8 
MUSEUM BULLETIN NO. 25. 
when dealing with current ripple-mark to determine the orient- 
ation of the ripple ridges and troughs whatever the depth may be. 
The very brief period during which the plaster mixture remains 
fluid after preparation limits the depth at which this method 
is practicable. 
Another device usable at any depth, which I have 
employed in obtaining information regarding the spacing 
or amplitude of ripple-mark in water too deep for taking ripple- 
mark moulds, consists of a thin piece of sheet iron or zinc 18 or 
20 inches square. A hole at each corner affords a means of 
attaching four short cords to the metal plate; these are united 
to a single cord by which the plate is lowered after being pro- 
perly weighted. The under side of the plate is coated with 
vaseline. When drawn up the plate will show parallel lines of 
sand corresponding to ripple-mark crests, if ripple-mark has been 
encountered. 
WIND RIPPLE-MARK. 
The ease with which every feature connected with the 
development of wind-made ripple-mark can be observed, makes 
it convenient to begin the study of ripple-mark with the examin- 
ation of this type of ripple-mark which finds its best develop- 
ment on the surface of sand dunes (Plates I and II). 
Sir Chas. Lyell's 1 description of the formation of ripple-mark 
by the wind can hardly be improved upon as regards the essential 
facts which are stated thus by him : 
“The following is the manner in which I once observed the 
motion of the air to produce this effect (ripple-mark) 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-mark 
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 
1 Elements of geology, 6th Ed., 1841, pp. 41-43. 
