Geological Trail Intro Part 3
in Idaho
Almo Pluton Most of the pinnacles at City of Rocks and Castle Rocks are made of granite from the Almo pluton. The pluton started out as an intrusive body of magma and has a convoluted contact with the Green Creek Complex. It is the youngest rock at City of Rocks and formed about 28 million years ago.
Unlike the granite of the Green Creek Complex, the Almo pluton has an equigranular texture. Notice in the photograph the small uniform size of the crystals in the Almo pluton in contrast to the variety of sizes in the Green Creek Complex. Both types of granite are cut in many places by dikes of pegmatite, a very coarse-grained granitic rock that usually forms during the last stages of crystallization of a large granitic pluton.
The pegmatites at City of Rocks contain large quartz and feldspar crystals about the size of a fist.
- States
- Idaho
- Trail type
- National Reserve trail
- Centroid nearest city
- Salt Lake City, UT · 131 mi · ~3.8 hr drive
- Centroid coords
- 42.0896°, -113.6881°
About City Of Rocks National Reserve
This trail is inside City Of Rocks National Reserve, a national reserve managed by the U.S. National Park Service. Conditions, road status, trail closures, and reservation requirements are published on the park's NPS page — check it before driving in, especially in winter or during major weather events.
Official NPS trail page: https://www.nps.gov/places/geological-trail-intro-3.htm
Park homepage: https://www.nps.gov/ciro/index.htm
Plan your hike
Maps + permits: long-distance trails like this often require permits for through-hiking, backcountry camping, or specific sections (especially in National Parks). Check with the maintaining organisation listed above and the relevant land manager before booking travel.
Water + supplies: water sources vary seasonally on most U.S. trails. Carry a filter and consult current trail-condition reports — through-hiker journals (PCT-L, AT Reddit, etc.) and the maintaining organisation publish regular updates.
When to go: hiking seasons vary widely with elevation, latitude, and snowpack. Through-hikers traditionally start the AT in March-April (Springer northbound) and the PCT in late April (Campo northbound). High-elevation western trails (CDT, JMT, Wonderland) generally aren't passable until July.
If you've hiked Geological Trail Intro Part 3 and have current notes (water sources, trail closures, permit changes), tell us at /contact — we update pages as we learn.
Other trails within 50 miles
Stripe Rock Loop - 3.5 miles
0 miles from this trail's centroid
Geological Interpretive Trail - 1 mile
0 miles from this trail's centroid
Geological Trail Intro Part 2
0 miles from this trail's centroid
Geological Trail Intro Part 1
0 miles from this trail's centroid
Creekside Towers Trail - 1.2 miles
2 miles from this trail's centroid
City of Rocks Loop - 6.8 miles
2 miles from this trail's centroid
Sources
Trail data on this page is compiled from OpenStreetMap contributors (ODbL), the maintaining organisation's public-facing materials, and Wikipedia (CC BY-SA where excerpts are quoted). Distance, terminus, and descriptive text for nationally-designated trails are hand-curated from federal land-manager websites and trail-association sources. We do not modify the underlying data; this page presents what is already publicly recorded. To suggest corrections, see our methodology page.