Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/153

Rh rather to pluvial than to fluvial action. Of marine remains in the Thames-valley gravels there are no traces.

In the south and east of England the hills are rarely higher than 1000 feet, and the valleys are deep and wide, and are adapted for rivers 20 times as large (or more) as those occupied by them now.

The banks of the old rapid and large rivers are now separated from the banks of the present small streams by a great extent of remanié alluvium; and modern gravel has accumulated in the old river-channels, owing to the decreased velocity of the present streams.

This is well shown in the large flat plains of gravel through which the Lea river flows, where the river, even in floods, only reaches the bottom of the 30-feet bank at Clapton, while the brick-earths and gravels of the ancient Lea reach to 80 feet above the level of the present stream.

Some modern streams have still force enough to cut horse-shoe bends and new courses out of the flat plains which were probably in flood-times constantly overflowed by the old rivers running in the gravel-period; but the covering of recent alluvium in the river-channel prevents any observation of the character of the gravel itself.

Large and deep side-valleys open into the Thames valley, containing small streams or rivers; but I do not know of any instance of a stream in this district having risen high enough, or had force enough, to cut through any of the Quaternary deposits, so as to expose a good natural section.

The absence of such natural sections is of important geological interest; for it is evidence that no marked changes of level occurred during the gravel-period. If elevations of the land had occurred during the latter part of the gravel-period, as has been supposed, and rivers had cut down these beds 30 or 40 feet, then all the side-streams would have had to adjust their beds to the new circumstances of level, and we should have had natural sections in every side-valley, as well as in the Thames valley itself. There are no waterfalls in the district; and every stream reaches the main river by a very gradual course in obedience to a definite law.

The fact that no good natural sections of this kind exist in the Thames valley, or in the valleys of its tributaries, is a proof that neither the principal river nor its confluent streams had force enough to break through the upper gravel and to expose the brick-earth and sands below. It is also evident from the same facts that the change in rainfall, and consequent decrease in volume and speed of all rivers and streams in the district, occurred immediately after the close of the gravel-period.

We find natural sections of gravels along the water-courses of the streams and rivers of South Wales, where the rainfall is still nearly 40 inches, and the side-streams fall with great rapidity to the main rivers from the high land. The materials of these gravels are essentially different from those of the south and east of England; and although the two sets of gravels may be identical in age, there would be as much relative difference between the deposits in high and low