposits were spread out; and in this area of the streams we may find at 

 present a tendency of the side branches to- turn downward before entering 

 the master stream, a habit characteristic of flood-plained valleys in general. 

 Several examples of this peculiarity in the Susquehanna tributaries have 

 been mentioned in my essay on the Rivers and Valleys of Pennsylvania.'^ 



The second large category of our topographic forms includes the slopes, 

 valleys and open lowlands that have been sunk into the Cretaceous lowland 

 after its elevation into an upland in Tertiary, probably early Tertiary time. 

 It may be well to repeat that in assigning a geological time-name to date 

 this change of level, it is not at present expected to reach a close accuracy of 

 statement. Tertiary time was so long and witnessed changes of such magni- 

 tude in the west that our problem cannot be considered more than opened 

 until a more definite date than " early Tertiary " is assigned for the time of 

 the elevation of the Cretaceous peneplain. 



We have in the foregoing paragraphs traced in some detail the develop- 

 ment of the valleys and lowlands to a depth permitted by the altitude given 

 to the old peneplain, and to a width allowed by the weakness of the rocks 



Nat. Geogr. Mag., vol. 1, 1889, p.' 241. 



