1911.] NATURAL SCIENCES OF PHILADELPHIA. 539 



The sandy strand was always ripple-marked after each tide, if 

 the surface of the water in the bay was somewhat ruffled by the 

 wind. The incoming tide pushed the sand along and, combined 

 with the wave motion in the shallow water, moulded the sandy 

 surface into a series of ripples which closely resemble those made 

 by the wind on dry, sandy surfaces. They are mainly ripples of 

 deposition, the sand surface moving with the advancing water as 

 the dry sand surface, thrown into wind ripples, advances under the 

 force of the wind. But there is also a little erosion in the shaping 

 of these ripples, although the main action of the water is to deposit 

 material. 



The sand ripples which form along the strand are quite large; 

 they range from three to four inches from crest to crest, and some- 

 times are even wider. The depth of the trough is f inch or more in 

 some cases. Their general appearance is well shown in fig. 1, which 

 is a photograph of this rippled surface taken looking vertically down 



Fig. 2. — Section of ripples of deposition. 



from above. Here the left of the picture is that portion of the 

 strand towards the open water of the bay and the right is towards 

 the land. When the tide is coming in the water and sand trans- 

 ported by it are moving from the bay towards the land. The slope 

 of the ripple towards the water or bay side is gentle; up this slope 

 the advancing water pushes the sand grains, which, after they pass 

 the crest, fall down the steeper landward slope of the ripple and 

 assume the angle of repose, making the ripple steeper on this land- 

 ward side. The advancing water strikes the bay slope of the next 

 ripple and removes a portion of its surface, from the fainter, smaller 

 crest seen on this slope in the photograph up to the crest itself. 

 The result is that the landward slope continually advances, while 

 parts are cut away from the bay-side slope so that the one grows by 

 deposition of the material removed from the other. The top of 

 the ripple is rounded with one steep side, sloping down to a rather 

 sharp trough, and one gently sloping side, running from this trough 



