142 
THE GEOLOGIST. 
bottom consists. The nature of the material of which these waves 
are formed is such, that, when the current ceases, their forms remain, 
and thus permanently record the direction of the current, which, of 
course, must have flowed in a line perpendicular to their trend. 
These wave-like forms are the well-known " ripple-marks," about the 
origin and nature of which there has often been much misunderstand- 
ing. They have too frequently been looked upon as having been 
invariably formed by the action of the waves of the surface of the 
water stranding on a sandy beach. They are, however, by no means 
necessarily connected with the waves of the upper surface, but are 
merely the effect of the movement of the current over the sand, and 
are the impressions of disturbances of a wave-like form affecting the 
bottom of the current, and generated by the resistance experienced by 
it in moving over the sand, under certain conditions of depth and 
velocity. Stranding waves produce " ripple-marks " on a sandy beach, 
because they give rise to a current ; and it is this current which 
produces the ripple-marks. Other facts must be taken into account, 
if we wish to decide whether any particular ripple-marks were formed 
by wave-currents, or by currents due to any other cause. 
. If we have merely the surfaces of the ripple-marks to guide us, we 
cannot always determine ft-om which side the current came ; but very 
commonly their wave-forms move forward and - progi'ess in the same 
direction as the current of water which generates them. This is 
owing to the sandy material drifted forward by the current being 
carried up the side of the ripple turned towards the direction from 
which the current comes, and then thrown down on the opposite side 
more protected from the action of the current. This will be more 
clearly understood by means of the diagrammatic section, fig. 2, which 
Fig. 2. 
f e d c h a 
is intended to show the connexion and gradual passage from a level 
surface at a, to low and round-topped ripples at h, which at c and d 
become crested and sharp-topped. Except in i-are cases such a ripple 
as d could not hold together ; the upper portion would be washed off. 
