(Information and pictures adapted from Digital Atlas of Idaho and Roadside Geology of Idaho by David Alt)
Overview from Table Rock
Ancient Waters
Lake Idaho
Fire and water built the western Snake River Plain. Fire came in the form of volcanic eruptions, which spread red-hot lava and rained searing ash across southern Idaho. Today the Plain is world famous for its vast, stark volcanic landscape.
Once the volcanic rock was laid down, water began to take a dominant role. Between 9 and 2 million years ago a series of lakes formed, then dried up and formed again as the climate shifted. At times, these lakes joined and filled the valley. This enormous body of water is known as Lake Idaho.
The Boise Foothills make up a portion of the ancient shoreline of Lake Idaho. Imagine its shimmering water stretching before you for 200 miles, from present-day Weiser, Idaho to Twin Falls! The rivers that fed Lake Idaho also slowly filled the valley with sediment from the surrounding mountains. The lake sediments, which in places accumulated to more than 4,000 feet thick, underlie most of the Treasure Valley today.
Lake Idaho existed for more than 6.5 million years. Where did it go? Geologists aren’t certain, but many think that, between 2-4 million years ago, melting glaciers caused the lake to overflow and drain west in a massive flood that gouged Hell’s Canyon, the deepest canyon in North America.
http://www.blm.gov/id/st/en/fo/four_rivers/recreation_sites_/hulls_gulch_national0/lake_idaho.html
Table Rock and Geothermal Heating
http://imnh.isu.edu/digitalatlas/counties/geomaps/BoiseGeo.pdf Kurt Othberg and Virginia Gillerman
Granite of the Idaho Batholith
The Idaho Batholith is a composite mass of granitic plutons (bodies of magma that cooled under ground) covering approximately 15,400 square miles in central Idaho. It is approximately 200 miles long in the north south direction and averages about 75 miles wide in an east west direction. There are two different pieces of the Idaho Batholith, the Bitterroot lobe in northern Idaho and the Atlanta lobe in southern Idaho. The Atlanta lobe formed 75 to 100 million years ago (Late Cretaceous); the Bitterroot lobe formed 70 to 80 million years ago.
Granitic outcrops of the Idaho Batholith are easy to recognize. When looking closely, granite has a salt and pepper appearance. The dark minerals are biotite mica and hornblende and light minerals are plagioclase feldspar and quartz (show samples from class mineral and rock collection). The minerals are up to an inch or more in diameter and can readily be identified without a hand lens.
Weathered outcrops of granite have a distinctive appearance and can in some cases be identified at a mile or more distance. Coloration tends to be very light gray to very light tan, and in some places chalk white due to leaching by hot water. Outcrops are generally smooth and rounded due in part to surface weathering. Granite and basalt are among the easiest rocks in Idaho to identify.