238 PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 



The manner in which this gap was developed is worthy of con- 

 sideration. 



Bordering the lake along its southwestern side is a very per- 

 fectly baseleveled plain from five to eight miles in width, which I 

 have called the Rivas plain (see map, page 242). This is proba- 

 bly a portion of the same peneplain which forms the funda- 

 mental topographic feature of the Nicaraguan depression, and 

 was at one time doubtless continuous with it. From the lake 

 shore where the waves have cut a narrow terrace backed by a 

 low cliff, the plain ascends toward the southwest at the rate of 

 about 8 feet per mile. Its even surface is interrupted by occa- 

 sional low residual knobs, which increase toward its inner 

 margin, passing into the continuous ridges and high hills of the 

 main continental divide. The Tola hills which border the Rivas 

 plain on the southwest here extend to the Pacific, although 

 further toward the northwest a narrow coastal plain is developed 

 similar to the Rivas plain on the opposite side of the range. 



The Tola hills doubtless correspond to the residual hills which 

 rise above the peneplain of the Nicaraguan depression. They 

 have a serrate outline, the altitude of their summits varying be- 

 tween 800 and 1,800 feet. While this range of hills still formed 

 a long, narrow point of land between the bay of Nicaragua and 

 the Pacific, the effect of deformations and wave erosion was such 

 as to make the position of the divide unsymmetrical. As shown 

 on the map, it was for a time located very much nearer to the 

 Pacific than to the head of the bay. Hence the streams which 

 headed upon the divide and flowed in opposite directions were 

 of very unequal length. Those flowing east to the bay must have 

 been five or six times longer than those flowing west to the ocean- 

 Such conditions rendered the divide unstable and the familiar 

 process of shifting toward a position of stable equilibrium took 

 place. A stream occupying the position of the lower portion of 

 the Rio Grande, by reason of the advantage which it possessed 

 in having its fall concentrated within a short distance, cut back 

 into the divide and diverted to its own basin successive portions 

 of the opposing stream. At the beginning of the process an 

 eastward-flowing stream occupied the valleys of the present Tola, 

 upper Rio Grande, Guiscoyol. and lower Las Lajas. A small 

 tributary headed against the Pacific stream on the divide in the 

 vicinity of La Flor. This tributary was first reversed and then 

 the upper portion of the original stream, the present Tola, was 

 diverted toward the southwest. The same process was continued 



