36 
MUSEUM BULLETIN NO. 25. 
sided cells. In the waters of small shallow ponds development 
of interference ripples frequently occurs in which the cells have 
rounded instead of quadrangular sides. A photograph of this 
kind of interference ripple-mark is shown in Plate XXIII A 
which shows the bottom of a small pond from which most of the 
water has evaporated leaving the interference ripple-mark 
structure exposed on the surface. Interference ripple-mark 
is frequently seen in miniature coves or identations of the shore- 
line but never along a straight uninterrupted shore-line. The 
Triassic sandstone of the Connecticut valley has furnished some 
striking fossil examples of this type of ripple-mark. It was 
originally described by Hitchcock from these beds as “tadpole 
nests.*' 1 It also occurs, though rarely, in the Berea sandstone, 
as shown by Plate XXX B. 
CURRENT-MARK. 
In preceding pages it has been shown that water currents 
passing over sand at certain velocities develop ripple-mark over 
the whole surface; at other velocities the surface film of sand 
is moved along in a uniform sheet without the formation of 
ripple-mark and the surface is left smooth. If the sand trans- 
porting current joins a slower current or enters quiet water its 
load is thrown down in a fan-shaped mass called a delta (see 
Plate XVI A). When converging currents meet or, where owing 
to irregular contour of the bottom or sides of the channel, vari- 
able or irregular currents are developed, there results an intricate 
interplay of the forces tending to produce ripple-mark and those 
tending toward delta formation. Instead of the regular rhyth- 
mic succession of ridges and furrows of ordinary ripple-mark 
variable currents often produce a great variety of irregularly 
mammillated and lip-shaped surfaces on sandy bottom which 
may be called current mark (Plate XVII and Figure 6). Jukes 
has described this type of current sculpture and its cause as 
follows : 
“In places where the current was troubled a modification 
of these rippled surfaces is sometimes produced, the bed being 
l ‘Tchnology of Mass.", ol. I, fig. 1, 1858. 
