LAND VALLEYS. 117 



America to 600 feet or less is much narrower than on the Atlantic side 

 of the Antilles, or is wanting (the basin of l^anania gulf, whicli is like a 

 submerged terrace, is an exception) ; but deep water often approaches 

 near the land, and in places fjords may be seen. A quite noticeable 

 feature occurs south of the isthmus of Tehuantepec and a similar one 

 south of the gulf of Panama, where broad, deep basins of the Pacific 

 ocean extend landward as if they Avere once continuations of tlie Mexi- 

 can gulf and Caribbean sea. 



This arra}^ of data in the geomorphy of the vast and only partly sur- 

 veyed Antillean region, although scanty in pro})ortion to the area, sug- 

 gests physical problems which should no longer be overlooked. Inter- 

 pretations may differ, but the facts are of undoubted significance and 

 seem to the author strongly to indicate vertical oscillations of great am- 

 plitude during the course of development of this and neighboring dis- 

 tricts. 



Characteristics of the lower Reaches of the Land Valleys. 



As the correlation of the vallej^s and the submerged canyons will fol- 

 low, the general characteristics of the depressions of the country traversed 

 by the lower reaches of the rivers may be noted, especiall}^ as the descrip- 

 tions of their forms are not readily available. After leaving the older 

 formations and entering the less coherent Cretaceous and Tertiary strata, 

 the rivers pass over the gently sloping coastal plains, which may have a 

 width of 200 miles or more and a descent of 400 or 600 feet before 

 reaching the existing coast. But where the plains are not over 250 

 feet above tide, the rivers occupy broad troughs. Even at 200 miles 

 from the sea the valleys may be from two to four miles wide, and where 

 only partly filled by the deposits of later date the flats are characterized 

 by flood-plains and swampy areas. Farther down their courses the 

 valleys widen and are delimited by bluffs rising perhaps 50 or 100 feet 

 above the streams which touch them at only occasional points. The 

 drainage of the swampv reaches is often retarded by the necessity of the 

 streams crossing durable rocks which have l^ecome exposed, owing to the 

 gentle deformation of the surface, during terrestrial undulations, or by a 

 change in the course of the stream. Nearer the sea the form of the valley 

 becomes obscure, owing to the filling with sand or alluvium during the 

 more recent epochs and to modern sedimentation. Here the shallow 

 valleys are apt to be s\vam[)y, Init limited by hills rising from 25 to 75 

 feet, more or less modified, owing to interruptions on account of the 

 entrances of great lateral branches. As all the features are low, but on 

 a broad scale, with the depressions from i\vQ to ten miles wide, on reach- 

 ing the coast the true characteristics of the valleys arc best api)reciated 



