FORMATION OF TERRACES. 57 



formerly was, and into which, a number of small streams formerly emptied, sevei'al 

 deltas are laid bare by the draining off of the water, and they are cut through by 

 the streams, which have worn deep chasms through the loose materials, and are 

 still wearing them backwards towards the Alps. 



18. We will now inquire, how, in like circumstances, lateral terraces may have 

 been formed. As the comminuted and sorted materials are projected into the 

 main valley, now an estuary, which, as it sinks, is putting on the characters of a 

 river, they will be swept towards the ocean by the current, a greater or less dis- 

 tance, according to the velocity of the stream. Thus will the delta terraces of the 

 tributary, become in part lateral terraces to the principal valley. 



19. There is another mode in which lateral terraces may be formed, as suggested 

 by Robert Chambers, in his paper on the Valleys of the Rhine and the Rhone. 

 In the successive basins that form the chain of lakes produced by the drainage 

 of a country, the detritus brought into the basins by their affluents, will more or 

 less be spread over their entire bottoms, although, as above suggested, banks may 

 be formed, also, along the shores. The materials there spread over the bottom, 

 may accumulate to a great depth, if the straits connecting the several expansions 

 of water are narrow, and the water not so deep as in the basins. . At length, 

 however, as the drainage goes on, the bed of the basins will be brought to the sur- 

 face, and the waters, narrowed into a river, will cut a passage through the detritus, 

 leaving probably on each shore a terrace of the same height. The current, how- 

 ever, might crowd so closely upon one side of the valley as to sweep away all the 

 detritus there, and leave a terrace on one side only. 



20. There is a third mode in which lateral terraces might be, and doubtless have 

 been formed. In the case last supposed, the river is represented as simply cutting a 

 chasm through its sandy, clayey, or gravelly bottom. But powerful freshets occur 

 not unfrequently on all rivers : and in their swollen condition, and with increased 

 velocity, they act powerfully upon their banks, especially if of alluvial materials. 

 And if the course of the stream be tortuous, as is always the case, one bank will 

 be acted upon more powerfully than the other. This action will produce a 

 meadow on one side of the stream, but little raised, it may be, above the river in 

 its ordinary state. Successive inundations will eat away the bank more and more, 

 and thus widen the alluvial flat. The stream will thus be spread out over a wide 

 surface during its floods, and of course its velocity will be lessened. This will 

 cause a deposition of suspended matter to take place, whereby the meadows will 

 increase in height. Meanwhile the stream will continue to wear its channel 

 deeper, the supposition being that the drainage is still going on. At length the 

 channel will become so deep, and the meadows so high, that even in freshets the 

 waters will not spread over the meadows. They have now become a permanent 

 terrace, bounded by the river on one side, and by a steep escarpment on the other, 

 that' leads to the higher terrace. 



As the river no longer rises over the meadows in time of floods, the process 

 already described is repeated, and a third terrace is the result; and so a fourth, a 

 fifth, &c, may be formed, if the river sink deep enough and time be given. 



21. A modification of the above process may in some cases be witnessed. The 



