20 EIPPLE MAEK. [Ch. IL 



at low tide, seems to originate in the drifting of materials along the 

 bottom of the water, in a manner very similar to that which may explain 

 the inclined layers above described. This ripple is not entirely confined 

 to the beach between high and low water mark, but is also produced on 

 sands which are constantly covered by water. " Similar undulating ridges 

 and furrows may also be sometimes seen on the surface of drift snow and 

 blown sand. The following is the manner in which I once observed the 

 motion of the air to produce this effect on a large extent of level beach, 

 exposed at low tide near Calais. Clouds of fine white sand were blown 

 from the neighboring dunes, so as to cover the shore, and whiten a dark 

 level surface of sandy mud, and this fresh covering of sand was beauti- 

 fully rippled. On levelling all the small ridges and furrows of this ripple 

 over an area of several yards square, I saw them perfectly restored in 

 about ten minutes, the general direction of the ridges being always at 

 right angles to that of the wind. The restoration began by the appear- 

 ance here and there of small detached heaps of sand, which soon 

 lengthened and joined together, so as to form long sinuous ridges with 

 intervening furrows. Each ridge had one side slightly inclined, and the 

 other steep ; the lee-side being always steep, as b, c, — d, e ; the windward- 

 side a gentle slope, as a, b, — c, d, fig. 9. 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 ^and up the slopes a b and c d, many of which grains, when 

 they arrived at b and d, fell over the scarps b c and d e, and were under 

 shelter from the wind ; so that they remained stationary, resting, ac- 

 cording 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. Occasionally part of a ridge, advancing more rapidly than the 

 rest, overtook the ridge immediately before it, and became confounded 

 with it, thus causing those bifurcations and branches which are so com 

 mon, and two of which are seen in the slab, fig. 8. We may observe 

 this configuration in sandstones of all ages, and in them also, as now on 

 the sea-coast, we may often detect two systems of ripples interfering with 

 each other ; one more ancient and half-effaced, and a newer one, in which 

 the grooves and ridges are more distinct, and in a different direction. 

 This crossing of two sets of ripples arises from a change of wind, and the 

 new direction in which the waves are thrown on the shore. 



The ripple mark is usually an indication of a sea-beach, or of water 

 from 6 to 10 feet deep, for the agitation caused by waves even during 



