644 whitmore. MILITARY GEOLOGY [Ch. 34 



traffic. But determination of these factors by no means solves the 

 problem, because cross-country movement is also influenced by natural 

 and artificial obstacles, snow cover, and the greatly varying ability of 

 the military (or other) driver. 



In preparing for a military operation, staff planners must be in- 

 formed about where vehicles can move and where they cannot. If the 

 expectable amount of precipitation in any season can be determined, 

 and the physical properties of the soil have been deduced, a reasonable 

 prediction can be made of the length of time necessary for the drying of 

 various soils after rain or snow to the point where cross-country move- 

 ment is again possible. 



During the recent war, the Military Geology Unit of the U. S. Geo- 

 logical Survey prepared trafficability maps for use in France and Ger- 

 many, using as sources a series of excellent published bedrock, soil, 

 and glacial drift maps. The trafficability maps were based on the 

 following legend: 



1. Mountains and steep hills (most slopes 25 percent or more); steep, rocky, 

 and terraced slopes. Vehicular movement generally confined to roads; 

 tracked vehicles can operate on some mountain slopes. 



2. (a) Hills with sandy or gravelly soils (slopes 7.5 to 25 percent). Traf- 



ficable in all weather. 



(b) Hills with loamy soils (slopes 7.5 to 25 percent. Trafficability ham- 

 pered by mud in wet weather, good in dry weather; soils dry quickly. 



(c) Hills with clayey soils (slopes 7.5 to 25 percent). Trafficability ham- 

 pered by deep mud in wet weather; good in dry weather; soils dry 

 slowly. 



3. (a) Gently sloping and level land; sandy or gravelly soils (slopes less than 



7.5 percent). Trafficability generally good to excellent in all weather. 

 Locally sand dunes hamper movement. 



(b) Gently sloping and level land; loamy soils (slopes less than 7.5 per- 

 cent). Trafficability hampered by mud in wet weather; good in dry 

 weather; soils dry quickly except where water table is high. 



(c) Gently sloping and level land; clayey soils (slopes less than 7.5 per- 

 cent). Trafficability hampered by deep mud in wet weather, good in 

 dry weather; soils dry slowly. 



4. (a) Flat, low-lying, wet meadowland. Trafficability very poor except when 



frozen or after prolonged dry spells; soils sandy to peaty; water table 

 generally near or at surface. 

 (6) Swamps and peat bogs. Cross-country vehicular movement difficult 

 or impossible because of boggy soil conditions. Foot movement very 

 difficult except when ground is frozen. 



5. Drained swamps. Untrafficable because of closely spaced canals and ditches. 

 Destruction of drainage system causes flooding and boggy soils. 



