SYMBOLS FOR A CHRONOLOGIC CARTOGRAPHY. 543 



Mr. Gilbert and myself have attempted to show '^ that certain terrace plains 

 on the Allegheny, Monongahela, upper Ohio, and tributary rivers are syn- 

 chronous with the earlier glacial epoch, and that certain lower terraces were 

 formed at the time of the later glacial incursion. To say nothing of other 

 determinations, here are four peneplains believed to represent four ages 

 stretching from the Cretaceous to the late Pleistocene. Between these pene- 

 plains are slopes contemporaneous with them in formation. It is more than 

 likely that other epochs than these are represented by the topography of 

 that region. Is it not as serviceable to geological history to map these plains 

 and slopes, and thus give tangible and vivid expression to the physiographic 

 history of the region, as it is to map chronologically the contemporaneous 

 sediments of the adjacent coastal region ? Such mapping will give sharp 

 expression to the determinations made and to the features that remain un. 

 determined ; will direct inquiry into the validity of what has been done ; and 

 will promote the developmont of physiographic study. 



I therefore propose that a system of physiographic mapping on the chrono- 

 logic basis be employed, and I submit the following system for discussion 

 and for trial. I am prompted to do this at this time in view of the approach 

 of the international congress of geologists, whose central themes of discus- 

 sion will be correlations and cartographies, and before whom this subject 

 may come appropriately if it has reached a sufficient state of maturity. 



The following genetic classes of plains or peneplains are involved: (1) 

 Plains formed by rivers approaching or at base-level ; (2) Plains formed by 

 shore-cutting and sublacustral or submarine deposition ; (3) Base plains 

 formed by subaerial degradation ; (4) Surfaces formed by glacial reduction 

 and deposition. Some minor classes will occasionally require representation, 

 as lava plains, delta fans, and orogeuic forms. There are also slopes con- 

 temporaneous with most of these that need appropriate means of represent- 

 ation. 



It is proposed that plains be represented by lines, and slopes by dotted 

 surfaces, both to be put on in colors varying according to the age of the for- 

 mation represented. It is proposed also that the direction of the action of 

 the agency, where practicable, be represented by arrow-heads. Thus, a base- 

 plain formed by a river will be mapped by covering the surface with arrows 

 (without feathers) pointing in the direction of the current, while lacustral' 

 plains will be mapped by parallel lines headed with arrow-points on the 

 margin, indicating thereby the shore action, the most characteristic agency 

 in its production. Where there is only a beach-line with a scarcely developed 

 bordering plain to be represented, a row of arrow-heads with their points set 

 against the beach-line on its seaward side may be used. In the case of sub- 

 aerial plains or peneplains, parallel lines will be used without arrow-heads. 



* Bulletin No. 58. U. 8. Geol. Survey, 1890. 



