File:The Quarterly journal of the Geological Society of London (12961444804).jpg

TTLOE — QTJATEBN-AEY aElVELS.
61
it to lower levels into the valleys. At page 124, Quart, Jonrn.
Geol. Soc. vol. xxiv., I described snch a flood, in Sind in the year
1866, when a rainfall of 24 inches occurred in ten honrs, laying all
the surface of the land under water, and moving very great weights
a considerable distance. The ancient river- deposits of the Thames
basin associated with this flood-borne detritus are generally covered
by an upper bed of well-rolled gravel, not exceeding 10 feet in thick-
ness. This bed is often only 3 or 4feet thick, and lies on the frequently
eroded surface of the stratified brick-earths and clays of the middle
part of the Thames Quaternary series. Even when for a short
distance this surface-bed contains few pebbles, there is no difficulty
in identifying it ; for it will soon be charged again with pebbles.
This thin upper coating of well-rolled pebbles, enc"'.osed often in a
stiff brown clay, but sometimes in sand or loess, extends from the
river-banks to near the tops of the hills over all the other parts
of the series of gravels, and often over the London clay and chalk ;
it covers a most extensive area, forming a marked contrast to
the middle series of laminated brick-earth, false-bedded sands, and
stratified clays, which are more confined to the low levels near
the rivers or brooks. This bed of upper clay and pebbles slopes
to the lower ground (I estimate on an average) at I in 260, and
falls therefore about 20 feet in a mile. It is washed over the
surface of the land smoothly, removing all abrupt heaps of gravel,
and leaving no greater irregularities of the surface in the London
basin than occasional low terraces. The smoothness and perfect
manner in wliich the gravel is deposited is shown by the fact that
the surface of the London clay or chalk is covered, notwithstanding
inequalities of the surface, by a bed only 25 feet thick over such a
laro-e district.
Fig. 1. — Map of part of the course of the river Aire, Yorlcshire.
RIVER AIRE YORKSHIRE
Map of the course of the E-iver Aire, from Bindley to Castleford. The
letters refer to the hnes crossing the river, and are for refei*ence to the
longitudinal section (fig. 2) of the River Aire, or rather of a line sup-
posed to be drawn at each point of its course 8 feet above the full-
water-mark of tli3 river. This map has been accurately reduced by
the pentagraph from the one-inch Ordnance sheets.