Highwood Bench. This same area was dissected and drained by the 

 ancestral Missouri River, which flowed in a northeasterly direction 

 to Hudson Bay. 



During the Pleistocene (one million to 15,000 years ago), 

 glaciers covered the entire region north of the Highwood Mountains 

 two or more times and left a mantle of unconsolidated, poorly sorted 

 deposits (glacial till) that filled most of the pre-existing valleys and 

 produced a gently rolling plain. The ice blocked the drainage of the 

 Missouri River and its tributaries, forcing the streams to change 

 course and cut new channels. During the last 15,000 years, erosion 

 established the present-day drainage pattern in northern Montana. 



The glacial till and underlying black shale of the Highwood 

 Bench are geologically important. Drill-hole information and exposures 

 along the Missouri River show till either absent or up to 70 feet thick. 

 Two distinct tills, representing two ice advances, are present--the 

 upper is normally the thicker and is buff to tan, whereas the lower 

 is generally only a few feet thick (absent in some localities) and is 

 light to dark gray. A pebble zone or meltwater layer is often found 

 between the tills. Except for the upper two or three feet- -the leach zone 

 of the soil profile--the entire till is loaded with salt crystals. 



The till is predominatly unsorted clay and silt with well- 

 rounded pebbles scattered throughout. X-ray analysis of the clay 



