﻿Vol. 64.] QUANTITATIVE METHODS TO THE STUDY OF ROCKS. 183 



Ripple-Marks, etc. 



My experiments at Woodbourne showed that when the velocity 

 of the current was only about O'l foot per second no ripples were 

 formed, even when saud was deposited from above. When the 

 velocity was from a quarter to half a foot per second, ripples were 

 produced when deposit took place, though not otherwise, but they 

 did not advance. This appears, therefore, to be the condition 

 necessary to produce such ripple-marks as are seen in some thinly- 

 bedded rocks, which show almost or quite symmetrical ripple-forms, 

 but little or rio effect of drifting. When this does occur a true 

 rippled surface is seldom visible ; but, in a section perpendicular 

 to the stratification, inclined laminae are seen, and a surface of 

 peculiar character is shown when the rock is broken parallel to the 

 plane of bedding. 



Production of Ripples and their Relations. 



As shown by my experiments and observations, when the mean 

 velocitv of a current decidedly exceeds 6 inches per second, ripples 

 are formed ; and when it is 1 foot per second they are well de- 

 veloped and advance about 3 inches per minute, by the Washing-up 

 of the exposed side and deposition on the sheltered side. The 

 formation of these ripples makes a considerable change in the 

 conditions, since the sand must be drifted up the slopes. Very 

 careful observation of excellent ripple-marks on the shore at Ryde 

 and Sandown showed that the angle of the slopes exposed to the 

 current was very nearly 18° in the case of both long and shorter 

 ripples. The surface would be very similar to that when a hori- 

 i^ontal one is tilted np 18*, and the grains would have to be washed 

 up this as well as over the small depressions between the grains, 

 so that for calculation we have 18° 4- 36^ = 54°. The sine of this 

 multiplied by the final velocity is -85 x •106 = -09 foot per second, 

 which multiplied by 6 gives "54 for the same velocity, in close 

 agreement with observation. 



Washing-away of Ripples. 



I found that in shallow water ripples are washed away when the 

 mean velocity of the current is 18 inches per second and upwards. 

 This is when, at the very bottom, it is so strong as not to allow sand 

 to remain at an angle of 18°. This result may not apply in the 

 ease of deep water. I fonnd, however, that some sand may remain 

 at the bottom with a current of 18 inches per second, grained in 

 the line of motion, though not in the form of ripples. Hence, 

 although the normal conditions for a horizontal grained surface are 

 not much above 6 inches per second, cases may occur when the 

 velocity is 18 inches, as may often bo seen in clear brooks; but it 

 would usually be easy to distiuguish the two conditions by studying 



