178 
THE AGE OF THE WYE. 
present occupied by the stream, is a mere accident of the time : 
it has, over and over again, in the course of time,, occupied all 
parts of the alluvium. 
This alluvial land is shown on the map by a pale-green 
colour, and the present channel of the river by a blue tint. It 
may be observed that the banks of the stream are, in some 
places, marked by a strong and dark line, and in others by a 
fainter line : the dark line shows the parts where the stream is 
now fretting or wearing the bank away ; the fainter line shows 
the part where the bank is either stationary or growing — for the 
river always preserves its average width. 
The alluvial land, it may be observed, is all along bounded 
by banks or cliffs of the old red sandstone, which is the rock of 
this part of the country. These cliffs vary considerably in height 
in different situations ; and, as this variation is a matter of some 
interest, the heights are accordingly shown on the map, approxi- 
matively by the breadth of the shading. Some figured heights 
are also added here and there. I he alluvial land is here from 
400 to 500 yards in width from cliff to cliff. 
In order to follow clearly what is said below, it will be well 
to bear in mind that when the term “ cliff” is used, in connec- 
tion with the river, it is intended to refer to the cliffs, or marginal 
banks, bounding the alluvial land all along the course of the 
river ; and that when tlie term “ general course ” of the river is 
used, the general direction of the river and the alluvium together, 
between these cliffs, is to be understood. The word river- 
channel” or “ stream” is used to denote the course of the river 
as contained between its alluvial banks. 
The water level of the river, in its usual or low water state, 
is about ten feet below the alluvial level, but the water rises 
rapidly up to, or above, the alluvial level after heavy rain 
amongst the mountains near the source of the river. 
The speed of the current, when the river is “ bankfull,” is 
