The materials of these beaches are usually sand and ^avel, 

 sometimes bowlders. In the latter case, the bowldery beach 

 usually ends rather abruptly near the base of wooded banks. ^ 



2 — Fluviatile sands. — These are the ordinary sand and 

 gravel bars along our streams, and they also grade into higher, 

 older bars or flats, now seldom reached by the waters of the 

 stream excepting for a short period during high water. The 

 materials on these shores also vary from fine sand to gravel and 

 bowlders. Illustrations may be found along most of our streams, 

 excepting the prairie streams of the western part of the state, 

 the Cedar river furnishing the finest examples. See also Plate 

 I, figs. 2 and 3. 



3 — Sand-dunes. — The sand-dune areas of Iowa are quite lim- 

 ited and are practically restricted to two sections of the state: 

 those along the Missouri river, chiefly in Harrison county; and 

 those which lie chiefly in Muscatine county. The latter are in 

 two distinct groups, one occupying a part of Muscatine Island 

 and extending into Louisa county, and the other extending along 

 the Cedar river valley, chiefly in the vicinity of Adams and 

 Bayfield. 



The Harrison county dunes ^ are typical low dunes, seldom 

 reaching a height of 20 feet. They are formed from sands blown 

 up from the bars of the Missouri river, and heaped up usually 

 around clumps of willows or cottonwoods. They present various 

 phases of development, some being almost bare (see PI. II, fig. 1), 

 and frequently shifting, others are densely covered with vegeta- 

 tion (see PI. II, fig. 3), while still others present intermediate 

 phases (as in PI. II, fig. 2). The dunes frequently break down 

 (see PI. Ill, fig. 2), especially during dry, windy periods, and 

 even the roots of trees may be exposed by the shifting of the 

 sand, as shown in Plate III, fig. 3. The sand of these dunes is 

 derived from nearby bars of the Missouri river. 



The dunes of Muscatine county are less pronounced, some- 

 what scattered, and with the exception of portions of the north 

 end of the Big Mound, mostly covered with well-established veg- 

 etation, though this is somewhat variable during different sea- 

 sons. Three principal groups may be noted : — those on Musca- 



- See ibid., plate V, fig. 3, and plate VI, fig. 1. 



^ See ^vTite^'s brief description, with illustrations, in Iowa Geological Survej", vol. 

 XX, pp. 411-412; 1910. 



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