52 SMITH 



distributed around the British Isles and the Mediterranean. In north Africa, there are numerous 

 areas of coastal dunes, and in some places the great areas of interior dunes come so close to 

 the coast as to enter into consideration of coastal problems. In South America, dunes are ex- 

 ceptionally well developed at many places in the desert coastal strip of Peru. 



GENERAL DESCRIPTION OF COASTAL DUNES 



The appearance of dune areas ranges from that of comparatively simple and well-defined 

 unit dune forms to complex mazes of ridges, mounds, and hollows with seemingly extreme de- 

 grees of disorder. The simpler forms comprise the following: 



(1) Foredune ridges, or elongate mounds of sand up to a few tens of feet in height, ad- 

 jacent and parallel to beaches. 



(2) U-shaped dunes, arcuate to hairpin- shaped sand ridges with the open end toward the 

 beach. 



(3) Barchans, or crescentic dunes, with a steep lee slope on the concave side, which 

 faces away from the beach. 



(4) Transverse dune ridges, trending parallel or oblique to the shore, and elongated in a 

 direction essentially perpendicular to the dominant winds. These dunes are asymmetric in 

 cross profile, with a gentle slope to the windward and a steep slope on the leeward side. 



(5) Longitudinal dunes, elongated parallel to wind direction, and extending perpendicular 

 or oblique to the shoreline; cross profile is typically symmetric. 



(6) Blowouts, comprising a wide variety of pits, troughs, channels, and chute-shaped 

 forms cutting into or across other types of dunes or sand hills. The larger ones are marked 

 by conspicuous heaps of sand on the landward side, assuming the form of a fan, mound, or 

 ridge, commonly with a slope as steep as 32 degrees facing away from the shore. 



(7) Attached dunes, comprising accumulations of sand trapped by various types of topo- 

 graphic obstacles. 



The above types of dunes may exist in either an active or a stabilized condition. The 

 active dunes have loose, bare sand on all or part of the surface, and undergo continuous change 

 in size, shape, and/or position under the impact of strong winds capable of drifting the sand. 

 Stabilized dunes are those so well covered by vegetation as to inhibit further drifting of the 

 sand. Different degrees or stages of stabilization may be distinguished, and in advanced stages 

 a well-developed soil is present. Any dune may become stabilized if conditions permit the 

 spreading of vegetation over its surface. Also, any stable dune may become active again if the 

 cover of vegetation is weakened or destroyed, either locally or generally, as by fire, deforesta- 

 tion, excavations, climatic changes, etc. Large-scale reactivation of dunes by the work of man 

 has been known to do great damage by overwhelming arable lands and settlements and remov- 

 ing the dune areas themselves from profitable uses. 



The simpler types of dunes, whether active or stabilized, exhibit a wide range of modifi- 

 cations and variations, and the over-all characteristics of dune assemblages are subject to 

 innumerable complications by the crowding or merging of individual dune forms, by alterna- 

 tions between activity and stabilization, by the juxtaposition or superposition of one type or 

 scale of dune form on others of different type or scale, by shifts in wind direction during dune 

 building, by wave erosion, and by other factors. Much remains to be learned both about the 

 general principles governing these complications and about the detailed features of specific 

 areas of coastal dunes. 



