Massacre Rocks, ID 2009


In May of 2009, I took a small group of students to camp at Massacre Rocks, Idaho. Massacre Rocks State Park is located 16 km west of American Falls, Idaho. Our plan for the trip, was to give the students a first experience with repelling in the close-by “City of Rocks”. Assisting me were two wonderful adult-assistants and trekking buddies named LaRue and David Fillerup.

Public domain map of the Massacre Rocks State Park area.

When wagon trains heading west in the mid-19th century approached the break in the rocks ahead of their trail, they were ready for attacks by the local Shoshone Natives. On August 9-10th, 1862, 10 emigrants were killed, and so the natural thoroughfare became known as the Gate of Death or Devil’s Gate, and the rock hills became “Mass Rocks”.

Our camp at Massacre Rock State Park in Idaho in 2009

Pictured above, the area is in the plain of the Snake River, and was once part of the Oregon Trail. Nearby Register Rock was a natural “rest stop” for travelers, and the names of these pioneers are carved into the stone.

The Devil’s Gate Pass, (Massacre Rocks) in Idaho

Pictured above, Devil’s Gate Pass is all that remains of a basaltic volcano. The pass itself was carved out at the time of the Bonneville flood about 15,000 years ago, when water from Lake Bonneville, (which covered most of Utah), burst through the pass ad along what is now the channel of the Snake River. The flow of water was thought to be four times that of today’s Amazon, making it one of the largest and most violent floods in history. The basalt boulders it broke away and carried are now scattered around the local landscape.

Devil’s Gate, (Massacre Rocks) in South Eastern Idaho.

Pictured above, the flood carved numerous natural box canyons and ‘alcoves’ leaving steep rock walls ranging from 10 to 67 m in height. Massacre Rocks State Park was a large natural-area of sagebrush, ancient cedar trees, sand dunes, and box canyons sculpted by the flood.

Public domain map of the “City of Rocks National Reserve”.

Our main destination to explore and train the students to repel was the “City of Rocks National Reserve”, 111 km southwest of Massacre Rocks State Park. The “City of Rocks National Reserve”, also known as the “Silent City of Rocks, is approximately 3.2 km north of the border with Utah. It is widely known for it’s enormous granite rock formations and excellent rock climbing.

City of Rocks National Reserve

The rock spires in the City of Rocks are largely composed of granitic rock of the Oligocene “Almo Pluton” and “Archean Green Creek Complex”. City of is also a popular rock climbing area, with over 1,000 traditional and bolt-protected routes.

My students at the City of Rocks National Reserve in 2009

The landscape of City of Rocks has been sculpted from granite that was intruded into the crust during two widely spaced times. The granite that composes most of the spires is part of the 28-million-year-old “Almo pluton”. However, some of the spires are made of granite that is part of the 2.5 billion-year-old “Green Creek Complex” that contains some of the oldest rocks in the western United States. The granite has eroded into a fascinating assortment of shapes.

Public domain geological map of the “City of Rocks”

The “Almo Pluton” is made of igneous rock formed by the slow cooling of magma that intruded into older rock. Granitic rock of the Almo pluton makes up most of the spires in the City of Rocks. The Almo pluton is cut in many areas by dikes of younger intrusive rocks. Pegmatite dikes in the Almo pluton commonly contain very large crystals.

David Fillerup helping a student repel off of a small ledge of the Almo Pluton Granite.

The Almo pluton originated as an intrusive body of magma and therefore has complex contact relationships with the surrounding country rock, (Elba Quartzite deposited during the Proterozoic Eon between 15,000 and 600 million years ago). The contact is best exposed on the eastern and southern margins of the pluton where the magma intruded Archean gneiss and granite of the Green Creek Complex. The most dramatic contact between these granitic rocks is in the saddle that separates the Twin Sisters. The ‘Green Creek Complex’ can easily be distinguished from the younger ‘Almo pluton’ by its darker color caused by a greater concentration of iron-bearing minerals.

La Rue Fillerup belaying for a student repelling at the “City of Rocks”

One of the most obvious features of the Almo pluton are the cracks, known as joints, that cut across every outcrop. The orientation and spacing of the joints plays a critical role in controlling the size, shape, and distribution of spires in the City of Rocks. There are three processes that that have created the joints at City of Rocks; contraction, extensional tectonics, and expansion related to the release of pressure as overlying rock is removed by weathering and erosion.

One of my students repelling at the “City of Rocks National Reserve”

Rock formations in the reserve developed through an erosion process called exfoliation, during which thin rock plates and scales sloughed off along joints in the rocks. The joints, or fractures, resulted from the contraction of the granite as it cooled, from an upward expansion of the granite as overlying materials were eroded away, and from regional tectonic stresses.

La Rue Fillerup assisting a student repelling at the City of Rocks National Reserve in 2009

The granite has eroded into a fascinating assortment of shapes as high as 180 m. The upper surfaces of many of the rocks are covered with flat-floored weathering pits known as pan-holes. The most notable pan-hole is located on top of Bath Rock and is continuously filled with water from rain or snowmelt.

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