CONTAINING- FLINT IMPLEMENTS, AND ON THE LOESS. 
297 
the present day ; but it is proceeding on a grander and larger scale in the vast regions 
of Northern America, where the streams, flowing through extensive champaign countries, 
have furrowed the land with deep and precipitous channels, generally much below the 
level of the great plains they traverse* * * § . In Europe, however, the connexion of cause 
and effect is by no means so apparent. The regularity so common in the former case is 
generally wanting in the latter, where the weathering is more excessive. 
I doubt also whether, without a change in the general level of the land, the full 
effects of the changes we are contemplating could have been produced. The excavating 
power of the rivers would, in a measure, depend upon the adjustment to be made 
between the inclination of the valley along which they flowed and the sea-level. 
The coasts of these opposite shores of England and France are fringed at places by a 
raised beach. Of this we have evidence at Sangatte near Calais f, Brighton J, the Sussex 
coast §, and possibly at Havre ||. In all these places it is about 5 to 10 feet above the 
level of high water. With this beach I would correlate the estuarine bed connected 
with the low-level valley-gravels at Menchecourt. But besides this zone we have in each 
district the higher-level gravels fringing the river-estuaries, and sometimes the coast, at 
an elevation of from 40 to 100 feet above the raised beaches. These mark the relative 
difference of water-level at our two valley-gravel periods. It is immaterial to our inquiry 
whether that difference resulted solely from an elevation of the land or partly from the 
encroachment of the sea on the coast. Probably both have contributed to the result. 
A slow elevation of the land may have commenced at the high-level gravel period, 
leading to an increase in the velocity and erosive power of the rivers until a state of 
repose again obtained in the low-level gravel period. During such a change of level the 
causes which we have above alluded to, acting upon the portions of the substrata suc- 
cessively subjected to the action of the maintained water- and ice-power, gradually 
effected the excavation of those deep and broad channels forming the valleys through 
which the present comparatively insignificant rivers of these districts now find their way. 
The sharper angles produced on the river-banks by the erosion of the stream have been 
rounded off and in great measure obliterated by the action of the severe cold combined 
with the periodical floods, — operations by which the exposed rock-surfaces were alter- 
nately disintegrated and denuded ; while at the same time the flood-waters in retreating 
from the higher platforms, before falling in with the main current, further grooved and 
furrowed the sides of the valleys, and, breaking the continuity of the river-terraces, helped 
to give our valley-sides their peculiar and varied outlines. 
The following diagram will serve to illustrate my meaning : — 
* The geological structure in both instances often greatly facilitates the operation, the country being con- 
stantly thickly covered by loose sands and gravels, offering little resistance to the erosive power of the rivers. 
t The author in Quart. Joum. Geol. Soc. vol. vii. p. 274. 
t Mantell, ‘ Fossils of the South Downs,’ p. 277. 
§ Godwin-Atjsten in Quart. Joum. Geol. Soc. vol. xiii. p. 61. 
|| Pass?, Desc. Geol. de la Seine Inferieure, p. 84. 
