12 
MUSEUM BULLETIN NO. 25. 
of th6 depth and width of troughs in wind-made and water- 
formed ripple-marks. This will be called the ripple-mark 
index. 
The index numbers of two examples of water-made ripple- 
mark found respectively in the St. Lawrence river and lake 
Ontario, and determined from moulds, are 6*3 and 4. The 
index numbers of two examples of moulds of wind-made ripple- 
marks taken at Wellington are 24 and 25, thus showing an index 
number four to six times greater than that of the water-made 
ripple-marks. The sharp contrast in the height of the crest in 
dune ripples and in subaqueous ripples is illustrated by the 
profiles of examples of both types which have about the same 
amplitude as shown in Figures 2A and 4E. 
ASYMMETRICAL RIPPLE-MARK. 
The action of a current of water flowing over a bed of sand 
resembles closely in its effects the work of air currents on sand 
(Plate III A). The surface becomes covered with ripple-mark 
ridges and troughs which are asymmetrical like those produced 
by the wind on sand dunes (Plate VII A and B). Wherever 
tidal currents or river currents traverse beds of sand such ripple- 
mark is found in abundance. The asymmetrical or current 
ripple-mark is characteristic of marine and fluvial deposits but 
it is seldom met with in lacustrine sediments. The ridges trend 
at right angles to the direction of the current. The upstream 
slope is long and gentle while the downstream slope is short and 
steep (Figure 3C-G and Plates VII and X to XI I) . Like the wind- 
ripples they travel with the water current which is continuously 
eroding the upstream side of the ripple-mark ridges and carrying 
the grains over the crest and dropping them in the trough. The 
obstruction to the current caused by the ridge of the ripple-mark 
creates an eddy in the lee of the crest and the return current of 
this vortex keeps a film of sand grains ascending the lee side of the 
ripple-mark against the general current thus preventing the 
troughs from filling, while ripple-ridges move forward with the 
current. Under the action of a strong current this movement 
is comparatively rapid. Where the current is barely perceptible 
