Fig. 123. Stratified Wind-Blown Sands,
Bermuda Islands
These islands are made wholly of limestone, the product of reef-building corals, and of lime from the sea water. The limestone sand of the beaches has been blown up into great dunes, some more than two hundred feet in height. Much of the loose dune sand has been changed to firm rock by percolating waters, which have dissolved some of the limestone and deposited it again as a cement between the grains.
Dunes occur not only in arid regions, but also wherever loose sand lies unprotected by vegetation from the wind. From the beaches of sea and lake shores the wind drives inland the surface sand left dry between tides and after storms, piling it in dunes which may invade forests and fields and bury villages beneath their slowly advancing waves. On flood plains during summer droughts river deposits are often worked over by the wind; the sand is heaped in hummocks and much of the fine silt is caught and held by the forests and grassy fields of the bordering hills.
Fig. 124. Cross Section of Transverse Dune after Reversal of Wind
Redraw diagram, showing by dotted line the original outline of the dune