Montana Earth Science Picture of the Week

The Perfect Alluvial Fan

The area shown on the map is located a 5 miles southeast of Ennis (30 miles northwest of Yellowstone Park). A photo of the area is shown to the right. The map, which represents an area about 5 miles x 5 miles, was copied from an older USGS contour map. Such maps, often called topographic, or "topo" maps, use contour lines to show the shape of the land. Older ones also used shading to help the map-reader visualize the topography.

What's so special about this area?. . .
The significance of this map is that the contour lines reveal a distinct fan-shaped formation in the area where Cedar Creek flows out of the Madison Range. The formation, called the Cedar Creek Alluvial Fan, is one of the best examples of an alluvial fan in the world and it is sometimes featured in geology textbooks.

How did it get there?. . .
Streams such as Cedar Creek transport a variety of rock materials, including clays, silts, sands and gravels. Where streams flow from steep slopes onto flatter areas they slow down, losing their ability to efficiently transport gravels and larger sands. The smaller pieces, including clays, silts, and some sands continue to go where the water takes them while the gravels begin to build up at the edge of the valley. Eventually so much gravel is deposited that the stream changes its course, and the process repeats itself over and over and the fan-shaped deposit takes form. That's exactly what has happened here. Since the last ice age ended about 10,000 years ago, Cedar Creek has carried gravels out of the mountains and deposited them on the edge of the valley, forming this classic alluvial fan.

Below: Move your cursor over this Google Earth image of the Cedar Creek alluvial fan shows what the area would look like if your were approaching (flying) from the west. You can see Ennis Lake, the town of Ennis, and Cedar Creek cutting through the alluvial fan. Cedar Creek is a tributary of the Madison River which flows northward (right to left across the center of the image) into Ennis Lake. Soils have developed on top of the gravels, and grasses have taken root. Cedar Creek continues to wind its way over the fan.

Term: alluvium


*More about alluvial fans (great photos)
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By Rod Benson
Earth Science Teacher at Helena High School

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