122 A SUPPOSED NEOLITHIC SETTLEMENT.
simple. The lowest formation visible anywhere is the London
Clay, which appears here and there in the river-valleys. Above
the London Clay, and chiefly in the river-valleys, may be seen
the sand and gravel of the Glacial Period. Above this sand and
gravel, and forming the surface of the plateau between the
valleys, is the Chalky Boulder Clay. Then, here and there on
the plateau, are patches of gravel and loam resting upon the
Chalky Boulder Clay, and usually occupying slight depressions
on its surface. These last-named deposits (which are post-
Glacial in the sense of being of later date than the Boulder Clay)
are extremely irregular in their occurrence and in the space they
occupy. They have sometimes been found to contain mammalian
remains of interest, as in the brickyard at Great Yeldham, of
which some account is given in the Essex Naturalist (vol. ix.,
pp. 115--118). All the beds hitherto mentioned are widely dis-
tributed, for though the lower are seldom visible, except in the
river-valleys, they are found over considerable areas of country
beneath the higher beds. In addition, there are deposits which
are wholly confined to the river-valleys, having been formed by
the streams at various periods during the erosion of their
valleys. These are all of later date than the others mentioned,
the most recent being the alluvium occupying the flat ground
close to the streams. No older river deposits, formed when the
river had not cut down its channel to the present level, are shown
on the Geological Survey Map as manifestly existing within three
or four miles of Braintree. For the older the terrace, the longer
the time since its formation during which it has been subject to
destructive influences, whether those resulting from the mean-
dering of the stream or from the ordinary action of the weather.
Then, while fragments of terraces cut in comparatively hard
rock often remain clear and distinct, those cut in soft material,
like London Clay or Glacial Gravel, soon cease to be traceable ;
and the difficulty of identifying them is at its highest in narrow
valleys like those around Braintree, where any river terrace can
never have been more than a few yards in breadth, and where,
consequently, its distinctively terrace-like aspect can never have
been very conspicuous. However, the obscurity that prevents
any beds of this kind from being mapped at Braintree by no
means implies that they do not exist there. In the Geological
Survey Memoir on Sheet 47† (which includes Braintree, Cog-
† The Geology of the N.W. part of Essex and the N.E. part of Herts., with parts of Cam-
bridgeshire and Suffolk. By W. Whitaker, W. H. Penning, W. H. Dalton, and F. J. Bennett.
London, 1878.