
STRATIFICATION 117 
effiaces old marks, and replaces these by new ones, it is difficult to 
understand how, under ordinary conditions, a rippled surface of sand can 
be preserved. Hence it has been surmised that many of the ripple- 
marked surfaces which appear in rocks of all geological ages may 
have been produced below low tide-level, in shallow bays or in estuaries, 
where sedimentation is more or less continuously carried on; so that 
rippled surfaces might be often preserved by the gentle deposition upon 
them of fresh accumulations of sediment. However that may be, it 
seems certain that not infrequently ripple-marked surfaces have really 
been formed between high and low water. In some cases these have 
probably been preserved by the deposition over their surface of a thin 
film of clay. In other cases the rippled sand (often to some extent 
argillaceous) had become sufficiently consolidated to resist the action of 
the next incoming tide. It must be remembered, that at low tide on 
gently shelving shores a wide expanse of beach is laid bare. Exposed 
to the rays of a hot sun, the fine-grained sand or sandy mud might thus, 
over wide areas, be so dried and hardened as to resist the obliterating 
action of the flowing tide, and under such circumstances it is conceivable 
that surface after surface might be covered up and preserved. Again, 
on flat shores, wide belts of rippled sand and mud might be exposed 
between the lines of spring and neap tides. Hence, the surfaces above 
high-water of ordinary tides might become dried and consolidated before 
they were eventually covered by newer accumulations. The layer 
immediately overlying a ripple-marked surface usually shows a more or 
less perfect cast, which, when removed from its position, is often hard to 
distinguish from the actual mould or original surface. Frequently, 
however, the hollows are more sharply pronounced than the ridges, and 
when such is the case the cast of a hollow would show a sharp crest, 
such as could not be formed on the summit of a ridge. 
Wave-marks.—These are seen forming on modern sea- 
beaches during ebb-tide. They are delicate outlinings 
which mark the limits reached by the waves as they die 
out. If the edge of the thin layer of advancing water be 
observed, it will be seen that it sweeps along with it fine 
grains of sand, and more particularly particles which, by 
reason of their shape (mica-flakes) or light specific gravity, 
(coaly matter) are readily carried forward. When the wavelet 
dies out, these materials are stranded so as to form a 
miniature ridge, which is often rendered conspicuous by the 
presence of black carbonaceous matter. Wave-marks of this 
kind are not infrequently seen on the surfaces of fine-grained 
_sandstones and flagstones, and are good evidence of a beach- 
formation. 
Rill-marks (see Plate XX V. 1).—These are small furrows 
