At localities where impervious formations (clay, etc.) occur at or near the surface, they hold the water and form a superficial zone of saturation. This condition makes trench construction and maintenance difficult, and cave shelters can usually only be made by providing means of sinking through the saturated zone. The surface saturated zone often dries out in summer.
In pervious, or almost pervious rocks, the zone of saturation, or ground-water level, lies at much lower depth, and may permit of the construction of field works as well as cave shelters above it.
Underground water bearing horizons and water bearing faults should be avoided in locating field works.
Wherever there is any uncertainty about the underground water conditions, test shafts or bore holes should always be made in advance of the construction of extensive deep works.
EFFECT OF THE WAR ON THE SCIENCE OF ECONOMIC GEOLOGY
In general, the war required an intensive application of geology along lines already pretty well established under peace conditions. Much was done to make the application more direct and effective, and a vast amount of geologic information was mobilized. The general result was a quickened appreciation of the possibilities of the use of geology for practical purposes. Perhaps the most important single result was a wider recognition of the real relations of mineral resources to human activities, and of the international phases of the problem. More specifically, there was a most careful stock-taking of mineral resources and a consideration of the "why" of their commercial use. Many new resources were found, as well as new ways to utilize them.
FOOTNOTES:
[60] Now known as Economic Liaison Committee.
[61] Military geology and topography, Herbert E. Gregory, Editor. Prepared and issued under the auspices of Division of Geology and Geography, National Research Council, Yale Univ. Press, New Haven, 1918.
[62] Davis, W. M., Handbook of Northern France, Harvard Univ. Press, Cambridge, 1918.