FLOODPLA.INS ON" THE PACIFIC SLOPE 293 



passes into an ordinary alluvial floodplain. Streams entering the lake 

 from the southwest in general flow in channels which were at one time 

 excavated to a very inconsiderable depth below the present surface of the 

 plain through which they flowed. This plain, it may be remarked in 

 passing, is not alluvial, but is a plain of degradation ; hence these streams 

 are bordered by very inconsiderable alluvial plains, and that only near 

 the lake. The streams entering the Pacific from this portion of the 

 isthmus are all short and consequently small, since the Continental divide 

 is near the west coast. They occupy valleys which have been cut to a 

 mucli greater depth than they have at present, and these old valleys have 

 been recently drowned and more or less perfectly filled with alluvial de- 

 posits. Wliere the filling is not quite complete an estuar}^ occupies the 

 old river valley and forms a harbor, as is the case at San Juan del Sur. 

 Where the filling is complete, as in the valley of the Rio Grande, the 

 headlands which mark the margins of the former deep valley are con- 

 nected by a curved beach, which does not indent the coast to any appre- 

 ciable extent. The depth of the alluvium in the Rio Grande valley varies 

 from about 40 feet at the head of the floodplain to something over 100 

 feet at the coast. The stream which has filled this valley carries at cer- 

 tain seasons an abundant supply of sediment, so that the seaward slope 

 of the floodplain is rather steep, a little over 10 feet to the mile. The con- 

 ditions in this region which determine the rate of erosion are much more 

 favorable to rapid degradation of the surface than in the region of much 

 greater rainfall to the east, where the rain is distributed evenly through- 

 out the year. The streams are alternately shrunken to mere rivulets and 

 swelled to torrents, and the resulting floodplain has somewhat the char- 

 acter of an alluvial cone. 



Dissected peneplain. — The group of topographic forms to be described 

 next in order after the alluvial plains consists of a more or less com- 

 pletely dissected plain or peneplain of degradation. In order to under- 

 stand the present topograph}^ it is necessary to consider the original 

 form of this plain and the manner in which it was developed. The 

 conditions which prevailed prior to its formation cannot be definitely 

 determined, but may be inferred in a general way. There was probably 

 a somewhat elevated plateau, growing broader and higher both to the 

 northward and the southward from a somewhat constricted region, now 

 occupied by the Nicaraguan depression. The Continental divide at that 

 time probably occupied a position near the central part of the isthmus, 

 crossing the present San Juan valley in the vicinity of the Castillo rapids, 

 and streams heading upon this divide flowed to the seas on either side. 

 Another important difference was in the form and position of the Pacific 

 coastline. These differences m the geography of the region, so far as 



XLIII— Bull. Geol. Soc. Am., Vol. 10, 1898 



