National Geographic : 1899 Jul
PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 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 h 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 rock. 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.