Geologic Techniques in Civil Engineering 1123 



type of structure, whether bedded or massive; and, if possible, the min- 

 eralogy of the grains are all helpful to the engineer. Another pitfall in 

 lithologic descriptions is the use of the term "sandstone." For example, 

 along the Front Range of the Rocky Mountains the Fox Hills sandstone 

 is in many places a very well-cemented material and forms prominent 

 hogbacks. In areas of North and South Dakota, however, where the Fox 

 Hills formation has been recognized, it is a weakly cemented to unce- 

 mented sand. The term "sandstone" fails to bring out the poor quality 

 of this geologic formation, which, to the engineer, is a very unfavorable 

 foundation material. An accurate description of the geologic formations 

 occurring at the site is one of the basic requirements of sound engineer- 

 ing geology. 



Care must be taken in describing formations exposed in outcrops. 

 A number of clay-shale formations tend to air-dry on exposure, thus 

 exhibiting a more competent nature than when found in their unweathered, 

 unexposed condition. Many sandstones have a tendency to case-harden on 

 exposure to weathering agencies, consequently exhibiting a much harder 

 outcrop surface than in their natural condition. For this reason some 

 subsurface exploration, even though a small amount, should be carried 

 out at any site to determine whether the surface exposures of certain 

 geologic formations are a true indication of their natural characteristics. 

 Caution must be used in predicting the character of a geologic formation 

 from a knowledge of its character at some other location. Normal geologic 

 quadrangle maps may give a very excellent picture of the geologic forma- 

 tions present in an area; however, their quality and physical properties 

 cannot be told from the average geologic map. One of the long-range 

 phases of engineering geology is to conduct a large-scale mapping pro- 

 gram that will more specifically delineate the physical properties of par- 

 ticular geologic formations as shown on existing maps. 



Stratigraphy 



Stratigraphy basically is not so important in engineering geology as 

 in other forms of geologic study. The engineering geologist must, how- 

 ever, study the stratigraphic sequence of sedimentary rocks in any geo- 

 logic investigation in order that intelligent predictions may be made as 

 to the possible occurrence of various lithologic types. As has been pointed 

 out previously, the occurrence of geologic outcrops at a particular con- 

 struction site may be so few or of such quality that definite information 

 cannot be obtained. Thus, geologic interpretations must be made from 

 areas outside the immediate construction area. An understanding of the 

 normal stratigraphic sequence in the area will permit the engineering 

 geologist to make more accurate interpretations as to the types of rock 

 likely to occur at the construction site. 



An important phase of the exploration program in its early develop- 

 ment is the drilling of a sufficient number of holes to establish definitely 



