Detailed geologic mapping (with emphasis on igneous units) at 

 1:48,000 scale shows the Ontario Creek and Telegraph Creek 

 headwaters sites underlain by undifferentiated Elkhorn Mountain 

 volcanics of late Cretaceous age. The wetland areas are near the 

 contact with the Boulder batholith ("Butte Quartz Monzonite" unit) 

 and are shown with a mappable (though probably thin) cover of 

 undifferentiated glacial deposits. 



The text includes extensive discussions of the mineralogy and 

 petrography of different phases of igneous rocks mapped in the 

 quadrangle. [Normative compositions of the Butte Quartz monzonite 

 are Ca>Na>K. ] Glacial deposits described include till and outwash; 

 granitic cobbles and boulders in till are described as relatively 

 deeply weathered, and the age of the glacial deposits is 

 interpreted as early Wisconsin. 



The Ontario Mine site is shown to be also underlain by the Butte 

 Quartz Monzonite (Boulder Batholith) near the contact of a mass of 

 aplite/alaskite underlying O'Keefe Mountain. The description of 

 mines within the quadrangle does not appear to include the mine 

 draining to the sample wetland, although the prospect is shown on 

 the map. 



♦♦Includes interesting mention of the 1898 injunction against mill 

 in the Tenmile Creek drainage by the City of Helena to protect 

 water supplies.** 



ping Byttg F^n 



Bibliographic source: Geoindex lat-long search 



Author: Mudge, M.R. , R.L.Earhart, J.W. Whipple and J.E. Harrison 



Title: Geologic and structure map of the Choteau 1 degree X 2 

 degree quadrangle, western Montana 



Series: U.S. Geological Survey Miscellaneous Investigations Series 

 Map 1-1300 



Date: 



Regional compilation at 1:250,000 scale shows Pine Butte proper 

 underlain by the Two Medicine and Horsethief Formations of Late 

 Cretaceous age. Ridges to the west of the fen are underlain by the 

 older Two Medicine Formation and underlying Virgelle Sandstone and 



57 



