Volcanos and rapids, the origins of Eagle Rock

Courtesy Library of Congress. Title: Great Falls of Snake River, Idaho territory / TM ; Prang’s American Chromo. Contributor Names: L. Prang & Co., Moran, Thomas, 1837-1926, artist. Created / Published c1876. Subject Headings: Waterfalls–Idaho–1870-1880, Cliffs–Idaho–1870-1880, Snake River (Wyo.-Wash.)–1870-1880

The location of Idaho Falls, Idaho, (initially Eagle Rock), along the Snake River in Eastern Idaho was not a coincidence. The geological formation of southern Idaho had been undergoing a major transformation that started long ago due to successive catastrophic volcanic activity.

In fact, this dramatic regional geology, for so long a formidable barrier to settlers in the western frontier, now draws landform fans for many reasons. Here’s a slideshow of “6 Stone-Cold Stunners” in Idaho from AtlasObscura (be sure to visit their site for locations and details), including Craters of the Moon which is close to Idaho Falls.

These formations are ancient. According to Paul Karl Link and E. Chilton Phoenix in Rocks, Rails & Trails, “The Snake River Plain-Yellowstone volcanic center originated about 17 million years ago near the southwest corner of Idaho. As the North American Plate moved southwestward, this volcanic center migrated in a northeast direction across southern Idaho, at a speed of 4.5 mm per year,” and it continues to move northwest to this day. 

A “hot spot” under the North American tectonic plate has been causing catastrophic bursts of lava and ashes for millennia. A hot spot is an area of weakness in the earth’s crust where super-heated magma rises, ruptures the earth’s crust, and forms a violent volcano or simply gushes out of cracks in the earth as mammoth lava flows.

The Yellowstone geysers are just one of the milder examples of what hot spots can cause. The most recent location of volcanic activity was near Yellowstone Lake where an eruption occurred just 174,000 years ago. An earlier eruption at this location distributed ash for thousands of years from Canada to Louisiana and west to southern California. After that volcanic activity, massive amounts of lava oozed up through the earth, both raising and leveling a large swath of land between Twin falls and Idaho Falls, Idaho, turning it into a high desert plain, now known as the Snake River Valley.

Although homesteading near a river could be considered advantageous, the lava formations and turbulence of the river, with its own unique geologic features, discouraged settlement in northern Idaho.

The Snake River is part of a moisture channel that allows humid Pacific air to flow through a gap between the Cascade Range and the Sierra Nevada all the way to the Continental Divide in Yellowstone Park. As this Pacific air rises, the moisture precipitates into heavy snow and rainfall that flows back to the Pacific Ocean via the Snake River. The often turbulent Snake River carved a path along the eastern edge of the high desert, eroded the lava, and created steep banks on both sides of the river. Early pioneers found crossing the river hazardous and at times impossible, so very few traveled this far north in Idaho to find homesteads.

Until the railroad bridges were built, there were very few people living in this region. However, at the location of what is now Idaho Falls, the river became very narrow allowing for the construction of Taylor’s Bridge in 1865. A lava island had remained in the middle of the river just south of this bridge and was known to be a home for eagles. In 1879, after the Union Pacific surveyed the best route to run a railroad line from the Salt Lake City area up to the profitable Montana mines, they chose this island as the center point for two bridges to span the river for the narrow gauge Utah & Northern Railroad. Being the approximate midpoint of this line, the east bank of the eastern bridge was made the Division headquarters for the line and named Eagle Rock after the island. 

As a division point, Eagle Rock required the rapid construction of a railroad depot and numerous maintenance facilities and housing for railmen. This drew construction crews, more railmen, and new opportunities for other early pioneer entrepreneurs. For the first time, there were two ways to safely cross the river within a few hundred feet of each other in northern Idaho.

In this way, the geology of the land made the little town of Eagle Rock an important destination for adventurers, settlers and much needed goods to cross the Snake River and continue on to the northwestern territories. Today, the renamed Idaho Falls is the eighth largest city in Idaho.

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