A STUDY OF LAKE BASINS 



403 



district great rectangular blocks of the earth's crust, which in their 

 upper portions at least are composed of basaltic lavas, have under- 

 gone vertical adjustments in level and have been tilted so that 

 the corresponding corners of neighboring blocks have been given 

 a similar degree of down- 

 tilt (Fig. 431). Lakes 

 formed in this way are 

 of triangular outline, are 

 bounded on the two 

 shorter sides by cliffs, 

 but have extremely flat 

 shores on their longest 

 side. From this shore the 

 water increases gradually 

 in depth and attains a 

 maximum depth at or 

 near the opposite angle. 

 Such lakes naturally be- 

 tray a tendency to appear 

 in series (Fig. 432), and are unfortunately much too often illus- 

 trated on a small scale after a shower by the tilted blocks of 

 imperfectly made cement sidewalks. 



Rift-valley lakes. Another type of lake basin which has its 

 origin in faulted block movements is known as the rift-valley lake, 



FIG. 432. Schematic diagrams to illustrate the 

 characteristics of basin-range lakes. 



FIG. 433. Schematic diagrams of rift-valley lakes, and the rift valley of the Jordan 

 with the Dead Sea and the Sea of Galilee as remnants of a larger lake in which 

 their basins were included. 



and is best exemplified by the great lakes of east Central Africa. 

 In this type a strip of crust, many times as long as it is wide, has 

 been relatively sunk between the blocks on either side so as to 

 produce a deep rift, or what in Germany is known as a Graben 



