The spectacular Bitterroot Mountains northwest of Sula expose granite rocks of the Idaho batholith, a major geologic feature that consists of a series of igneous intrusions that pushed their way toward the surface between about 80 and 53 million years ago. The molten magma tha formed these intreusions forced its way into older rocks and crystallized more then ten miles below the surfce. As the magma rose upwards, it reaised up the overlying rocks, which sloughtedoff an enormous block that slid to the east, forming the Sapphire Range on the east side of the Bitterroot Valley. It took about sevenmillion years for this block to slowly slide along a surface that forms the eastern slope of much of the Bitterroot Range. The granite rocks exposed along U.S. Highway 93 in the Sula area are part of this block that was once in the presetn Bitterroot RAnge. About 50 million years ago, magma again rose up through the crust of the earth resulting in teh eruption of large volumes of volcanic rock in the southern Bitterroot Range southwest of Sula.

Glaciers capped much of hte Bitterroot Range and carved dramatic U-shaped profilesinto side drainages that flow eastward into the Bitterroot Valley. The last glaciation ended about 15,000 years ago. Multiple times during the glacial ages, a glacier dammed the Clark Fork River near the Idaho/Montana border,forming Glacial LakeMissoiujla. The highest lake stand reached an alittude of 4,200 feet above sea level, forming a lakeshore only a few miles downstream of Sula. As you travel down the Bitterroot Valley, imagine the ice-age scene: a lake hundredsof feet deep lapping onto the base of the mountains on either side of the valley and icebergs breaking off from the glaciers flowing in fromthe Bitterroots.

Credits and Sources:

Sign text.

Montana's Historical Highway Markers, Jon Axline, Montana Historical Society Press. 2008.