Agency (EPA) relating to water quality problems arising from saline 

 seep. 



HYDROGEOLOGICAL SETTING 



Much of the definitive hydrogeological work relative to saline 

 seep has been conducted on the Highwood Bench near Fort Benton. 

 The details of geological history and mode of seep formation vary from 

 area to area, but the situation on the bench may serve as a general 

 model for the entire northern Great Plains region. 



The geological history of northern Montana and the bench 

 included long periods of sedimentation, emplacement of volcanic and 

 plutonic igneous rocks, regional uplift, erosion, and glaciation. 

 Throughout most of the Paleozoic and the Mesozoic (600 to 70 million 

 years ago), thousands of feet of predominatly marine sediments were 

 deposited in this region. At the end of the Mesozoic (late Cretaceous) 

 and extending into the early Tertiary (70 to 50 million years ago), 

 the entire area was uplifted, faulted, and folded, producing the Rocky 

 Mountains to the west and south, gently tilting the sedimentary rocks 

 to the northeast, and subjecting the area to erosion. The emplacement 

 of volcanic and plutonic rocks that form the Highwood and Bearpaw 

 Mountains also occurred during this time. Continued erosion during 

 the Tertiary stripped away the uppermost Cretaceous sediments and 

 exposed the black shale of the Colorado Group over what is now the 



