34 



coulees and "seeped" drainageways were included, the affected area 

 would be much greater. 



17. Geological conditions, that is, a variable thickness of 

 glacial till underlain by thick sequences of black marine shale (Colorado 

 and Bearpaw Formations), are similar over vast areas of Montana 

 (12, 500 square miles), North Dakota and South Dakota (45, 500 square 

 miles) and the three prairie provinces of Canada (70,000 square miles). 

 In addition to the above-mentioned region, saline seeps are also spreading 

 in areas underlain by siltstone, sandstone, shale, and coal (Fort Union 

 Formation). Here again excess water is moving downward and accumu- 

 lating on thin impermeable shale beds forcing the water to move laterally 

 along coal seams until it breaks out at the surface. This area covers 

 an additional 100,000 square miles making a total of 228,000 square 

 miles of potential saline- seep development in the northern Great Plains 

 region of the United States and Canada. This is the major grain-growing 

 region for both countries. The cropping sequence over this entire 

 region--generally an alternate crop-fallow system--is the same one 

 being used on the Highwood Bench, northcentral Montana. 



