

116 STRUCTURAL AND FIELD GEOLOGY 
does not indicate that of the series of strata in which it occurs. 
The structure is common in littoral deposits and accumula- 
tions formed in shallow water, where there is much shifting 
and eddying of current-action. Not infrequently a highly 
false-bedded sandstone is directly underlaid and overlaid by 
evenly bedded strata, which give evidence of quiet and 
undisturbed sedimentation. 
False-bedding of a pronounced character is characteristic of the 
deltas formed by torrential streams and rivers. Such deltas advance 
more or less rapidly, and usually present a somewhat steeply sloping 
front. As already explained, gravel and coarse detritus are shot 
forward by the current, and roll down this steep bank—the finer sediment 
being carried further and coming to rest on the bed of the lake or 
estuary, so as to form approximately horizontal accumulations. Thus in 
time, as the bank advances, steeply-inclined beds of gravel (g) come to 
overlie horizontal sheets of sand (s) and silt (wz), (Fig. 9). | 

Fic. 9.—DELTA FORMED BY TORRENTIAL STREAM. 
g, gravel; s, sand; m, silt. 
Surface-Markings.—The surfaces of derivative rocks 
often exhibit interesting markings, among which the most 
notable are current-marks. These are of precisely the same 
character as the ripple-marks seen on modern sea-beaches 
(see Plate XXV. 2). Inthe shallow water of a sea-beach the 
ripple-marks slowly advance with the inflowing tide. They 
usually present a long, gentle slope seawards, and a short and 
more abrupt slope towards the shore. With the ebb-tide, the 
crests of the ridges tend to be smoothed off or truncated. 
When the movement of the water is irregular, as between 
skerries, boulders, etc., the result is the formation of numerous 
miniature hummocks and dimples, or straggling hollows and 
rounded ridges. 
Although current-marks are most commonly associated with beach- 
deposits, they are not confined to these, but may originate at any depth to 
which the agitation of the water extends. They have been observed forming 
in clear water at depths of 50 feet and more. Ripple-marked surfaces are 
common in many sandstones and argillaceous beds, and often occur one 
over another throughout a thick series of strata. As each advancing tide 
