290 C. VV. HAYES GEOLOGY OF NICARAGUA CANAL ROUTE 



were at one time islands fringing the shore before the alluvial deposits 

 extended out to tliem and connected them with the mainland. 



The inner margin of the deltaplain is extremely irregular. The isolated 

 hills increase in number and size and finally merge with the dissected 

 interior highland, while the delta})lain itself merges with the broad flood- 

 plains of the streams. 



The surface of the deltaplain in its seaward portion is but a few feet 

 above tidelevel. Its extreme outer margin is marked by low ridges . 

 parallel with the shore, formed by the sand thrown up during excep- 

 tional storms. From the shore margin the surface of the plain ascends 

 toward the interior at a fairly uniform rate of about 28 inches to the mile. 



The surface of the deltaplain is also diversified by numerous small 

 lakes and lagoons. These are produced chiefly by the formation of 

 sandspits and by unequal sedimentation. 



Sediment is delivered by the larger streams slightly faster than it can 

 be distributed by the littoral current; hence it tends to build out a 

 delta; but this is deflected in the direction of the current and forms a 

 curved sandspit, which for a time makes a well sheltered harbor. As 

 the sandspit continues to grow, however, its point eventually joins the 

 mainland, and the harbor is converted into a closed lagoon. This com- 

 plete cycle of changes has taken place at Greytown during the last cen- 

 tury and a half. The cycle has also been repeated at the same point 

 several times previous to the last, giving rise to the several distinct 

 lagoons which occur back of the one last formed. 



The second method by which lagoons are formed on the deltaplain is 

 by unequal sedimentation. As the coast was built outward by additions 

 to its outer margin it advanced past numerous islands fringing the shore 

 and which in some cases prevented the uniform deposition of sediment 

 by interrupting the littoral sand stream, and these areas, in which little 

 or no deposition took place, subsequentl}'' formed lakes. Perhaps the 

 best exami)le of a lake formed in this manner is lake Silico. This occu- 

 pies what was evidently at one time a bay sheltered by the Silico hills, 

 which then formed a group of islands. As the deltaplain was built out, 

 connecting these islands with the mainland, the sheltered bay was not 

 filled by sediment, but its opening was cut off and a lake thus formed. 



Another class of lakes or lagoons formed b}'' unequal sedimentation is 

 found about the margins of the deltaplain and the river floodplains. 

 The rivers which head upon the Costa Rican volcanoes carry a much 

 more abundant supi)ly of sediment than the smaller streams which flow 

 from a region composed of compact residual clays protected b}" a heav}^ 

 mantle of vegetation. Hence the floodplains of the larger streams, as 

 the San Juan, are built up more rapidly than those of their tributaries. 



