234 PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 



summits reach a tolerably uniform elevation on north and south 

 lines, but increase in height from either side of the isthmus to- 

 ward its axis. In the vicinity of the San Juan these hills have 

 steep slopes and rounded summits. Some distance back from 

 the San Juan the valleys which separate them are narrower, and 

 there are considerable areas of level or undulating surface at an 

 altitude corresponding with the summits of the hills nearer the 

 center of the depression. It is evident that if the valleys were 

 filled even with the summits of these hills, there would be formed 

 a broad undulating plain, sloping gradually up from either side 

 toward the axis of the isthmus. It is entirely probable that such 

 a plain once existed, and that it has been converted into a series 

 of even-topped hills and ridges by the subsequent cutting of 

 stream channels below its surface. The manner in which this 

 plain was originally formed is manifestly by the long-continued 

 action of streams when the land, stood considerably lower than 

 now — that is, by the process of stream degradation or baselevel- 

 ing. It was therefore a gradational, not a constructional, plain. 

 If it were reconstructed by the filling of the stream valleys, its 

 present altitude would vary between 100 and 200 feet. 



As indicated above, numerous valle3*s now intersect the sur- 

 face of this old plain. Except in the case of the San Juan they 

 vary with the size of the stream which they carry. The reasons 

 for this exception will be pointed out later. The valleys are 

 broad in proportion to the extent to which the old plain has been 

 destroyed, and they grow narrower with increasing distance from 

 the axis of the depression. The smaller streams generally head in 

 narrow gorges. In some cases they have not completely dis- 

 sected the old plain, but flow upon its surface in shallow valleys 

 which, lower down, give way to narrow gorges, and these in turn 

 to the rather wide alluvial vallej'S near the trunk stream. The 

 greater part of the erosion which has dissected the surface of the 

 old plain was accomplished when the land stood somewhat 

 higher than at present. The valle} r s were then much deeper 

 and none had extensive floodplains, except perhaps the largest 

 streams near the sea. The recent change in the altitude to the 

 land has brought the valleys below sea-level, changing the rivers 

 from corrading to aggrading streams. They have since silted 

 up the estuaries which were thus formed, producing the wide 

 alluvial plains through which the}' now meander. 



Corresponding in some degree to the valleys incised within 

 the old plain are eminences rising distinctly above its surface. 



