Geolocrv and Paleoecology 



The Sweetgrass Hills originated during Tertiary mountain-building 

 activity as three igneous intrusions, i.e. lacoliths, that penetrated 

 sedimentary deposits, and have subsequently been exposed at the 

 surface. The highest summits are dome-shaped lacoliths, including the 

 West Butte summit and the twin peaks of East Butte, Mt. Royal and Mt. 

 Brown, composed of Tertiary intrusive rock consisting mainly of 

 syenite-diorite porphyry. The summits are made up of, exposed as 

 coarse sharp-edged talus rather than bedrock outcrop. The flanks of 

 all three buttes are Colorado Shale (Ross et al. 1955) with few 

 outcrops. In addition. East Butte has extensive areas of Madison 

 Group limestone, and Kootenai Formation sandstone, shale and mudstone. 

 West Butte has small areas of Eagle sandstone and the Madison Group 

 limestone surface geology. 



The igneous bedrock of the Sweetgrass Hills were more resistant to the 

 weathering affects of wind and water as well as the abrasion of 

 glaciers, compared to the prevailing sedimentary bedrock. The 

 Sweetgrass Hills were completely surrounded and overtopped by glaciers 

 about 54,000 years ago, shaping the north-south trending ridge 

 systems. It was noted that melting glaciers left behind giant 

 boulders in places, i.e., glacial erratics, and that some of these 

 granitic erratics also bear the marks of giant floods, with deep 

 globe-shaped potholes scoured into their surface. In subsequent 

 glacial episodes, the Sweetgrass Hills effectively represented islands 

 above the ice sheets during the more recent advances. These islands, 

 or "nunataks", provided habitat set apart from the surrounding plain 

 at some stage in glaciation. It has been suggested that migrating 

 belts of coniferous forest recolonized these islands following the 

 receding ice sheet (Thompson and Kuijt 1976) . In keeping with this 

 hypothesis, it was noted that the surrounding plains have abundant 

 potholes and closed drainage topography, representing areas of 

 stagnation moraine topography where glaciers were mantled by deep 

 layers of till, and suggesting that the most recent glaciers melted in 

 situ. In this paleoenvironment, regardless of whether or not the 

 nunataks were open or covered by snow and ice, such till deposits as 

 would have surrounded the Hills have been known to support boreal 

 floras (Love 1959) . The Sweetgrass Hills montane and subalpine flora 

 may have persisted locally, so that it is not necessary to invoke 

 forest migrations of hundred-mile distances in this paleoenvironment 

 model. If montane and subalpine flora persisted in the immediate 

 Sweetgrass Hills vicinity during the most recent glaciation, then the 

 Sweetgrass Hills is a refugia in the strictest sense, with local 

 shifts in species' distribution patterns accompanying the major 

 climate changes. 



During an interval of climate warming about 5,000 years ago, elements 

 of Great Plains and Great Basin floras expanded their range north, 

 establishing or increasing their presence on the tillplains and alkali 

 bedrock outcrops surrounding the Sweetgrass Hills. It appears that 

 many of the alkaline outcrops correspond with Eagle Sandstone 

 Formation along Eagle Creek and some Willow Creek tributaries. This 



