252
THE ESSEX NATURALIST.
bearing gravel thus occurs between the lines c—d and e—f.
It is distinct from the related gravels to the north-west and
south-east respectively, not only in respect of pebble-
constituents, but also in its heavy accessory minerals which
include, inter alia, abundant kyanite, resembling that known
to occur in the Lower Greensand. These facts point clearly
to a difference of provenance, and inspection of the map suggests
that a river of major dimensions entered the region from the
south. It would appear that a similar river entered from
Fig. 1.
the west, bringing the small quartz pebbles, derived either from
the Reading Beds or from the Lower Greensand of the northern
outcrop.6 The southern and the western suites of detritus mingled
east of the line c—d, i.e., beyond the confluence of the rivers,
and passed on together along a north-easterly course towards
the Pliocene sea of East Anglia. Neither suite passes beyond
the line e—f, the evidence for whose position has already been
noted. In the Boulder Clay country of northern Essex evidence
6 Note.—It may be pointed out that chert is restricted to the southern or Wealden Green-
sand outcrop. Quartz pebbles occur in both northern and southern Greensand terranes, though
more conspicuous in the north (Wilts, Bucks, Beds, etc.).