THE GEOLOGICAL DATES OF ORIGIN OF CERTAIN TOPO- 

 GRAPHIC FORMS ON THE ATLANTIC SLOPE 

 OF THE UNITED STATES. 



BY WILLIAM MORRIS DAVIS. 



(Read by title before the Society Decetnber 31, 1890.) 



Introductory Note. 



It has lately been suggested to me* that the geological age of topographic 

 forms might be indicated on maps by means of a geological coloring, similar 

 to that employed commonly to denote the age of the rocks themselves. The 

 suggestion is closely pertinent to certain work that I have had on hand for 

 two years past, and in order to experiment upon it, as well as to place it 

 before the geologists and geographers of the country for discussion, I desire 

 to submit the following essay to the consideration of the Society. 



The Classification of Topographic Forms according to Age or 

 Degree of Development. 



Basis of Classification. — In the attempt to classify geographical forms on a 

 natural basis, I have come to consider thera as follows: Every geographic 

 unit, or area of essentially single structure, may be conceived as passing 

 through a complete cycle of topographic forms in its life history, from the 

 time when it is first presented to the destructive forces of the atmosphere, of 

 rivers and of the seashore, to the remote end when it is worn down to a flat 

 surface at baselevel. At the beginning its form is determined by processes 

 of accumulation, elevation and dislocation. Its topography is then con- 

 structive, and may be recognized as such by the absence of elaborated 

 drainage forms, and by the unworked outline of its coast. Later on, when 

 the drainage has had time to develop, the surface is of vastly greater variety 

 of form, every part of it having been wrought by the processes of waste 

 under the guidance of the main streams, while its coast is cut and filled into 

 outlines on which the waves of the sea may swing in long, smooth curves. 

 At this time it has the greatest variety and intensity of relief. Finally, 

 when the waste of the valley sides has consumed the hills and reduced the 

 Avhole to a nearly featureless surface of denudation, a peneplain, the early 



*Cf. this volume, p. 542. 



(545) 



