Dr. Trving—Surface- Changes in London Basin. 215 
of such a change of view, beyond such as is based upon a misunder- 
standing of the theory under dispute. 
There remains to be considered some rather puzzling cases of 
gravels at rather high altitudes, to which I have hesitated to assign a 
place among the true plateau-gravels. A reference to my 1890 paper’ 
will show that I have omitted the gravels of Hartford Bridge Flats 
from that category. I rejected them, because, notwithstanding their 
altitude and even their composition (the same as that of the plateau- 
gravels), I have never been able to see one, among a dozen or more 
gravel pit sections on that, plateau, which answered to the structural 
test of stratification. Many of them are simply pockets of gravel 
in the eroded hollows of the Upper Bagshot Sands, as above Cricket 
Hill, and in every case, where I have seen them exposed, they have 
given me the impression that, as reconstructed material of an older 
plateau-gravel, they have been degraded into mere talus, or have 
been lowered by “soil-cap’’ movement due to the removal of sand 
from beneath them by the action of “ high-level”? springs. In many 
cases erosion of this nature has in course of time been sufficiently 
concentrated locally for small landslips of inversion to occur; and 
this is probably the explanation of the curious contorted masses of 
loamy ferruginous sand, which are so often met with in the gravels, 
and very often occur near the present surface of the ground at 
rather high altitudes.2 I have not much doubt that some such 
explanation applies to the few exceptional cases of contortion and 
of the absence of stratification in gravels of the plateau, several of 
which are cited by Mr. Monckton in his paper. These things being 
considered, I cannot admit that I have ‘“‘made too much of the 
question of stratification and contortion” (p. 35). 
It seems to be too little realized often by writers on the superficial 
geology of this part of England, how great the time interval is, since 
the plateau-gravels were laid down; and unmindfulness of this 
seems to make them slow to recognise the great extent and variety 
of the surface-changes, which must have been brought about by 
the action of ordinary “meteoric” agencies. Many of these have 
been long since wholly or partly obliterated; and a very great deal 
of this kind of erasure of surface-phenomena is no doubt to be 
referred to the Glacial Period, when the action of certain agencies 
of change was intensified. Erosion of sands beneath the gravels by 
high-level springs, and consequent ‘“soil-cap” movement has been 
already referred to; and a little thought will suffice to show how, 
in the earlier stages of the sculpture of the present country, small 
ravines would be developed out of what were at first mere rain- 
water gullies. As surface-erosion continued, co-operating with the 
action of springs, numerous small landslips must have occurred, and 
this would scarcely have happened without the surface-drainage 
of some of the wider upland valleys being in places impeded, the 
1 Q.J.G.S. (doc. cit.) p. 560. : 
? Such contortions in the gravels themselves do not furnish the evidence of glacial 
action which contortions in the wnderlying laminated argillaceous Eocene beds do 
furnish. 
