34 



LYELL'S ELEMENTS OF GEOLOGY. 



Ripple Mark, how formed. 



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 eflect on a large extent of level beach, 

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

 were blown from the neighbouring dunes, so as to cover the 

 shore, and whiten a dark level surface of sandy mud, and this 

 fresh covering of sand was beautifully rippled. On levelling all 

 the small ridges and furrows of this ripple over an area 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 appearance 

 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 



Fig. 7. 



inclined, and the other steep ; the lee side being always steep, 

 as 6, c, — e; the windward side a gentle slope, as a, &, — c, d. 

 Fig. 7. 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 slopes a h 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, according to their shape and nnomentum, on 

 different parts of the descent, and a few only rolling to the bot- 

 tom. In this manner each ridge was distinctly seen to move 

 slowly on as often as the force of the wind augmented. Occa- 

 sionally 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 common, and two of which are seen in the slab Fig. 6. 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 sys- 



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