Montana Earth Science Picture of the Week

Helena has its faults.

Photo courtesy of Mike Stickney
Montana Bureau of Mines and Geology

A torn surface . . .
This aerial photo shows the western edge of the Helena Valley just a few miles north of Helena. If you look closely you can see an elevation change in the surface along a line between (and parallel to) the railroad tracks and Green Meadow Drive. If you can’t tell where it is, scroll down to see the labeled photo near the bottom of this page. The linear feature, called the Iron Gulch fault scarp, is where the ground surface has dropped three to four meters above a “normal fault” as a result of earthquakes over the last 130,000 years. It is not known how many quakes it took to form the Iron Gulch scarp, but it could probably be determined by careful study if a trench were dug across the scarp.

One quake at a time . . .
Earthquakes happen when there is movement along a fault, however, not all movements cause scarps to form. For example, during the historic quakes that rocked Helena in 1935-1936 there were no surface ruptures. In contrast, a big earthquake near West Yellowstone in 1959 caused several new scarps, including one that was 14 miles long and as high as 21 feet (see Hot Link below). Like most fault scarps, the Iron Gulch scarp is not in solid rock, but rather in gravels and soil above the bedrock. The actual fault is located within bedrock, buried beneath soil and hundreds of feet of gravel.

Map courtesy of Montana Bureau of Mines and Geology . . .
The map at the right shows faults (dashed lines) that have been identified in the Helena area as well as post-1982 epicenters (yellow circles). The stars mark epicenters of post-1900 earthquakes with magnitudes of 5.5 or greater. Some geologists believe that the Iron Gulch fault (#683) extends to the southeast in the subsurface, all the way to East Helena. All of the faults shown on the map played a role in the formation of the valley. Like many basins in the northern Rockies, over millions of years, the block(s) of bedrock that the Helena Valley sits on has dropped down one earthquake at a time along these faults to form a basin surrounded by mountains (called a graben). Since its formation the Helena Valley has filled with gravels, sands, and silts washed down from the surrounding mountains. As Helenans learned during the 1935 quakes, structures built on these deep, loose sediments shake much more violently than structures built where bedrock is not far below the surface. .

Supported by Internet Montana

Terms: graben, normal fault


About this site
Past pictures of the week
*Fault Scarp near West Yellowstone
*Earthquake Studies Office for Montana
Students of Benson: click here to print worksheet
Other students: Click here to print worksheet
*Helena High destroyed by 1935 quake
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By Rod Benson
Earth Science Teacher at Helena High


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