
Part f.] : RIPPLE-MARKS, _ 483 
up sedimentation went on as before, and there is usually an ob- 
vious close sequence between the continuous strata. Though it 
may be impossible to decide as to the relative length of the in- 
terval that elapsed between the formation of a given stratum and 
that of the next stratum which lies upon its eroded surface, or to 
ascertain how much depth of rock has been removed in the erosion, 
yet, when the structure occurs among conformable strata, evidently 
united as one lithologically continuous series of deposits, we may 
reasonably infer that the missing portions are of small moment, and 
that the erosion was merely due to the irregular and more violent 
action of the very currents by which the sediment of the successive 
_ strata was supplied. 
The case is very different when the eroded strata are inclined at 
a different angle from those above them, and are strongly marked off 
by lithological distinctions. In some of the coal-mines in central 
Scotland, for instance, deep channels have been met ,with entirely 
filled with sand, gravel, or clay belonging to the general superficial 
drift of the country. These channels have evidently been water- 
courses worn out of the Coal-measure strata at a comparatively recent 
geological period, and subsequently buried under the glacial accu- 
mulations. There is a complete discordance between them and the 
paleozoic strata below, pointing to the existence of a vast interval 
of time. 
Surface-markings.—Rip ple-mark.—\The surface of many beds 
of sandstone is marked with lines of wavy ridge and hollow, such as 
may be seen on a sandy shore from which the tide has retired, on 
the floors of shallow lakes and of river pools, and on surfaces of dry 
wind-blown sand. Water (or air) gently agitated in a given 
direction, throws the surface of sediment into ripples, which tend 
to run at right angles to the course of movement. If the wind blows 
with little variation towards a given point, the sand ripples have a 
long gentle slope towards the wind, and a short steep slope away from 
it (Fig. 196). Considerable diversity in the form of the ripple (as at 
_abcin Fig. 197) may be observed, depending on conditions of wind, 
water, and sediment which have not been thoroughly studied. As 
the wind veers from point to point, producing corresponding changes 
in the direction of the water currents, the ripples on the bottom are 
not strictly parallel, but often coalesce, intersect, and undulate in 
their course. Their general direction, however, suffices to indicate 
the quarter whence the chief movement of the water has come. No 
satisfactory inference can be drawn from the existence of ripple-marks 
as to the precise depth of water in which the sediment was 
accumulated. -As a rule,it is in water of only a few feet or yards in 
depth that this characteristic surface is formed. But it may be 
produced at any depth to which the agitation caused by wind on the 
upper waters may extend (p. 423). 
An examination of asandy beach brings before us many modifica- 
tions of the perfect ripple-mark. The ridges may be seen 2 eTow 
21 
