RECENT AND FOSSIL RIPPLE-MARK. 
9 
of sand, which soon lengthened and joined together, so as to 
form long sinuous ridges with intervening furrows (Figure 1). 
Each ridge had one side slightly inclined, and the other steep; 
the lee-side being always steep, as be and de; the windward-side 
a gentle slope, as ab and cd. When a gust of wind blew with 
sufficient force to drive along a cloud of sand, all the ridges were 
seen to be in motion at once, each encroaching on the furrow 
before it, and in the course of a few minutes filling the place 
which the furrows had occupied. The mode of advance was by the 
continual drifting of grains of sand up the slope ab and cd, many 
of which grains, when they arrived at b and d, fell over the 
scarps be and de, and were under shelter from the wind ; so that 
they remained stationary, resting, according to their shape and 
momentum, on different parts of the descent, and a few only 
rolling to the bottom. In this manner each ridge was distinctly 
seen to move slowly on as often as the force of the wind aug- 
mented.” 
* 
Figure 1. Wind ripple-mark, profile after Lyell. 
The diagrammatic figure here reproduced after Lyell, although 
the troughs are too deep for their width, very well illustrates the 
mode of formation and movement of ripple-mark. The lee 
slope is much longer in proportion to the windward slope than 
it is ever found in nature. The actual relations and appearance 
of the lee and windward slopes in wind ripple-mark are shown 
in Figure 2 A and B. These show the very short lee slope and 
shallow trough which are characteristic of wind ripple-mark. 
Examination of a large number of sand dunes along the north 
shores of lakes Ontario and Erie shows the surface of the sand, 
where free from vegetation, to be nearly everywhere covered 
by ripple-mark in dry weather (Plate I). These, of course, are 
obliterated by heavy rains but reappear soon after the surface 
dries. The only areas which are free from the ripple-marks are 
those where the slope is very steep, and where the action of the 
wind is interrupted by trees or other obstructions. Whenever 
the wind is strong enough and the surface of the dune sufficiently 
