Newland lakes.—On land recently elevated from the sea, basins of lakes may be merely the inherited slight irregularities of the earlier sea floor, in which case they may be assumed to be largely the result of an irregular distribution of deposits derived from the land. Lakes of this type are especially well exhibited in Florida, and are known as newland lakes ([Fig. 430]). Such lakes are exceptionally shallow, and are apt to have irregular outlines and extremely low banks. Under these circumstances, they are soon filled with a rank growth of vegetation, so that it is sometimes difficult to properly distinguish lake and marsh.
Fig. 431.—View of the Warner Lakes, Oregon (after Russell).
Fig. 432.—Schematic diagrams to illustrate the characteristics of basin-range lakes.
Basin-range lakes.—Newland lakes may be said to have their origin in an uplift of the land and sea floor near their common margin. A lake type dependent upon movements of the earth’s crust but within interior areas has been described as the basin-range type and is exemplified by the Warner Lakes of Oregon. In this district great rectangular blocks of the earth’s crust, which in their upper portions at least are composed of basaltic lavas, have undergone 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 betray a tendency to appear in series ([Fig. 432]), and are unfortunately much too often illustrated on a small scale after a shower by the tilted blocks of imperfectly made cement sidewalks.
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.
Rift-valley lakes.—Another type of lake basin which has its origin in faulted block movements is known as the rift-valley lake, 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 (trench). Such a basin when occupied by water yields a lake which is long, straight, deep, and narrow, and is in addition bounded on the sides by steep rock cliffs. At the ends the shores are generally by contrast decidedly low. If the hard rock at the bottom of the lake could be examined, it would be found to be of the same type as that exposed near the top of the side cliffs. The valley of the Jordan in Palestine is a rift of this character and was at one time occupied by a long and narrow lake of which the Dead Sea and the Sea of Galilee are the existing remnants ([Fig. 433]).
