OF THE THAMES VALLEY.
61
there confront us. In such a case it is easy to see that on each
side of the valley there are two terraces developed which are
each closely connected with the major bendings of the river ; on
the convex or cliff side of each bend occurs the highest and most
ancient terrace, while ou its concave side the lower terrace
would occur, the latter doubtless being carved into many smaller
terrace structures.
Such terraces as these must always bear a more or less
obvious relationship to the curvature of the river. They are
best developed or preserved in valleys eroded in the harder strata,
and they may be regarded as the true "river" terraces. For
in such valleys there is more resistance offered to those
accelerating influences with which we shall deal hereafter, and
which produce very important effects in valleys eroded in softer
and less resisting materials. Moreover, these terraces
can never be continuous to any great extent along the valley
sides ; their opposition both as regards altitude and in relation to
the plan of the valley must of necessity be always imperfect. In
all these points the terraces which are produced purely by
unaided fluviatile action differ from hose which are so largely
developed in the lower valley of the Thames, and thus it may be
held that what may be termed the Huttonian philosophy does
not afford a complete explanation for the structures under con-
sideration in this paper.
Having obtained a clear idea of the nature of the terraces
produced by unaided river action, it will be convenient, before
proceeding further with the argument, to briefly recall those
characteristics which are common to the terraces of the Lower
Thames Valley. These terraces are built up of deposits of
gravel, sand and brickearth, and extend continuously along either
side of the valley for many miles. In the lower portion of the
valley the terraces on the one side coincide with those on the
other in the elevations which they respectively reach above the
river, and thus they are truly opposed and are said to be paired.
Darwin, in 1844, described certain terraces occurring in
South America. In these he was able to trace a connection or
confluency with the raised beaches occurring on the coast, and
he ascribed their formation to the action of the sea on river-
borne detritus as the land was being elevated.3 Robert
3 Geological Observations in South America, pp. 258, 291