22 
MUSEUM BULLETIN NO, 25. 
the sand is removed leaving only rough angular rock boulders 
on the bottom. Near the west bank, where the current has 
somewhat less power, the sand remains and ripple-mark forms on 
sand waves, which have an amplitude of 15 to 20 feet and crests 
rising 5 to 20 inches above their troughs. The sand flats above 
these and the monster ripple-mark ridges themselves are covered 
at low tide with small ripple-mark pattern having an amplitude 
of 3 inches. A photograph of the large ripple-mark at the 
Avon River bridge is shown in Plate VIII B, It will be noted 
that the large ripple-marks are covered with small ripple-marks. 
The latter were doubtless formed near the end of the ebb tide 
when the current was comparatively weak and held a smaller 
volume of sediment than earlier. Both sets trend at right 
angles to the direction of the current. Plate VII shows the type 
of ripple-mark formed on the wide sand bars below the bridge 
by a lighter current. The ripple-mark of both views was 
formed by the currents of the same tide. Near the east bank 
above the bridge the swift current, which is strengthened by a 
wing dam from the west side, keeps the sand swept clean leaving 
a hard bottom of coarse gravel and small stones. The sand 
plain which fills most of the river bed elsewhere shows an area 
of 3 acres or more adjacent to the stony bottom where the 
currents have been presumably too strong or too irregular to 
form the sand waves described above. Here the sand is cut 
into a series of basin-like depressions most irregular in shape 
and depth (Plate XIII A and B). These range in depth from 3 
to 5 feet and in diameter from 3 to 40 feet with outlines of 
extremely variable shape. This is the type of structure which 
would result in cross-bedded sandstone. It is in another zone 
near the west shore where the current must be less powerful 
that regular “sand waves” are developed. 
Vaughn Cornish 1 has found by observation of large sand 
waves in tidal estuaries in Great Britain that they advance with 
the tidal current. An average advance of 13 inches, with one 
ebb tide was found in one case. Cornish also found by a series 
of observations on large sand waves extending through spring 
i Ibid. 
