244 ECONOMIC GEOLOGY
under the North Sea, stood on the western side of the Spurn
Head of its day.
The effect of spit formation in a less powerful river than
the Humber is shown by the Yare,® which in Roman times
had a wide opening to the North Sea (Fig. 61 a-¢). A shoal
formed across the estuary, and had become an island by
495, when the Saxon, Cedric, landed upon it. Its growth
into a large delta left the Yare with two mouths. The
southward drift of shingle closed the northern mouth after
1066. The filling of the estuary by silt diminished the tidal
flow, which no longer stopped the southward growth of
the spit; by 1347 the spit extended 10 miles 5. of Yarmouth
and the Yare mouth was near Lowestoft. The Yare fre-
quently breached the spit until a channel was cut in 1560
at Gorleston, and has since been artificially maintained.
The southward migration of the shingle then ceased, and the
spit from Gorleston to Corton was swept away as the material
was not replaced.
WarPING—THE SiLTinG OF Estuaries 2—Silting in the
stagnant water behind a bar or spit converts an estuary into
land. The salt in the sea promptly precipitates the silt
carried into it by rivers. If fresh water be stirred up with
mud, it remains turgid far longer than salt water. The salt
coagulates the particles of silt, which become larger and
heavier, and sink more quickly. The mud banks thus
formed are raised above tide level; their rise is aided by vege-
tation, which acts as a sieve and catches the sediment carried
against it.
The lowland beside an estuary may be raised by warping,
as more silt is carried on to it by the rising tide than is
removed by the gentler ebbing water. Fach tide deposits
a thin film of “warp.” Carey and Oliver report (Tidal
Lands, 1018, p. 212) that 184 acres along the Trent were
raised by warping from 1-4 feet in 3 years; grass was grown
on the land at the end of the first season, and in the fourth
year it yielded large crops. Natural warping along the
Norfolk coast near Blakeney (ibid., pp. 200-1) varied from
1 foot in 0} years to 1 foot in 120 years, in a position only
1 For recent study of the coast of East Anglia, J. A. Steers, Geogr.
Journ., 1927, xix, pp. 24-48.
2 A Beazelev. Reclamation of Land from Tidal Waters, 1900.
