262 black. PERMAFROST [Ch. 14 



engineer or both on a job encounter "cut and dried" situations, and 

 it is obvious that personal discretion must be exercised in modifying 

 the outlines to meet the situation at hand. 



In reconnaissance or preliminary survey to select the best site for 

 construction in an unknown area, it is recommended that the approach 

 be one of unraveling the natural history of the area. Basically the 

 procedure is to identify each land form or terrain unit and deter- 

 mine its geologic history in detail. Topography, character and dis- 

 tribution of materials, permafrost, type and distribution of vegetation, 

 hydrology, and climate are studied and compared with known areas. 

 Then inferences, deductions, extrapolations, or interpretations can be 

 made with reliability commensurate with the type, quality, and quan- 

 tity of original data. 



Thus the solution of the problems depends primarily on a complete 

 understanding of the thermal regime of the permafrost and active 

 layer. No factor can be eliminated, but all must be considered in a 

 quantitative way. It is understandable that disagreement exists on 

 the mean annual air temperature needed to produce permafrost. Few, 

 if any, areas actually have identical conditions of climate, geology, 

 and vegetation; hence, how can they be compared directly on the 

 basis of climate alone? Without doubt the mean annual temperature 

 required to produce permafrost depends on many factors and varies at 

 least several degrees with variations in these factors. For practical 

 purposes, however, units (terrain units) in the same climate or in 

 similar climates may be separated on the basis of geology and vegeta- 

 tion. Thus there is a basis for extrapolating known conditions into 

 unknown areas. 



The advantages of aerial reconnaissance and study of aerial photo- 

 graphs for preliminary site selecting are manifold. Aerial photographs 

 in the hands of experienced geologists, soils engineers, and botanists 

 can supply sufficient data to determine the best routes for roads and 

 railroads, the best airfield sites, and data on water supply, construc- 

 tion materials, permafrost, trafficability conditions, camouflage, and 

 other problems. Such an approach has been used with success by the 

 Geological Survey and other organizations and individuals (Black and 

 Barksdale, 1949; Wallace, 1948; Woods et al., 1948; Pryor, 1947). 



Emphasis is placed on the great need for expansion of continuing 

 applied and basic research projects as outlined by Jaillite (1947) and 

 referred to by Muller (1945) for a clearer understanding and evalua- 

 tion of the problems. 



