Mount Helena Rocks! EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
Mount Helena is the city's backyard. Dominant on the skyline as you approach the city from the North, East or West, the mountain is a recreational mainstay of the community. Covered with trails, wildlife, varied topography and ecosystems it offers the locals a pleasant reprieve, any time of year, to the downtown/valley hubbub. At the time of the posting of this earthcache, it's also home to over 30 active geocaches. This cache is designed to allow you to know a little bit more about this iconic mountain whether or not you choose to seek out it's caches!
Helena sits on many beds of rock that date back to the Late Precambrian period, about 1,400 million years ago. Helena sits at the northeastern most part of the Basin and Range geological province. Here, as in Utah and Nevada, the continental crust has been stretched, resulting in a series of nearly parallel mountain ranges and valleys. Mountain building began in the late Cretaceous period and continues today. This area often experiences small seismic tremors.
Helena is a town that was built as a result of the mining boom of the 1800s, specifically the discovery of gold in 1864 near Last Chance Gulch. While the geology of Helena gave it a jump start, it was the people who built the city and community have made it a great place to live, work, recreate and raise a family today.
The major portion of the city overlies the Helena Dolomite of the Belt Super-group, a Late Precambrian formation of thick yellow-weathering dolomite. Deposits of Early Cambrian sediments have eroded, giving way to Middle Cambrian strata. Helena also sits on the northernmost zone of an immense granite or granodiorite field collectively known as Boulder Batholith. The main body of Helena is situated at the base of the northeast slope of Mount Helena on a pediment that extends northward into the Prickly Pear Valley.
The valley was named "Prickly Pear Valley" by Lewis and Clark in 1805 for its abundant, and irritating, prickly pear cactus. The main topographical feature of the Helena valley is the ever-so-formidable Mount Helena, rising almost 1,400 feet above the valley. Mount Helena City Park at its peak is 5,468 feet above sea level.
Mount Helena exhibits its strata like a layer cake, exhibiting six distinct decompositional layers. One of the Country's largest city parks, Mount Helena, offers a few differences compared to surrounding geologic formations. It is made up of ancient 520 million-year-old Cambrian rocks including limestone, shale, and sandstone. As you descend from the top, you traverse across the following formations:
Pilgrim Limestone > Light to medium grey limestone and dolomite with minor talc
Park Shale > Dull greenish-grey shale in saddle between north cliffs and peak
Meagher Limestone > Light-grey limestone forming the north cliffs
Wolsey Shale > Pale green, grey, reddish-brown shale; exposed on north slopes and in Reeder's Alley
Flathead Sandstone > Yellow/reddish-brown/white, cross-bedded sandstone and quartzite
Helena Dolomite > Pale tan-grey dolomite bedrock
Structurally, the layered rocks of Mount Helena form a syncline, which is basically a trough-shaped fold that in this case plunges down toward the southeast. At the base of Mount Helena, the now inactive Woolston Fault trends north-south under the reservoirs and truncates the east side of the syncline. Using a bit of imagination, you can get a sense of the layout of the rocks while viewing the mountain from the Spring Meadow Lake area. Think of it as a layered cake...slanting SE.
The parking coordinates for this cache place you at the Adam's Street Trailhead for Mount Helena. This area is accessed via S. Park Ave to the Reeder's Village Subdivision's city street system. The Adam's street approach is for foot and bicycle use only. The cache coordinates are several hundred feet away from the parking area. From this general area a variety of trails will take you to the summit of the mountain if you wish to tackle the trip. (See Map Board at site) Mt Helena has many miles of routes that will allow you to visit the different "layers", or rock formations, that make up the mountain if you feel inclined to do some field exploration in conjunction with this cache.
Logging the cache does not require a trip through all the rock formation zones but just a short walk, an eye for rocks and an email to the cache owner answering the following questions.
1. What is the name of the city reservoir(s) located at this location?
2.The reservoir area is carved into the slope. What rock formation (based on your understanding of the info above) has been excavated to allow the siting of the tank building?
3. You have walked southeast up a small hill from the parking area. This "nob" is
known as Daisy Hill. Are the rocks on top of Daisy Hill the same as those dug out to site the reservoir? What do you think these rocks indicate about top of Daisy Hill?
4. Describe the rocks at your feet. How do they feel to the touch? What color are they?
5. From here you can clearly see the excavation of the site. Describe the angle, in degrees, of the excavated formation visible beyond the water retention buildings to your NW.
Have Fun!
Congrats to derikkathryn on the FTF!
References:
Numerous sources used can provide more detailed information, and
are available at local bookstores, libraries, and the USGS:
Exploring Mount Helena (geology section by Ray Breuninger, 1997)
Hydrogeology of the Helena Valley-Fill Aquifer System (Dave Briar
& James Madison, USGS, 1992) Unpublished Paper - Young
Naturalists Awards
(visit link)
Profiles of Montana Geology (Montana Bureau of Mines and Geology
SP89, 1984) Roadside Geology of Montana (David Alt, 1986)
Additional Hints
(Decrypt)
Lbh pna jnyx nebhaq gur erfreibve srapr ba gur abegu fvqr naq neevir ng gur gnvyvatf cvyr sbe gur fvgr qvt va n srj zvahgrf. Lbh'yy trg "hc pybfr naq crefbany" jvgu gur qvfcynprq ebpx. Svir zvahgr jnyx jvgu n fubeg fgrrc frpgvba. Sbyybj gur srapr.