Lost River Range Volcanoes.
by Rick Baugher
Volcanoes and related igneous flows are a staple of the Pacific Northwest landscape, so it is interesting to observe that Idaho's highest mountain range presents a 60 mile front of sedimentary limestone.
How did these Lost River mountains align the way they did, and how did
they reach their current height? Much can be explained by the movements of
the North American tectonic plate over the last 11 million years, when
compression and extension forces initially faulted the surface crust. Also,
two glaciations over the past 100,000 years left their mark. But a far more
powerful source, arising from deep within the earth, are the volcanoes.
Their presence can be seen along much of the backside of the Lost River
Range. The largest igneous flow begins at the mouth of Pass Creek, continues
north then northwest, to form a large apron around the range. The keen
observer, with a little imagination, can trace the roots of long gone
Mesozoic stratovolcanoes on the order of Rainier and Orizaba. The limestone
which piled up against their eruptive flanks gave the birth to our Lost
River mountains. Three stratovolcano locations, tentatively identified are:
Arco Valley (between King Mtn & Arco Hills), Wet Creek Basin, and Upper
Pahsimeroi.
Here's a look at some remnant Lost River volcanic peaks. Clockwise from
top left.
VULCAN PEAK 10297' The highest volcanic summit in LRR. View S to USGS Peak,
1999.
APRON MOUNTAIN 9382' View NE to Hawley Mtn & Lemhi Range, showing Wet Creek
stratovolcano caldera, 2007.
EAST BURNT PEAK 10149' A prehistoric cairn? View SW to Cleft Peak, 1999.
THE CAPE 8387' Iron rich andesite of Eocene Challis Volcanics. View S to
Massacre Mtn, 2007.
GROUSE PEAK 8464' The northernmost Pahsimeroi/LRR summit overlooks the Round
Valley caldera. A TM Bannon 1914 triangulation station, 1994.
VICTORIAS SECRET 8860~ Beauty is in the eye of the beholder, 2007.
