National Geographic : 1899 Jul
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 flat alluvial plain about a mile in width between the abrupt margins of the older valley. This plain extends out nearly even with the headlands, which are connected by a long curving beach.