DETAILS OF TOPOGRAPHY 289 



The various topographic features mentioned in the above primary 

 classification may now be taken up and described somewhat more fully. 

 Since the oldland occupies the largest area and has more complex forms 

 of relief, which are intimately connected with the recent history of the 

 region, greater attention \vill be devoted to this division, and particularly 

 to that portion of it which constitutes the Nicaraguan depression. 



When examined in detail the surface of the oldland is found to have 

 considerable diversity in its relief, and its topographic forms naturally 

 fall into th^ee classes. These are (1) fairly well developed peneplains, 

 which rise gradually from, either coast toward the axis of. the isthmus 

 until recent geologic time occupied by the Continental divide; (2) many 

 valleys which intersect the surface of the peneplain, having been cut 

 during a period of high level and subsequently depressed below sealevel, 

 and (3) residual hills which rise distinctly above the peneplain surface 

 and are most numerous toward the axis of the isthmus along the former 

 Continental divide. 



For convenience of description the valleys will be taken up first, and 

 since these have been silted up by recent alluvial deposits, the floodplains 

 and the coastal deltaplains will be described at the same time. 

 ■ Alluvial plains. — The coastal plain on the Atlantic side of the isthmus 

 increases from a mere fringe at the base of the mountains in Costa Rica 

 northward to a belt from 10 to 15 miles wide in the vicinity of Greytown. 

 It is formed wholly of materials brought down by the rivers heading in 

 the Costa Rican volcanoes — is in fact a series of coalescing deltas, of 

 which the largest is that formed by San Juan river. The sediment 

 brought down to the sea by streams north of the San Juan is very small 

 compared with that brought down by those to the south. The more 

 rapidly growing southern deltas would therefore be extended seaward 

 except for a strong northward littoral sand current set up by the oblique 

 direction at which the prevailing winds strike the shore. The true lit- 

 toral current in this portion of the Caribbean sea is to the southward, 

 l)ut its capacity for transporting sediment is more than neutralized by 

 the active northward sand drift within the zone of surf action. This 

 sand drift tends to distribute the sediment evenly along the coast and 

 preserve gently curving coastlines. Notwithstanding this tendency, the 

 San -Juan delta has been built out a short distance into the Caribbean, 

 forming a shallow embayment to the northward of Harbor Head. 



The level surface of the deltaplain is interrupted by numerous low 

 rounded hills composed of residual clay derived from the decay of rock 

 in situ, and differing decidedly in appeq,rance and composition from the 

 surrounding alluvium. These hills have the form and appearance of 

 islands rising above the level deltaplain, and it is quite probable that they 



