RECENT DEPRESSION AND ALLUVIATION 339 



The recently deserted gap between the diverted upper Rio Grande and 

 the beheaded Guisco3^ol is a broad shallow valley, its highest point being 

 154 feet above sealevel. It is occupied during the wet season by a swamp, 

 from which the water appears to flow in both directions. That flowing 

 toward the lake occupies a shallow channel, evidently once the bed of a 

 larger stream, while that flowing west soon finds itself in a narrow, 

 sharply cut ravine with rapid descent to the rather deep channel of the 

 Rio Grande. 



The process of diversion above outlined was inaugurated at the begin- 

 ning of the high-level period now being considered, but it doubtless 

 continued during the succeeding period after the formation of the lake. 

 It is evident that the process is still going on, and that the Continental 

 divide is now moving eastward at a rate which may be regarded as ex- 

 tremely rapid compared with most drainage changes, and, with the de- 

 cided advantages possessed by the Rio Grande, it is somewhat surprising 

 that the latter stream has not already tapped the lake. 



RECENT DEPRESSION AND ALLUVIA TION 



The process of gorge-cutting which characterized the period of high 

 level just described was terminated by a depression of the region, amount- 

 ing to a little more than half the elevation which had inaugurated the 

 preceding period. The effect of the depression was to drown the lower 

 portions of the river valleys, converting them into tidal estuaries. At 

 first the depression affected only those portions of the river vallej^s which 

 were brought below sealevel, while in the upper portions the deepening 

 of the stream channels continued as actively as before. The waste from 

 the land, however, instead of being carried out to sea and distributed 

 by littoral currents, began at once to shoal and fill up the heads of the 

 estuaries. With the consequent lengthening of the streams their beds 

 were raised, and consequently the influence of the depression was ex- 

 tended up their valleys at a rate corresponding to the extension of their 

 lower courses. It is probable that the depression of the surface was com- 

 paratively slow, and the filling of the estuaries may have very nearly kept 

 pace with their formation. As soon, however, as the depression of the 

 land was at any point slower than the filling of the estuary the influence 

 of the depression would proceed upstream at a rate depending upon the 

 extension of the lower course of the river. 



The depression of the land appears to have been accompanied by a 

 moderate local warping of the surface. This warping may have affected 

 the entire isthmus, but the means of detecting it are not at hand, except 

 in the western portion. The Rivas plain has evidently suffered a gentle 

 tilt to the northeast, and it is more than probable that this tilting was 



