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Eastern Pioneer Sedimentary Mountains

Ecoregion in Rocky Mountains, MT

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The rather dry Eastern Pioneer Sedimentary Mountains ecoregion lies south of the Continental Divide and is mostly forested. It is characterized by Paleozoic and Mesozoic sedimentary rocks and is lithologically distinct from the intrusive and extrusive rocks of the neighboring Elkhorn Mountains-Boulder Batholith (17ai). Limestone is common and strongly affects water quality, available water quantity, soils, and aquatic biota. Granitic intrusions occur and have locally reacted with limestone to produce zones of skarn that have been mined for the tungsten mineral scheelite. Average annual precipitation ranges from 12 to 20 inches. The Eastern Pioneer Sedimentary Mountains (17ah) ecoregion was glaciated but has fewer lakes than the Pioneer-Anaconda Ranges (17ag) ecoregion which is wetter and more westerly. Climax vegetation is mapped as subalpine fir and Douglas-fir forests.

— EPA

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