RECENT AND FOSSIL RIPPLE-MARK. 
21 
This rock floor is fairly smooth over a portion of its surface 
and thinly covered with sand in some places; in other places 
sand and gravel are mixed. In the latter areas the straight 
parallel lines of ripple-mark formed by the currents of the outgoing 
tide are beautifully shown. Where sand without gravel is present 
in the area east of the cliffs the strong outgoing currents develop 
good examples of large current ripples. These are all of the asym- 
metrical type, most of them showing an amplitude of 6 inches to 
1 foot. But over one stretch of the sand area a mammoth type 
of ripple-mark similar to that at the head of St. Mary bay is 
seen : the crests are 5 to 8 feet apart and the troughs 3 to 5 inches 
deep. A characteristic of all the smaller examples of ripple- 
mark seen here is the graded character of the materials in them; 
for the current has left the coarse materials and fine gravel 
in the troughs, and the sand on the crests and upper slopes 
(Plate IX B). The mammoth ripples have their crests more 
scalloped and less nearly parallel than the smaller ones but still 
show a general parallelism. These large ripple-marks are in 
many places broken up into an intricate pattern and lose the 
ordinary ripple-mark characteristics by irregularities in the 
currents which have breached the crests of the ripple-mark with 
various irregular slashes not easily referable to any definite 
type of water sculpture. 
In estuaries subject to these unusually strong currents during 
ebb and flow of the tide, terrace-like waves of sand are often 
formed, resembling ordinary ripple-mark in all respects except 
their extraordinary size. This gigantic kind of ripple-mark 
is developed on the west side of the channel of Avon river near 
the railway bridge at Windsor, N.S. The abrupt contraction 
and increased grade of the channel of the Avon river near the 
Canadian Pacific Railway bridge accelerates the current of the 
ebb tide as it passes this point. The effect of the swift current 
heavily loaded with silt in producing "sand-waves” 1 or ripple- 
marks of enormous amplitude is manifested at low tide when 
the extensive sandbanks above and below the bridge are laid 
bare. In the middle of the channel at the bridge the whole of 
* Cornish, Vaughn, "Waves of sand and snow," 1914, p. 300. 
