RH YSIO GRA PHY OF NICARAGUA CANAL RO UTE 235 



These are residual hills which, by reason of the harder rocks of 

 which they are composed or their position on the divide away 

 from the main drainage lines, were never reduced to the level 

 of the plain. Where the plain was best developed, that is, near 

 the sea margin on either side, these residual hills are infrequent 

 and inconspicuous. To the southward of the San Juan, in the 

 region lying between the Sarapiqui and the San Carlos, there is 

 also an extensive area in which the hills are almost wholly rem- 

 nants of the dissected plain, their summits in general presenting 

 but little variation in altitude. To the northward of the San 

 Juan the residual hills occur with increasing frequency and 

 greater altitude, and finally merge with the mountains of north- 

 ern Nicaragua. They also increase in number and height from 

 either side of the isthmus toward its center, being most abundant 

 along a line which crosses the San Juan valley in the vicinity 

 of Castillo. If the old plain were reconstructed by the filling 

 of the present valleys, it would not be continuous across the 

 isthmus, but its eastern and western portions would be separated 

 by an irregular line of these residual hills, the low gaps between 

 them being slightly above the level of the plain. 



The relations of these three classes of topographic forms will 

 perhaps be made somewhat clearer by a reference to the accom- 

 panying idealized sketch and section on page 236. The sur- 

 face of the peneplain is indicated by the even summits of the 

 hills to the right. Residual hills are represented to the left, 

 rising abruptly and distinctly above the surface of the peneplain. 

 The profile shows a transverse section of the San Juan valley 

 and a longitudinal section of the valley of a tributary stream. 

 The latter is represented as rising in the residual hills to the left 

 and flowing for some distance in the narrow gorge a b. From 

 b to c the stream flows in a broad shallow valley at about the 

 level of the peneplain. From c to d it is in a narrow gorge re- 

 cently cut and still being actively deepened within the pene- 

 plain. It emerges from this gorge at d and thence to the margin 

 of the main river valley at e it meanders through an alluvial 

 plain continuous with the San Juan floodplain ef. The bottom 

 of the valleys which the tributary and the trunk stream occupied 

 before the recent depression of the region is represented in the 

 profile by the solid line between the alluvium and the underly- 

 ing lock. When these valleys were formed they were consider- 

 ably above sea-level and the streams had a much more rapid 

 fall than at present, but they are now somewhat below sea-level. 



