PENEPLANATION OF THE NICARAGUAN DEPRESSION 295 



its topograph}^ has been entirely changed and its older rock formations 

 concealed by the recent eruptions of its volcanoes. From this combina- 

 tion of circumstances it followed that the surface was most completely 

 degraded and the divide most rapidly lowered along a belt extending 

 diagonally across the isthmus and now forming the great Nicaraguan 

 depression. A broad river basin was developed on the east side of the 

 divide, occupying the present position of the lower San Juan basin. 

 The land between its various southern tributaries was reduced to low 

 relief. Its northern tributaries were separated by somewhat higher 

 hills, probably the result chiefly of the greater original elevation of this 

 portion of the region. Another river system developed a similar basin 

 with its outlet to the west. The several upper tributaries of each of 

 these two river systems headed on the Continental divide in low gaps 

 against the tributaries of the other system. The basin of the western 

 system was somewhat larger than the one on the east of the divide. 

 Its lower portion was separated from the Pacific by a range of hills 

 which continued northwestward, forming the cape between the then 

 existing bay and the ocean. The southern portion of the present basin 

 of lake Nicaragua was occupied by this river system, and extensive 

 plains were developed on either side of the axis extending up the tribu- 

 taries as broad valleys well back into the surrounding hills. 



The foregoing brief account of the original extent of this peneplain 

 and the manner in which it was formed is an essential preliminary to 

 an understanding of the present topography. At the conclusion of the 

 long period of degradation, during which the surface of the region now 

 occupied by the Nicaraguan depression was reduced to a low relief, the 

 land was slowly elevated until it stood some hundred feet higher than 

 before and perhaps 200 feet higher than now. The elevation stimulated 

 the streams to renewed activity, and they began trenching the valleys 

 which they had previously formed. The erosion was at first most active 

 near the coast, and worked backward toward the interior most rapidly 

 along the largest streams. The portions of the peneplain most com- 

 pletely dissected were therefore its outer margins. Here the surface was 

 almost entirely reduced to the lower baselevel, and only a few rounded 

 hills on the divides retained any trace of the former plain. The first of 

 these remnants seen on ascending the San Juan are in the vicinity of 

 the delta head where low hills approach the river on the north side. 

 This region, however, has been so deeply dissected that the hilltops 

 scarcely suggest the existence of a former plain. Other hills of similar 

 character occur along the river, chiefly on the north side, although the 

 most prominent hills which come down to the river do not belong to the 

 group now being described, but to the residual hills which rose above 



