e waves, may cause the progressive motion of a shingle beach in
one direction. I have already alluded to the erection of piers and
groins at certain places on our southern coast, to arrest the course of
the shingle and sand (see p. 318). The immediate effect of these
temporary obstacles is to cause a great accumulation of pebbles on one
side of the barrier, after which the beach still moves on round the end
of the pier at a greater distance from the land. This system, however,
is often attended with a serious evil, for during storms the waves throw
suddenly into the harbor the vast heap of pebbles which have collected
for years behind the groin or pier, as happened during a great gale
(Jan. 1839) at Dover.

The formation and keeping open of large estuaries are due to the
_combined influence_ of tidal currents and rivers; for when the tide
rises, a large body of water suddenly enters the mouth of the river,
where, becoming confined within narrower bounds, while its momentum is
not destroyed, it is urged on, and, having to pass through a contracted
channel, rises and runs with increased velocity, just as a stream when
it reaches the arch of a bridge scarcely large enough to give passage to
its waters, rushes with a steep fall through the arch. During the ascent
of the tide, a body of fresh water, flowing down in an opposite
direction from the higher country, is arrested in its course for several
hours; and thus a large lake of fresh and brackish water is accumulated,
which, when the sea ebbs, is let loose, as on the removal of an
artificial sluice or dam. By the force of this retiring water, the
alluvial sediment both of the river and of the sea is swept away, and
transported to such a distance from the mouth of the estuary, that a
small part only can return with the next tide.

It sometimes happens, that during a violent storm a large bar of sand is
suddenly made to shift its position, so as to prevent the free influx of
the tides, or efflux of river water. Thus about the year 1500 the sands
at Bayonne were suddenly thrown across the mouth of the Adour. That
river, flowing back upon itself, soon forced a passage to the northward
along the sandy plain of Capbreton, till at last it reached the sea at
Boucau, at the distance of _seven leagues_ from the point where it had
formerly entered. It was not till the year 1579 that the celebrated
architect Louis de Foix undertook, at the desire of Henry III., to
reopen the ancient