390 R. M. Deeley — Erosion hy Rivers and Glaciers. 



according as it is deflected by the curves of the river. At such 

 bends the stream is deep and narrow, but in the straight reaches 

 this rotary motion does not occur, and the river shallows and 

 broadens. Gravel and coarse sand are not thrown up, as a rule, 

 on the inside of a curve to a height much exceeding the ordinary 

 water-level of the river. On the top of the coarser material fine 

 sand and mud then collect, mainly at flood times, between the 

 grass reeds and rushes where they root in the shallows, and in 

 this manner the level is raised to flood-line. It thus comes about 

 that the river deposits a bed of alluvium, the lower gravelly portion 

 of which has a thickness equal to that of the river in ordinary flood, 

 whilst the upper surface of the sand and brick-earth on the top 

 indicate the extreme flood rise. But in course of time the sinuosity 

 of the curves becomes so pronounced that the bends approach each 

 other ; the river then cuts through the narrow peninsula and deserts 

 a portion of its course, which slowly silts up, and often leads to 

 the preservation in the clay and mud of mammalian and other 

 remains. 



The active nature of this looping tendency may be seen by 

 examining any good map of a district where two counties are 

 separated by a river, for in such cases we shall find patches of 

 a county stranded on the opposite side of the stream. As long as 

 the river moves bodily in a lateral direction it, of course, carries 

 the boundary-line with it, but, when a loop is cut off, an island 

 is first formed, and then by the silting up of the old channel 

 through which the boundary passes a piece of land which stood 

 on the right bank is transferred to the left bank, or vice versa. 

 A river by wandering about in this way from side to side may 

 excavate a broad valley and spread out a sheet of alluvium having 

 a width of several miles and a thickness of twenty feet or more. 

 When vertical as well as horizontal erosion is going on some 

 of this gravel is often left on the valley-sides, and forms wide 

 terraces, the highest at any point above the river being the oldest. 



It is important to note that the river, although it is spreading 

 sheets of gravel and sand over its valley floor, is also deepening 

 and widening it, i.e., we have erosion and deposition going on 

 hand in hand under the action of one and the same agent. The 

 gravel is everywhere of nearly the same thickness as it is traced 

 away from the stream, is covered by brick-earth except where it 

 has been denuded from the edges of the higher terraces, and 

 rests upon older rocks, which may be of any age, including 

 Pleistocene Boulder-clays, etc. 



As a valley becomes deep and approaches the sea-level the fall 

 becomes small and vertical erosion ceases. The river then attacks 

 the sides of its valley only, rapidly carries away the gravel terraces, 

 and we eventually have produced an approximately flat alluvial 

 plain bounded by long steep escarpments. It must be remembered 

 that terraces, which are removed in this final stage, by no means 

 necessarily indicate that the river has paused at any particular level, 

 for gravel must have been left at all heights below the highest 



