Adel Mountain Volcanics EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
These coordinates are for the information signs in the north bound I-15 rest area The the same information may be found at the south bound rest area.
While traveling along Interstate 15 between Wolf Creek and Cascade you are passing through the northern edge of the Adel Mountains. These are all that remain of a volcano that was active about 75 million years ago. The Adel Mountains Volcanics occurred along the Great Falls Tectonic Zone. This was a zone of weakness caused by the collision of two continental plates over a billion years ago.
The predominate rock of the Adel Mountains is shonkinite. A study of these of these rocks reveals large shiny black crystals of augite, a silicate mineral of the pyroxene group. These augite crystals are unique in that augite typically has a dull surface whereas the crystals found in the shonkinite are glossy. The large augite crystals are found in a matrix composed of very small crystals of augite and feldspar.
Although not much of a recognizable volcano is left after all these millions of years of erosion you can still see some traces. Among these are the remnants of the some of the plumbing in the form of dikes that are visible running up the mountain sides in several spots in the canyon. It’s also possible to see the results of ash and lava in some of the road cuts lying on sandstone or shale.These cuts reveal the base of the old volcano resting on older Crestaceous sedimentary rock.
Although having nothing to do directly with the volcanics, an interesting thought is the feature known as The Gates of the Mountains located nearer to Helena. When reading the journals of the Lewis and Clark Expedition they refer to the dark rock of what they named the Gates of the Mountains while the area now known as the Gates of the Mountains are very light colored limestone. Is it possible that the Gates of the Mountains are not properly sited?
To claim this cache email the owner the names of the prominent lacoliths in the area and how these laccoliths were formed.
Additional Hints
(No hints available.)