SUBSIDENCE OF THE THAMES ESTUARY. 161
Congestion of the existing habitation site may have resulted
in a compromise ; the continuance of the native settlement on
the shore and the foundation of a foreign colony on a natural
or artificial island (crannog or Packwerkbau) in the Eastern Mere,
connected with the shore by a causeway.
The Causeway. In 1929 the construction of an outfall
sewer in Thorpe Hall Avenue exposed a landing-stage and
causeway, south of the stream, leading from the position of the
shore of the prehistoric settlement into the site of the Eastern
Mere. In 1930 the continuation of the causeway was followed
to a point over 60ft. from the landing-stage, by excavation in
the adjacent marshland. As the frontage line of some projected
buildings had been reached further exploration was then arrested.
The Causeway showed unmistakable evidence of subsidence
of the area since its construction and yielded pottery in situ
which fixes its approximate date. It presented three methods
of construction. (Plate VIII.)
(a) Landing-Stage. Two trimmed oak trunks, 8 and 12 in. diams.
the smaller split longitudinally in halt, were laid horizontally at an
angle splaying towards the shore ; they rested on basic fascines.
(b) Corduroy Road. Crossed a water channel separating the shore
from an islet. Width, 7 ft. ; four parallel trimmed saplings, 4-5 in.
diam., were laid horizontally in the line of the causeway, 16 in.
apart, bedded in soil ; beneath were basic fascines.
(c) Islet Road. The causeway, 5-6 ft. wide, turned southwards in
crossing the islet. At the summit it made a zig-zag bend to resume
nearly its original direction; the bend being marked by a vertical
pile. The road was composed entirely of basic fascines, laid on the
boggy surface.
The causeway was covered with structureless black mud, probably turf
or soil impregnated with decayed organic matter. The basic fascines
were uniform ; but the constituent layers varied in thickness ; they were
laid transversely to the line of the causeway and, from below upwards,
consisted of:
1. Reed-and-Rush Layer. Reeds, rushes, grass and water-loving
plants from the margin of the mere were tied in bundles and laid
horizontally. Layers of gravel were placed between the bundles
to bed and weight them down.
2. Brushwood Layer. Fine twigs, trimmings of the next layer, were
laid horizontally to level the surface. This layer contained the fruit
of elder, hawthorn and blackberry ; their presence together indicates
that the causeway was constructed at the end of the summer, when
the mere was at its lowest level before the autumnal rains.
3. Wattle Layer. Layers of trimmed stems of willow, alder, elder
and hawthorn, 1-3 ft. long, were laid horizontally in parallel lines
about a foot apart, bedded in soil ; at the bend of the islet road