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whitmohe. MILITARY GEOLOGY 



[Ch. 34 



surface expression of known rocks, to a close approximation of the 

 geology of the rest of the area. This was done during the war in the 

 preparation of most of the Strategic Engineering Studies of the Military- 

 Geology Unit, U. S. Geological Survey, prepared for the Office of the 

 Chief of Engineers. An example is the terrain-intelligence study of the 

 Marianas Islands, prepared in 1944. No detailed information on geol- 



Fig. 2. Oblique aerial photograph of Aguijan Island, Marianas Group, looking 

 south. (U. S. Navy photograph.) 



ogy or soils was available, and the most complete Japanese articles 

 were available only in abstract. There were no surveyed topographic 

 maps. However, the geologists had access to low-level oblique aerial 

 photographs, taken one month before the study was begun (Fig. 2). 

 These showed the southern islands of the group (Guam, Saipan, Tinian, 

 Aguijan, and Rota) to consist largely of raised coral-reef terraces. The 

 higher terraces, generally above 600 feet, are much dissected; the lower 

 terraces are broad and flat. Volcanic extrusives form rugged spines 

 on some of the islands. Alluvium fills some small coastal embayments. 

 Brief published notes, plus knowledge of volcanoes to the north, led 

 to the identification of limestones with interbedded tuff; elsewhere, un- 

 interrupted limestone sequences were known to exist. This knowledge 

 of the approximate distribution of the few rock types allowed pre- 

 diction of the composition of the alluvium. The known mode of 

 formation of reef rock made it a relatively simple matter to define 



