94 



THE NATURE BOOK 



"ON THE INNER PART OF THE BEND THE GROUND IS 

 LOW AND SLOPES GRADUALLY TO THE WATER'S 

 EDGE." 



the meandering portions of the river ? 

 Before deahng with this question it will 

 be well to acquaint ourselves with 

 certain features which can be observed 

 at a bend. On the concave or outer 

 part of a bend we note the banks are, 

 as a rule, lofty, cliff-like and steep. 

 whereas on the convex or inner part 

 the ground is low and slopes gradually 

 to the water's edge. The strand on the 

 inner curve consists of sand or gravel, 

 and it is usually triangular in form. 

 American geographers call it the " toe- 

 cap," from its resemblance to the end 

 of a pointed boot. If the sloping bank 

 is followed down and across the stream, 

 it is found to end in a pool of deep water 

 which lies immediately under the cliff 

 on the outer curve. We have no term in 

 general use in the English language 

 which applies to this feature, but I once 

 heard a Welshman describe it by a word 

 which translated into our tongue means 

 " turnpool," and in North Lancashire 

 it is occasionally called the " wheel." 



We may take it as a general 

 rule that the stream is deepest 

 under the lofty cliff and shal- 

 lows towards the low banks. 



If we make experiments to 

 test the flow at a bend, we 

 note striking differences as 

 compared with the results 

 obtained in the straight parts. 

 The line of maximum flow 

 does not follow the middle 

 line of the stream, but takes 

 a curve more sharply bent as 

 shown by the dotted line in 

 the diagram. If we watch the 

 foam bells carried down by 

 the current we see that they 

 cling to the turnpool side and 

 seem to avoid the toe-cap, and 

 reeds and grasses growing in 

 the brooks show, by the trend 

 of their blades, the direction 

 in which the current is flowing. 

 We can test the bottom 

 flow by means of prepared 

 sugar. On dropping a cube 

 into the turnpool we find the 

 movement is very sluggish as 

 compared with the surface. An 

 aureole of coloured liquid grad- 

 ually forms round the sugar 

 and then begins to drift with the current 

 across the bed towards the toe-cap. 

 (Indicated in the diagram by arrows.) 

 The toe-cap is the direct product of this 

 cross bottom current, and has been 



Toe-cap 



1----T 



TOE-CAP 



DIAGRAM SHOWING THE FLOW OF A STREAM. 



formed by the gradual rolling of material 

 towards the inner bend. Why should the 

 undercurrent at a curve be across stream? 



