1911.] NATURAL SCIENCES OF PHILADELPHIA. 541 



Where these strand ripples of deposition are reworked by the 

 receding tide they are deepened in the trough, the steep side becomes 

 rather steeper and the crest crumbles away, becoming sometimes 

 narrower, sometimes even sinking locally by the softening and 

 collapse of the sand when impregnated with water. The waves 

 in the water undoubtedly condition the formation of the ripples in 

 the sand. In fig. 3 the rippled strand is shown after the tide has 

 partly receded and the small water waves seen on the surface of the 

 bay are similar to those that have rippled the sandy shore. The 

 sands here are derived from the raised beach of the Saxicava sands, 

 which formation is formed on both sides of the estuary of White 

 Bear River. They are being continually washed down from the 

 banks of this raised beach upon the Leda clays, which they overlie 

 stratigraphically. 



The clays of the flats are finely rippled in some places over large 

 surfaces. The appearance of these rippled surfaces is well shown 



'0&$&&jtUty&'\ y^f^^W^M^^^^^ 



Fig. 4. — Section of ripples of erosion. 



in the photograph, Plate XLI, fig. 1, which was taken on the flats 

 about one-quarter of a mile to the west of our camp. The boulder- 

 strewn surface of the flat is very noticeable at the back of the picture. 

 These ripples were carved out of the hard, tough surface of the Leda 

 clay, and were so firm that in walking over them in the Eskimo 

 skin boots we wore we left scarcely any impression; and even a 

 bear, traversing this ripple-marked surface, did not flatten down 

 the crests of the ripples very much. They were quite uniform in 

 dimensions, as the picture shows, with a width from crest to crest 

 of the ripples of about one and three-quarters to two inches and a 

 depth amounting to not more than one-quarter of an inch. Unlike 

 the ripples of deposition, these erosion ripples had a rather sharp 

 crest, about one-quarter of an inch broad; the trough was concave, 

 nearly or quite symmetrically so, and rounded out with a gentle 

 curve (see fig. 4). They did not cover the entire surface of the flat, 

 but were more pronounced where the water had to travel a greater 

 distance over the flat, and where, therefore, there was more current. 

 36 



