16 
MUSEUM BULLETIN NO. 25. 
the ebb tidal current has reached approximately its maximum 
strength, that ripple-mark is generally abundantly developed. 
Some of the illustrations of ripple-mark parallel with the shore 
and the general trend of the tidal current (Figure 9 l ) which are 
introduced to prove his theory are undoubtedly chiefly the 
product of wave action and cannot be admitted to furnish any 
evidence of the work of supposed transverse currents. In brief 
this author entirely overlooks the fact that asymmetrical ripple- 
mark can be produced in any properly equipped laboratory 
by the direct action of a water current, or may be seen on the 
bed of any stream with suitable bottom, and that these will be 
oriented at right angles to the current. Symmetrical ripple- 
mark can likewise be developed by wave action under laboratory 
conditions. Comparison of Figures 3 C and 4 A and Plates 
VII and XXI will show the sharp contrast between these two 
types which have been confused by Epry. 
Under the simple conditions existing in the bed of a shallow 
stream flowing over fine sand quantitative studies of the factors 
determining ripple-mark formation by current action have been 
made by two or three careful observers. Sowerby, 2 as a result 
of a study of the work of a small brook flowing over sand, has 
recorded the following observations regarding the relations 
between current velocity and ripple-mark formation: 
“By these experiments, and by observations made in a 
clear brook at Fulwood (near Sheffield), I came to the conclusion 
that, when the velocity of the current is about 6 inches per 
second, sand with grains about a hundredth of an inch in diameter 
is drifted along slowly, and a surface is produced, grained in the 
line of the current, but no ripple-marks are formed. When the 
velocity is somewhat greater than 6 inches per second, ripples 
are produced. When it is about 1 foot per second, these are well 
developed and advance about 3 inches per minute, by the sand 
being washed up on the exposed side and deposited on the other; 
which velocity may be looked upon provisionally as an average 
for undoubted drifted ripples. If the velocity attains 18 inches 
per second the ripples are destroyed by the washing away of the 
l Qf Epry. 
•Henry C. Sowerby, “On the application of quantitative methods to the study of the 
structure and history of rocks,” Jour, Geol. Soc., vol. LX IV, 1908, p. 180. 
