National Geographic : 1899 Jul
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.valleys 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 valleys 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 valleys 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 they now meander. Corresponding in some degree to the valleys incised within the old plain are eminences rising distinctly above its surface.