RECENT AND FOSSIL RIPPLE-MARK. 
37 
irregularly mammi Hated on its surface, which is pretty equally, 
although irregularly divided into small hollows and protuber- 
ances of a few inches in diameter.” “It might be called 
dimpled current-mark .” 1 Strong currents which impinge upon 
each other over a sand bar which lies between two channels are 
apt to produce this phase of water sculpture showing a peculiar 
pendant and elongated lip-like effect. Examples of this phase 
of current mark are shown in Plates XVI B and XVII A and B. 
Plate XVII B and C represents current-mark which was made by 
a small brook near its junction with the St. Lawrence river. Plate 
XVII A is from the mould of a cast taken on the sand bar at the 
mouth of the Avon river. Plate IV A shows the appearance of the 
surface of a small brook near Ottawa which is flowing over a sandy 
bottom and producing in parts of its channel a variety of current- 
mark. Two lines of tongue-like ripples on the surface of the brook 
indicate the zones of maximum bottom ridging. Where these two 
lines of ripple unite below the small bar current-mark is formed. 
This photograph clearly shows the varying intensity of the 
action of the current in different parts of the bottom of the narrow 
channel shown. The different types of current mark are all 
apparently the result of irregular or “troubled” currents. Some 
of these, like the structure shown in Plate XVI B,have the appear- 
ance of mud flows. 
Many geologists have been puzzled by these curious features 
on the surface of stratified rocks and they are frequently errone- 
ously called “mud flows” in geological reports. Jas. Hall 2 * 
figured examples of this structure as “casts of flowing mud.” 
David Dale Owen 8 figured a fine example of current mark but 
wrote “I am rather disposed to the belief that the material of 
which the rock is composed was once volcanic mud and that 
while in a viscid state, it congealed suddenly, or became fixed 
in the very act of flowing down the hillside; transmitting to us a 
lapidified memento of the action of some mud volcano in the 
vicinity.” The peculiar impressions left by the swirl of waters 
on sand in eddying currents resembles often the sweeping 
1 J. B. Jukes, Manual of geology, 1872, pp. 163-164. 
1 Geology of New York, pt. 4, fig. 10, 1843, p. 233. 
1 Illustrations to the Geol. Rept. of Wis., Iowa, and Minn., table I, fig, 1, 1852. 
