SMITH 53 



RECOGNITION AND INTERPRETATION OF DUNE TERRAIN 



Identification of dune terrain is generally possible from analysis of landforms as shown 

 on air photos. For the simpler types of dunes, this may be done in an empirical way by per- 

 sonnel of limited scientific training. For the more complex types, however, and for dune areas 

 which have undergone modification during long intervals of stabilization, the experience and 

 training of the specialist are required. 



Interpretation follows after identification and may be concerned with one or more of the 

 following factors: 



(1) Direction of the effective sand-moving winds. For areas in which no meteorological 

 records are available, dune forms may provide the best available data on wind direction. Some 

 caution is necessary in making interpretations, however, for such a factor as unequal exposure 

 to winds from different directions may introduce qualifications, particularly in the case of 

 partially stabilized dunes. 



(2) Offshore conditions. Analysis of the source of sand supply for dune building may 

 lead to significant conclusions as to nature of the bottom on the seaward side. In some places 

 it has been determined that the sand now incorporated in stabilized dunes could have been de- 

 rived only from sandy flats some distance from the present shoreline and exposed at a time 

 when sea level was much lower. 



(3) Present trend in coastal development. Tendencies toward erosion or fill along the 

 shore, important in the planning of permanent installations, may be recorded in the form and 

 pattern of dunes. 



(4) Ground and underground conditions. The range in size of sand grains that are readily 

 moved by the wind is comparatively small, thus dune sand is characteristically well sorted and 

 generally is composed mainly of grains between 1/2 and 1/8 mm in diameter. Identification of 

 dune topography, therefore, leads at once to the conclusion that incoherent, fine sand lies at or 

 near the surface. Some implications of this fact are outlined in a following section. Although 

 the characteristics of the subsoil, as noted above, are relatively constant, the nature of the 

 soil on stabilized dunes ranges widely, particularly with respect to thickness and degree of 

 stability. Correlation of these characteristics with the details of dune morphology is a matter 

 for continued research, and it is hoped that this project will contribute useful data. 



(5) Other terrain conditions affecting military operations, as discussed in the next sec- 

 tion of this paper. 



The general directions in which interpretation, particularly photo interpretation, of dune 

 terrain for practical purposes may proceed are thus marked out. Progress in these various 

 directions, however, will be conditioned to a large extent by continued basic research on the 

 geology and geomorphology of the widest possible variety of coastal dune areas and types. 



ENGINEERING AND MILITARY ASPECTS OF DUNE TERRAIN 



Dune sand in general has the following characteristics which are important from the 

 engineering standpoint: 



(1) It is easily excavated, but sides of excavations require support. 



(2) Drainage is excellent, except in low-lying hollows or depressions. 



(3) It provides good foundations for roads and other structures, if properly confined. 



(4) It provides excellent fill material, and has been used extensively for this purpose 

 near Chicago. 



