fraction indicates 80-percent montmorillonite (highly plastic, sodium- 

 rich, expanding clay), 15-percent illite, and five-percent kaolinite 

 or chlorite (2). A few sand and gravel lenses were found in some 

 test holes, but they are normally thin, discontinuous, and small in 

 area. Numerous vertical joints are believed to enhance the vertical 

 movement of water through the till. Infiltration studies on poorly 

 drained upland (recharge) areas indicate down>vard movement of over 

 five inches of water a day. These high infiltration rates greatly 

 exceed previous estimates. 



The black bentonitic marine shale of the Colorado Group that 

 underlies the entire Highwood Bench is 950 to 1,850 feet thick. Owing 

 to erosion and the gentle dip to the northeast, the shale thins to the 

 southwest toward the Little Belt Mountains. A weathered zone, one 

 to eight feet thick at the till- shale contact, is commonly saturated 

 and appears to be the only slightly permeable shale layer. The 

 unweathered shale beneath is completely dry. 



Close correspondence between local precipitation and water- 

 table fluctuations means that excess water is moving through the soil 

 to beneath the root zone, through the remainder of the till, and eventually 

 accunnulating on the bedrock. Most of this movennent occurs in spring, 

 particularly April, May, and the first part of June. The water table 

 may rise a few inches to several feet in years with average or above- 



