30 
MUSEUM BULLETIN NO. 25. 
broken or interrupted by the troughs of the current ripples. 
This type of ripple-mark is shown in Plates IX A, XV B, XXVII, 
XXVI 1 1 B , and XXIX. The size or prominence of these superim- 
posed ripples will depend on the strength of the waves producing 
them. Under the influence of a strong wind they will develop to 
the extent of breaking across the current ripple pattern and produce 
a knobby surface like that shown in Plate XV C. The ripple-mark 
superimposed on the current mark by wave action is always of the 
asymmetrical type owing to the influence of the current upon it, as 
will be seen by an examination of Plate IX A. The current ripples 
and superposed wave ripples shown in the photograph on Plate XV 
were formed under water and air conditions which were closely 
observed. These conditions are indicated in the following excerpt 
from my notes: The mud bank of the east shore of the Avon river, 
l j miles below Windsor, slopes gently toward a broad sand plain 
100 to 250 yards wide which is nearly flat and uncovered at low 
tide. This sand flat is beautifully ripple-marked throughout at 
low tide, when there has been little or no wind, by regular ripples 
with an amplitude of 3 to 3| inches. These are to-day rippled at 
right angles by very small ripples caused by the waves set up by 
the strong westerly breeze which has been blowing during ebb tide. 
These small ripples of wave origin over considerable areas are 
of one inch amplitude. At lower levels where the ripple-mark 
formed after the breeze dropped somewhat, these superposed 
ripple-marks are much smaller, making only small ridges one- 
fourth to one-third inch apart on the steep and gentle slopes of the 
current-ripples. 
Wave ripple-mark is subject to modification by current 
action, the resulting form depending on the strength of the 
current. The symmetrical crests of wave-ripples are extremely 
sensitive even to very gentle current action. A current which 
is barely perceptible is sufficient to modify wave-made ripple- 
mark and give it a slightly asymmetrical form. These slight 
modifications of the symmetry of wave ripple-marks can seldom 
be clearly recognized in observations made through the water 
but are easily seen in the plaster moulds. Many observations 
on the influence of current on wave-made ripple-mark were 
made along the St. Lawrence river. The breadth of the river, and 
