46 
MUSEUM BULLETIN NO. 25. 
1 
with the same strength in successive tides the ripple-mark result- 
ing from each tide would doubtless be always obliterated by the 
migrating ripples of the succeeding tide. But tidal currents 
may be interrupted and even completely checked in certain 
areas by winds or flooded rivers. In the latter case large acces- 
sions of sediment from the land, in the temporary absence of the 
regular tidal current, might result in the ripple-mark of the last 
tide being covered and permanently preserved under the influence 
of a current from a flooded river strong enough to check the 
tidal current but too weak to produce ripple-mark. The flood 
tidal current is thus sometimes held in abeyance in San Francisco 
bay. In the Golden Gate the current during spring tide has a 
maximum velocity of 6 or 7 miles per hour. “At periods of great 
freshets in the Sacramento and San Joanquin rivers there 
have been instances of very slight surface current or none at all 
on the small flood in the Golden Gate.” 1 In such a case as this 
the sediment discharged by the rivers would be likely to bury the 
ripple-mark formed by the outgoing tidal current and thus 
protect it from destruction by later current action. It is safe 
to assume that during the deposition of those sandstones in 
which ripple-mark is absent such special conditions as these 
never occurred. 
Probably the simplest and most frequently applicable 
explanation of the presence or absence of ripple-mark in sand- 
stone formation is to be found in the fact that ripple formation 
depends on current velocity. Dr. Owen 2 finds that in shallow 
water a current of less than 0 • 85 feet per second does not move 
ordinary sea sand. A speed of current between 0-85 and 2-5 
feet per second moves the sand in the form of ordinary current 
ripples. With a current above 2 * 5 feet per second the sand moves 
in a sheet without forming ripples. Current velocities between 
these produce ripple-mark. The absence of current ripple- 
mark from parts or the whole of a formation would, therefore, 
suggest that the beds without ripple-mark had been deposited 
under the influence of a current velocity which was above the 
1 Coast. Pilot, p. 74. 
1 “Experiments on the transporting power of sea currents,” Geog. Jour., rol. XXXI, 
Jan.-June, 1908, pp. 415-420. 
