PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 243 



suffered another elevation. The streams were thereby stimu^ 

 lated and began to trench the surface of the baseleveled valleys 

 which they had previously formed. Shortly after this elevation 

 there was a renewal of the volcanic activity, which had been 

 quiescent for a long time. This second distinct period of activity 

 has continued down to the present time. It was manifested 

 along two nearly parallel lines of vents. One of these lines gave 

 rise to the Costa Rican volcanic range, and the other to the Nica- 

 raguan range. The vents forming the latter were submarine. 

 They occupied a line nearly parallel with the Pacific coast, ter- 

 minating to the southward near the southern end of the bay 

 which then indented the Pacific coast. Vast quantities of lava 

 and ashes were thrown out from these vents, and their position 

 was such that the ejected materials formed a dam, cutting off 

 the bay from the ocean. Since the precipitation was greater 

 than the evaporation, the waters collected behind this barrier, 

 and, as their surface was gradually raised, encroached upon the 

 basins of the streams which had been tributary to the bay. The 

 water doubtless continued to escape westward for a long time 

 after the volcanic activity began, but successive eruptions finally 

 raised the dam to such a height that the impounded waters 

 found a gap in the continental divide, which was lower than the 

 dam to the westward. They then escaped eastward to the At- 

 lantic. The continental divide, which had previously occupied 

 a position near the axis of the isthmus, was thus abruptly 

 shifted to its present position near the Pacific coast west of the 

 lakes. When the waters of the lake first overtopped the conti- 

 nental divide they were doubtless considerably higher than at 

 present. The material forming the divide, however, was resid- 

 ual clay and deeply weathered rock, and the outlet was quickly 

 lowered to the solid rock, where it has been held practically 

 unchanged to the present time. 



The latest episode in its geologic history has been the depres- 

 sion of this portion of the isthmus to the extent probably of one 

 hundred or two hundred feet. By this depression the lower por- 

 tions of the river valleys were drowned, forming long tidal estua- 

 ries. The streams flowing to the Pacific have in most cases entirely 

 filled these estuaries with sediment. Thus the Rio Grande val- 

 ley, which is followed by the western portion of. the canal route, 

 is a fiat alluvial plain about a mile in width between the abrupt 

 ma rgina of the older valley. This plain extends out nearly even 

 with the headlands, which are connected by a long curving beach. 



