DUNES 19 



interval used on tin- atlas sheets of the Geological Survey is 5 feet. This is used 

 for regions like tin- MisMsMppi delta and the Dismal Swamp. In mapping great 

 mountain masses, like those in Colorado, the interval may be 250 feet. For 

 intermediate relief contour intervals of 10, 20, 25, 50, and 100 feet are used. 



"Drainage. Watercourses are indicated by blue lines. If the streams flow 

 the year round the line is drawn unbroken, but if the channel is dry a part of the 

 year the line is broken or dotted. Where a stream sinks and reappears at the 

 surfare. the supposed underground course is shown by a broken blue line. Lakes, 

 marshes, and other bodies of water are also shown in blue, by appropriate con- 

 ventional signs. 



" Culture. The works of man, such as roads, railroads, and towns, together 

 with boundaries of townships, counties, and states, and artificial details, are 

 printed in black." From folio preface, U. S. Geol. Surv. 



Explanation of Plate I. In Fig. i, Plate I (Five Mile Beach, 8 miles north- 

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

 contour line (the lo-foot contour), though this appears in several places. Since 

 this line connects places 10 feet above sea-level, all places between 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. i 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 de- 

 pression contours (4280 and 4260) within the contour of 430x3, near Spring Lake. 

 The bottom of the depression is therefore lower than 4260, but not so low as 4240. 



Migration of dunes. 1 By the transfer of sand from its windward 

 to its leeward side, a dune is moved from one place to another, 

 though continuing to be made up, in large part, of the same sand. 

 In their migration, dunes may invade fertile lands, causing so great 

 loss that means are devised for stopping them. The simplest meth- 

 od is to help vegetation to get a foothold in the sand. The effect of 

 the vegetation is to pin the sand down. 



Where dunes migrate into a timbered region, they bury and kill 

 the trees (Fig. 1 1). On the coast of Prussia a tall pine forest, cover- 

 ing hundreds of acres, was destroyed between 1804 and 1327. At 

 some points in New Jersey orchards have been so far buried within 

 the lifetime of their owners that only the tops of the highest trees 



1 Beadnell, Sand Dunes of the Libyan Desert, Geog. Jour., XXXV, 379, 1910. 



