202 GREYWETHERS AT GRAYS THURROCK, ESSEX.
hard Tertiary stones. Mr. Whitaker (Geol. London, etc., pp. 478-
80) has some remarks on "Greywethers and Pudding Stones,"
in which he discusses the distribution and probable origin of
both. He agrees with Professor Prestwich in thinking that
they have been largely derived from the Woolwich and Reading
Beds; though at the western end of the London Basin, where
they are most numerous, "the origin of the greywethers may be
traced to the Bagshot Sands." At Grays it seems most probable
that the greywethers have been derived from the Woolwich and
Reading Beds, which occupy a considerable area at the surface
between Stanford-le-Hope and Wennington, and were once
continuous with the beds of the same age on the Kentish shore
opposite. Between Orsett and Stifford, and thence towards
Aveley, they occupy much less of the surface than between
Orsett and Stanford-le-Hope. But this results mainly from the
fact that from Orsett westward their outcrop is much more
largely hidden by the Old Thames river deposits than from
Orsett eastward. And in these old river deposits, formed
when the Thames was flowing about 100 feet above its
present level, the Grays greywethers were seen.
It is curious to note in the section at Grays showing these
greywethers in place, the absence of any signs of disturbance
caused by the entrance of these weighty masses of stone into
the fine gravel and sand. During the gradual changes in the
position of the channel of the old Thames these blocks seem to
have been deposited in it most gently and gradually, having
travelled but a few yards from the place at which they were
originally formed. A stream bringing down to a certain part of
its course, in time of flood, a debris of tree trunks with other
organic remains mixed with gravel and sand would show a much
more disorderly channel section in later times, from the perishable
nature of much of the material, than would be afforded by
the gradual letting-down of some weighty but almost indes-
tructible greywethers in the same place.