RECENT AND FOSSIL RIFFLE-MARE. 
3 
statement is made that “ripple-marks are the work of the tide 
and of that alone.” 1 Various authors, who evidently are not 
aware that certain types of ripple-mark may be formed at great 
depths, speak of fossil ripple-mark as an evidence of shore or 
very shallow water conditions. Geologists are also told that 
observation of fossil ripple-mark affords in areas of great orogenic 
disturbance a means of determining whether the beds have been 
overturned or not, without being told what types can and what 
can not be used in this way. These and other considerations 
have led to the belief that a detailed discussion of the conditions 
under which ripple-mark is formed in nature would be useful 
to geologists. This belief appears to be well supported by one 
of the ablest investigators of ripple-mark who wrote as late as 
1904 that “The apparent contradictions of writers on ripple- 
mark are so surprising that one fails to see how the student 
or even the text-book writer can find his way through the mist.” 2 
Consideration of ripple-mark on calcareous beds will be 
omitted from this discussion because the writer has had no 
opportunity to study its formation on recent deposits. 
RECENT RIPPLE-MARK. 
GENERAL CONSIDERATIONS. 
Beds of sand when exposed either to wave action or to 
currents of air or water of the proper degree of intensity, become 
covered by the beautiful flutings known as ripple-marks. On the 
sea beach at low tide the sandy strectches, with their innumerable, 
equally spaced ridges and furrows look as if they might have been 
combed with a giant comb. The parallelism of the troughs and 
ridges of ripple-mark distinguish it from other forms of water 
sculpture (Plate VII). Wherever beds of sand occur which are 
subject to the action of subaqueous or subaerial currents with 
neither too low nor too high a velocity, or to wave action, ripple- 
mark is formed. Material in which there is little or no cohesion 
between the particles composing it is essential to ripple-mark 
formation. Mud, marl, and slimy sediments are never ripple- 
marked. It sometimes happens that a bed of mud and sand is 
1 Ann. Rept. Smith. Inst., 1913 (1914), p. 310. 
* Hunt, A. R., Nomenclature of ripple-mark, Geol. Mag., vol. I, p. 417, 1904. 
