32 
trickled, a tiny rivulet, over the tide-washed fore-shore of the emerging 
chalk. In so soft a stratum erosion is rapid and on a rising land every 
ebh is on the whole a little longer than its corresponding flood, and has 
therefore a slight advantage in carrying out to sea the excavated material. 
The volume of water increased with the elevation of the ground, and the 
excavation of the channel would go on rapidly until the velocity of the 
stream became retarded by its diminished inclination. A brook if left to 
itself is incessantly leaving its old bed and cutting a new one, as slight 
observation is sufficient to show. In this way a bank of shingle and sand 
deposited at one time is often undercut, carried away, and re-laid 
elsewhere on a lower level, at another. The existing beds, therefore, may 
be merely the relics of larger ones formed by the river at different times 
and places along the valley. Nor is it necessary to assume that the volume 
of water when this took place was very much larger than now. It may 
have been so, but evidence of the fact is wanting. Large rivers are less 
effective than smaller ones in moving gravel. A certain velocity in the 
stream is required which cannot usually be maintained except when the 
volume of water is small. Moreover, how swift soever the current of a 
great river may be in the middle and at the surface, yet at the bottom and 
sides the movement is always sluggish owing to the friction against the 
bed and the banks. Its carrying power is thus reduced and few rivers 
of any large size are capable of cutting away and removing anything 
larger than fine mud. Even a small stream must be watched in flood in 
order to be convinced of its eroding power, for the amount actually carried 
down at any other time is quite insignificant. 
On this view a regular succession of gravels must have been deposited at 
varying heights along the valley. These may however have been since 
then almost entirely removed and consequently the now existing 
relics may be very different in age from one another. This difference is 
also indicated by the presence of the quartzose pebbles in the middle 
gravel and their absence from the upper. The natural inference is that 
during the interval this kind of stone, a stranger to the district, had been 
brought in from some foreign source. In reference to this point, the 
following extract may be of interest : — 
" These hills (Bromsgrove Lickey) are the source whence the pebbles 
met with in far distant places have been drifted. 'They are accu- 
mulated,' says Dr. Buckland, 'in immense quantities over the plains 
of Warwickshire and the ' Midland counties and are found also 
on the summits of some hills in the neighbourhood of Oxford 
