120 Primaeval Man in the Valley of the Lea.
to believe that they had been thrown up by floods on the
banks of a large river such as the Thames." A mile to the
west, at Highbury, other molluscan genera are represented.
A list of the Highbury shells is given by Mr. John Evans—
'Stone Implements,' p. 524. Drift wood, twigs, and even
carbonized leaves are commonly found with the shells; this
drift wood must not be confounded with the silicified wood
which also occurs. Both the drift wood and the leaves are
beautiful objects under the microscope, the cellular structure
being perfectly preserved.
I now come to the bed of gravel indicated at b (fig. 5), and
a (fig. 8). It is found at an average depth of 12 ft., and
descends to 20 or 30 ft. from the surface; this drift contains,
chiefly in its upper parts, lustrous subabraded Palaeolithic
implements of medium age. All these tools have been more
or less moved and relaid by the agency of water; none are
quite unabraded; bones, teeth, and tusks of the Mammoth
also occur, with other mammalian remains, drift wood, &c.
This deposit has been described by Prof. Prestwich in the
'Quarterly Journal of the Geological Society,' 1855, vol. xi.,
p. 107, where he also gives a list of Shacklewell fresh-water
shells. The material is remarkable for containing immense
blocks of sandstone, probably never moved by water alone;
that these stones fell from blocks of drifting ice seems
extremely probable. They must have been brought from the
north long prior to the deposition of the "trail," and pro-
bably long after the time when other immense blocks found
at 20 and 30 ft. at the bottom of the gravel were deposited.
The stones forming the Lea-gravel are principally derived
from the glacial beds at the North of Middlesex and South
Hertfordshire, which in their turn chiefly originated in the
breaking up of the chalk; the flints are generally subangular
smoothened blocks; quartz and quartzite blocks and pebbles
are frequent, hornblendic granite less so; silicified wood often
occurs with numerous fossils from the chalk. Generally in
the deepest pits the third and oldest class of implements is
found; the examples are rudely made, mostly massive, and
deeply ochreous in colour.