OF THE THAMES VALLEY. 69
courses of the valley above them. Below Grays the Thames
soon cut down to the lowered base-level of erosion. But the
Chalk brought up by the small anticline in the Grays district
formed such a resistant barrier for a time as to produce a local
base-level of erosion, the effects of which are seen in the drifts
occurring above the anticline. Thus at Crayford, deposits,
usually classed with the Middle Terrace, are found at elevations
of as much as 80 feet O.D., while at Grays similar portions
creep up to about 70 feet O.D.8 These deposits, in our opinion,
are not to be regarded as belonging to the Middle Terrace, but
rather as representing the interval between High Terrace and
Middle Terrace times. To this local base-level of erosion the
production of the apparently complete passage from High
Terrace to Middle Terrace deposits developed in the Upminster
district is due. In other localities, e.g., the Ilford and Wanstead
district, these passage beds have in large measure disappeared,
their obliteration being attributable to the lateral erosion of the
Thames during Middle Terrace times, and to other forms of
subsequent atmospheric denudation.
Having at length brought its bed down to the effective base-
level of erosion, the river dropped its tools—the stones forming
the bed of coarse gravel which is so constantly found at the base
of the Middle Terrace deposits. The admirable sections in these
deposits at Grays, Ilford, Crayford, and other places afford us a
very complete record of the history of the river during this stage.
We meet everywhere, in the gradual transition from coarse to
finer material, evidence of a current gradually becoming feebler
as a whole, yet withal liable to flooding. Thus, in the Orsett
Road section described in Part I., we have an illustration of
these floods. The beds there exposed showed firstly, in order of
deposition, the thick clays deposited intermittently in some reach
by flood water, to which method of deposition their laminated
character is probably due. Next there is evidence of a
small secondary channel being eroded in them and being kept
open, as shown by the presence of shells of Anodon, formerly its
inhabitants. Then a flood brought down the sand and fine
gravel, filling up this little channel, and which brought with it
the carcases of hundreds of small vertebrates, whose bones occur
so abundantly in this part of the section. Then in succession we
8 "B" in figure 2.