29G C. VV. HAYES — GEOLOGY OF NICARAGUA CANAL KOUTE 



the surface of the old plain at the time of its most perfect development. 

 The remnants of the dissected })lain increase in number and in the reg- 

 ularit}^ of their summits until, in the vicinity of Ochoa, their uniformity 

 is such tluit the position of the old pene{)lain can be accurately deter- 

 mined. The dense tropical forests mask tlie minor topographic features, 

 so that the uniformity in the summits of the hills is not at once apparent. 

 The detailed contour maps, however, of those portions of the region which 

 have been actually surveyed exhibit the uniformity in a striking man- 

 ner. The present elevation of the hilltops in this region is about 150 

 feet above sealevel. To the south of the river the old plain was very ex- 

 tensively developed, and while it has suffered much subsequent dissec- 

 tion, there is a large area in which its former position can be readily 

 determined by the summits of the present hills. To the north of the 

 river it was less extensive, forming only broad valleys between the resid- 

 ual hills which occupied the divides. Although not so extensively de- 

 veloped here as south of the river, the plain has been somewhat better 

 preserved, and many streams are found which have not yet lowered 

 their valle3's appreciably below the old surface. Heading on the steep 

 residual hills, their upper courses are in sharply cut V-shaped val- 

 leys. Emerging from these, they flow in shallow valleys across the rem- 

 nant of the old plain, their channels meandering and obstructed by 

 swamps. Farther down they enter narrow gorges which they have cut 

 and are still deepening in the old peneplain. Still farther down they 

 are bordered by alluvial plains, where the valle3's which they cut in the 

 old plain have been depressed below baselevel and so silted up. 



Continuing westward from Ochoa, the summits of the hills become less 

 uniform in altitude, corresponding with the originally less perfect develop- 

 ment of the old peneplain in the vicinity of the former Continental divide. 

 Along the upper portion of the river, west of the Toro rapids, are numer- 

 ous low, rounded hills merging on either side of the valley with a more 

 continuous upland, and these probably mark the position of the former 

 peneplain. It slopes gently westward and probably passes beneath the 

 waters of lake Nicaragua. The broad valle3^s bordering the streams 

 which enter the northeastern side of the lake and the level plain which 

 forms the western margin of the lake basin probably constitute parts of 

 this old plain, which have here almost entirely escaped dissection. 



In connection with the remnants of this old peneplain, the topography 

 of the surface now concealed b}'' the alluvial deposits should be consid- 

 ered. At the close of the period of high level, during which the plain 

 was dissected, the valleys were rather narrow with steep slopes except 

 near the coast. If the subsidence which inaugurated the period of allu- 

 viation had occurred all at once, tidewater would have extended up the 



