RECENT AND FOSSIL RIPPLE-MARK. 
27 
On the seashore, where the current of the ebbing tide nearly 
always develops the asymmetric or current ripple-mark type, the 
retreat of the tide removes every obstacle to close examination. 
The numerous photographs of ripple-mark uncovered by the 
tide, which have been published, consequently show current 
ripple-mark but not oscillation ripple-mark. The photographs 
and profiles of wave ripple-mark, which are here shown, have been 
obtained by making, under water, plaster moulds of these ripple- 
marks. The method used in taking these preserves perfectly 
the finest details, notwithstanding the fact that they are com- 
posed of material so unstable as sand. 
Symmetrical ripples do not travel like the asymmetrical or 
current ripple-mark. They maintain a permanent position so 
long as the conditions which produced them remain unchanged. 
In specimens of fossil ripple-marks in the Joggins, N.S., section, 
I have observed symmetrical ripple-marks directly superimposed 
throughout a bed of sandstone 6 inches thick. Gilbert records 
an observation in which the lamination of the strata showed that 
a set of ripple-marks had held the same position while 2 feet of 
sediment were accumulated. 1 
All waves have a tendency, whatever their direction may be 
at a distance from the shore, to change gradually the direction 
in which they are trending as they approach land and enter shoal 
water, so that they reach the beach with troughs and crests 
approximately parallel to it. “The action of the bottom on a 
wave may be compared to that of a friction brake on a machine.” 
The end of a wave which first “feels the bottom” in shallow water 
will slacken its speed while the opposite end in deeper water 
travels faster, thus causing the entire wave in approaching 
shore to swing round and face the beach. As a result of this 
tendency of waves to face the beach in approaching land, the 
ripple-marks made by them always trend with the direction of 
the strand line near the shore. On the shores of lakes where 
ripple-mark is due entirely to wave action it always runs parallel 
with the coast-line. Ripple-mark along the sea coast is generally 
the work of tidal currents which follow the shore-line. These 
* Bull. Phil. Soc., Washington, rol. 2 , 1874-78, pp. 61-62. 
