SUBSEQUENT MODIFICATION OF LAKE NICARAGUA 345 



At some points the steeply inclined sedimentary rocks contain certain 

 beds of sandstone which are much more resistant than the mass of the 

 formation, and these form parallel ledges which extend into the waters 

 of the lake in some cases a mile or more, the softer rocks between hav- 

 ing been removed by the wave action to a considerable depth. 



Some estimate may be made as to the extent of the wave-cut terrace 

 along the lake shore west of Ometepe from the hight of the cliff. The 

 Rivas plain has an average slope of about 8 feet to the mile, and it is 

 assumed that this plain extends to the eastward under the waters of the 

 lake. If it retain.s the same slope a cliff 24 feet in hight would repre- 

 sent a terrace at least 3 miles broad. It is probable that the wave cut 

 terrace varies between 2 and 4 miles along this portion of the shore. 



From Zapatera northward to Granada the wave action is more efficient 

 than on any other portion of the lake by reason of the greater sweep 

 which the prevailing winds and waves possess. Since the shore is here 

 composed of only partially consolidated volcanic ash, the modification 

 of its outline, due to wave action, has been very considerable. It is prob- 

 able that this action has severed Zapatera from the mainland, and that 

 the many islands surrounding it were originally portions of that volcanic 

 cone. They probably represent the more resistant lavas from which the 

 softer materials have been washed away. 



The modification of the northeast shore of the lake by wave action 

 has been extremely slight. This portion of the lake shore is without a 

 beach, and only rarely is there any considerable surf; hence only a few 

 points which project well out into the lake show any effect of wave action. 



The material eroded by the waves from the western shore has been 

 carried north by the action of the waves and deposited in the upper end 

 of the lake. A bar has been built across the point of the lake, enclosing 

 a broad, shallow lagoon behind it, and the outlet of lake Managua has 

 been pushed northward by the sand drift well toward the northern margin 

 of the valley. 



Alluviaiion. — The third way in which the outline of the lake has been 

 modified is by the building out of its shores by material brought down 

 by ti'ibutary streams. The effect of this is seen almost exclusively along 

 the south and east shores. Elsewhere the constant surf and consequent 

 littoral currents have been sufficient to distribute the sediment as rapidly 

 as brought down by tributary streams, so that not only have no addi- 

 tions been made to the lake shore, but the new material added has not 

 been sufficient to compensate for the wave erosion. When the waters first 

 occupied the depression behind the barrier to the northwest, the outline 

 of the lake must have been quite irregular, since it filled a river basin 

 some portions of which had rather strong relief. Much of its basin occu- 



