Ch. 34] TRAFFIC ABILITY 643 



it in terms of porosity, toughness, and mode of occurrence (large coral 

 heads with matrix of fine-grained coral fragments). 



In turn, the soil types of the Marianas were defined, on the basis of 

 parent materials, vegetation, and relief, as follows: 



(1) Stony clays, clay loams, and loams derived from limestone. 

 Residual soil, containing friable clay (the result of laterization) , with 

 sand content dependent on amount of sand in parent material. 



(2) Clay soils derived from volcanic rocks. Generally thin mantle 

 of friable red clay. 



(3) Coarse-textured alluvial soils of coastal plains (texture and 

 composition varying with source material, distance transported, and 

 relief) . 



(4) Fine-textured alluvial soils and swamp soils. 



When the origin of these sediments is known, it is possible to predict 

 their engineering properties with reasonable accuracy. For example, 

 the residual friable lateritic clay of the Marianas, Ryukyus, and other 

 tropical and subtropical islands is known to be fairly well-drained and 

 to dry rapidly when not disturbed by vehicles or other heavy use ; it is 

 generally shallow, of irregular depth, with numerous rock outcrops. 



Even more closely related to the study of sedimentation is the 

 identification in aerial photographs of depositional features, such as 

 dunes, bars, spits, beaches, glacial moraines of recent origin, stream- 

 terrace deposits, active alluvial fans, and some volcanic deposits 

 (Hack, 1948). These are especially recognizable in dry or cold clim- 

 ates where they are not obscured by vegetation, and their texture and 

 composition are determined easily enough to be well-defined in simple 

 terms. However, where vegetation mantles the earth, it can often 

 be turned to the advantage of the terrain analyst as an indicator of 

 the underlying materials. Vegetation is being used in this way in the 

 study of permafrost (see Chapter 14 in this symposium) , and it was 

 utilized with great profit during the war to predict tropical ground 

 conditions. For instance, the nipa and sago palm of the southwest 

 Pacific indicate poor drainage conditions wherever found. Unfortu- 

 nately, most plants are not restricted to a single type of substratum, or 

 their ecology and relation to bedrock and soil are not yet sufficiently 

 understood (U. S. War Department, 1944a). 



Tkafficability 



The possibility of cross-country vehicular movement depends pri- 

 marily on the slope, porosity, and permeability of the ground, and 

 on the cohesion of its constituent particles under repeated passes by 



