11 



POTENTIAL REGIONAL PROBLEM 



As noted earlier, Montana has already lost over bO, 000 acres 

 of cropland to saline seep and the area affected is increasing by over 

 10 percent a year. Geological conditions that favor saline seep-- 

 a variable thickness of glacial till underlain by thick sequences of 

 black marine 8hale--are similar over vast areas of Montana (12,500 

 square miles), North and South Dakota (45, 5C0 square miles), and 

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

 seeps are spreading in that entire region and also in farming areas 

 underlain by the siltstone, sandstone, shale, and coal of the Fort Union 

 formation (21). Here again, excess water is moving downward and 

 accumulating on thin impermeable underclays, in this case forcing 

 the water to move laterally along coal seams until it breaks out at 

 the surface. The farmed portion of the Fort Union area cover another 

 100,000 square miles (4,500 in Montana), making a total of 228,000 

 square nniles (17, 000 in Montana or about 10. 5 million acres) of 

 potential saline seep in the northern Great Plains (Figure 2). These 

 plains are the major grain-growing region for North America. The 

 cropping sequence over the entire region--generally an alternate crop- 

 fallow system- -is the same as that on the Highwood Bench, 



Over 90 percent of eastern Montana's cultivated dryland is in 

 the Missouri River Basin. Based on discharge records at Fort Benton 



