RECENT AND FOSSIL RIPPLE-MARK. 
13 
and too weak to initiate ripple-mark it will still be able to slightly 
modify the shape of wave-made ripple-mark as it is developed 
and to cause it to migrate so long as wave action is active. Two 
cases were observed and timed near Lanoraie on the St. Lawrence 
by placing pegs in the ripple crests. In one case ripple-mark 
ridges with an amplitude of 1 J inches in 3 inches of water moved 
If inches in 20 minutes, a distance equal to their amplitude. 
In the other observation at the same locality ripple-marks with 
an amplitude of 2 inches in 6 inches of water moved 2 inches in 
20 minutes. This ripple-mark was made by onshore waves 
working in the presence of a barely perceptible current. Many 
clear rivers with broad shallow streams and sandy bottoms 
afford good examples of current ripple-mark. In such streams 
the ripple-mark trends across the channel at right angles to the 
current and moves down stream when the current is sufficiently 
strong. In clear shallow streams the sharp crested parallel 
lines of ripple-mark may often be seen extending from bank to 
bank. The ripple-mark produced by river currents and that 
formed by tidal currents are both asymmetrical and easily dis- 
tinguished from the symmetric type produced by wave action. 
The photographs reproduced in Plate IV A and B show a small 
brook flowing over a bed of sand near Ottawa. In this case the 
volume and velocity of the water was sufficient to develop 
ripple-mark only over the middle portion of the channel where 
the stream had its maximum velocity and depth. In this 
middle zone the photograph shows the water breaking into 
tongue-shaped ripples which are directly over corresponding 
ridges In the sand. 
There is, according to Vaughan Cornish, remarkable exception 
to the usual downstream migration of current ripple-mark in 
the case of currents maintaining a velocity of about 2 * 2 feet per 
second. Cornish states that “The most remarkable property 
of the sand waves produced in very shallow water when the 
velocity attains about 2*2 feet per second Is that they travel 
up-stream.” 4 It is very curious to see a group of 
these sand ripples moving in procession up-stream against the 
current which produces them.” 1 This phase of ripple-mark 
1 “Waves of sand and snow,” 1914, p. 278, P. Fisher Unwin, London. 
