RECENT AND FOSSIL RIFFLE-MARE. 
23 
and neap tides that the length or amplitude and the height of 
the waves were directly related to the stage of the tide. He 
states that: 1 “As the tides diminished after springs, the length 
of the waves diminished very slowly, but their height fell off 
rapidly, and at the time of neap tides they were nearly obliterated, 
the surface of the sandbank being almost smooth. The average 
level of the sandbank was, however, unchanged. When the 
tides increased after neaps, well-defined steep sand-waves were 
again formed which grew in height and length/' 
The reason for the formation of the mammoth sized current 
ripples is not altogether clear at first sight since small ones are 
developed near them. It has been observed, however, in 
certain cases that the sand waves lie on a surface sloping with the 
direction of the main currents where the velocity would prob- 
ably have its maximum rate. The small ripple pattern asso- 
ciated with the larger forms is in all probability developed in the 
last stages of the ebb when the velocity of the current is much 
reduced. 
All available observations appear to indicate that current 
velocity is an important factor in determining the amplitude of 
current or asymmetric ripple-mark, the amplitude increasing or 
decreasing with the current velocity. Volume of sediment in 
suspension is certainly another factor and depth may be a third. 
With decrease in current a point is reached where ripple-mark 
is not developed while increase in velocity results, where the 
bottom is sandy silt, in rapid addition to the load of sediment 
carried. A strong current carrying a maximum load of sand 
probably forms ripple-mark of large amplitude where a slightly 
loaded current having the same velocity would leave no ripple- 
mark. 
SYMMETRICAL RIPPLE-MARK. 
A type of ripple-mark which differs distinctly in its perfect 
symmetry (Figures 3 and 4 and Plate XXI) from the asymmetric 
current ripple-mark is produced by the oscillation of water resulting 
from wave action. This is the type of ripple-mark which charac- 
terizes the sandy bottoms of lakes and ponds but which is seldom 
1 “Waves of sand and snow" 1914, p. 316. 
