90 



J. W. SPENCJ2R — GEOGKAPIIICAL EVOLUTION OF CUBA. 



Rivers deserting their old courses and cutting out new channels is 

 another imitative feature, which will be noticed under the head of harbors. 



Rifts across ridges, made by waves opening narrow valle3^s, have already 

 been referred to on page 87. 



Rock-basins like others of the north, and produced by modern fault- 

 ing, also occur. They are illustrated on page 92. 



Harbors. 



A few harbors such as that of Matanzas are simply the heads of deep 

 baj^s which extend out into fiords. Others are the products of corals 

 building reefs leaving shallow lagoons. A few are closed by sandbars 

 forming hooks such as that of Casilda, the port of Trinidad. There is 

 another class of harbors, however, which are ver}^ common throughout 

 the West Indies ; the}'' are land valleys, often of considerable magnitude, 

 depressed below the water-level and closed by rocky barriers. ^Vliile 

 their breadths may be great, tlieir outlets are through narrow canyons 

 of recent formations. Without carrying the generalizations beyond those 

 harbors seen, the writer will offer one notable example for future study. 



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Figure 9. — Bay and Valley at Santiago. 



The narrow and deep canyon which forms the egress of the broad bay and valley is 

 700 leet wide and 350 feet deep to waterline. (After Hydrographic Office chart.) 



Xagua bay, or the harbor of Cienfuegos, is the united continuation of 

 several smaller valleys depressed below the sealevel. The bay is ^bout 

 12 miles long and from three to four miles wide, while the canyon form- 

 ing the outlet is only 1,200 feet wide at the narrowest point. The bay 

 increases in depth from a few feet to about 120 feet near its egress, but 

 the canyon becomes a fiord 168 feet deep (see figure 11). The south- 

 western boundary of the valley and also the plateau separating the bay 

 from the sea are composed of Matanzas limestones which rise to 100 or 

 150 feet above tide. Upon the northeastern side of the bay the land 

 rises gradually and is underlaid by Miocene clastic deposits. This val- 

 ley (now the bay) was excavated by atmospheric erosion after the Matan- 

 zas epoch or during the high Pleistocene elevation already described. 



