Cliffs of the Shonkin Sag Laccolith

May 16, 2016


PhotographerRod Benson
Summary AuthorRod Benson

The formation and flooding of Glacial Lake Missoula are two of the most famous events that occurred during the most recent glaciation. However, a lesser-known incident that shaped the cliff shown in the photo above was also quite dramatic. The circumstances began when the Laurentide Ice Sheet grew southward from Canada, reaching the north slopes of the Highwood Mountains of central Montana. As the ice sheet pinched against the Highwoods, it blocked the flow of the ancient Missouri River, causing the formation of Glacial Lake Great Falls. The lake filled with meltwater, eventually growing high enough to spill over the dam near the present-day town of Highwood. Once spilled over, the water flowed along the southern edge of the ice sheet (along the north side of Highwoods), across existing drainages, carving the impressive channel known today as the Shonkin Sag. Although the path and the volume of the overflowing water certainly varied, and probably included occasional outburst floods, the Shonkin Sag is the most distinct channel visible today. The wide, deep, dry valley with scattered brackish lakes winds its way between the small towns of Highwood and Square Butte.

For geologists who visit this remote area the Shonkin Sag, the cliff shown in the photo, is a must see. Here, two events separated by tens of millions of years (a period of volcanism and an ice age) teamed up to provide a unique cutbank. This is where ice age flows of water in the Shonkin Sag cut into the Shonkin Sag Laccolith, exposing the spectacular cross-section represented by the cliff. Such views of a plutonic formation are rare - something normally only seen in textbook diagrams. On the photo above, the thick, dark part toward the left is the eastern edge of the laccolith, and the dark stripes on the right are sills formed as magma from the laccolith squeezed between layers of sandstone. The Shonkin Sag Laccolith is about 200 ft (61 m) thick and a mile (1.6 km) long. Photo taken on April 9, 2016.
Photo Details: Camera Maker: Panasonic; Camera Model: DMC-LX7; Focal Length: 4.7mm (35mm equivalent: 24mm); Aperture: ƒ/3.2; Exposure Time: 0.0006 s (1/1600); ISO equiv: 80; Software: QuickTime 7.6.6.