GEOLOGICAL WORK OF THE ATMOSPHERE 97 



it is always distinctive. Where the dunes take the form of ridges 

 (Fig. 1, PL I), the ridges are often of essentially uniform height 

 and width for considerable distances. If there are parallel ridges, 

 they are often separated by trough-like depressions. Where 

 dunes assume the form of hillocks (Figs. 2 and 3, PL I), rather 

 than ridges, the topography is even more distinctive. In some 

 regions depressions (basins) are associated with the dune hillocks. 

 Occasionally they are hardly less notable than the dunes them- 

 selves. 



Explanation of Plate I. In Fig. 1, Plate I (Five Mile Beach, 8 miles 

 northeast of Cape May, N. J.), the contour interval is 10 feet. There is here 

 but one contour line (the 10-foot contour), though this appears in several 

 places. Since this line connects places 10 feet above sea-level, all places be- 

 tween it and the sea (or marsh) are less than 10 feet above the water, while 

 all places within the lines have an elevation of more than 10 feet. None of 

 them reaches an elevation of 20 feet, since a 20-foot contour does not appear. 

 It will be seen that some of the elevations in Fig. 1 are elongate, while others 

 have the forms of mounds. (From Cape May, N. J., Sheet, U. S. Geol. Surv.) 



Fig. 2 shows dune topography along the Arkansas River in Kansas (Larned 

 Sheet), and Fig. 3, dune topography in Nebraska (Camp Clarke Sheet), 

 not in immediate association with a valley or shore. In Fig. 2 the contour 

 interval is 20 feet. All the small hillocks southeast of the river are dunes. 

 Some of them are represented by one contour, and some by two. In Fig. 

 3, where the contour interval is also 20 feet, there are, besides the numerous 

 hillocks, several depressions (basins). These are represented by hachures 

 inside the contour lines. In some cases there are intermittent lakes (blue) 

 in the depressions. There are two depression contours (4280 and 4260) 

 within the contour of 4300, near Spring Lake. The bottom of the depression 

 is therefore lower than 4260, but not so low as 4240. 



Migration of dunes. By the continual transfer of sand from 

 its windward to its leeward side, a dune may be moved from one 

 place to another, though continuing to be made up, in large part, 

 of the same sand. In their migration, dunes sometimes invade 

 fertile lands, causing so great loss that means are devised for stop- 

 ping them. The simplest method is to help vegetation to get a 

 foothold in the sand. The effect of the vegetation is to pin the 

 sand down. As a dune ridge along a coast travels inland, another 

 may be formed behind it, and successions of dune ridges are thus 

 sometimes formed. 



