Lost Lake

July 14, 2017

LostLakeEPOD (1)

Photographer: Rod Benson  
Summary Author: Rod Benson

Lost Lake, found hidden below the prairie in central Montana, is one of Montana's most unique bodies of water. The lake and its 250 ft (76 m) cliffs tell of a fascinating past, including a time when an ancient sea covered the area, an episode of volcanic activity, and a more recent period when ice age floods shaped the landscape.

The lighter-colored rocks apparent in the cliffs around the lake are layers of sandstone; part of the Eagle formation, made from sands deposited during the late Cretaceous Period (over 65 million years ago) when the Western Interior Seaway inundated North America. Dinosaurs wandered along its coastal plains. The dark cliffs and stripes between the sandstones are igneous rocks, formed as magma cooled underground.

During volcanic activity, magma was moving around below the surface, working its way into cracks, between layers or simply melting its way through surrounding rock. The magma forced its way between layers of rock at Lost Lake, forming a sill. Dark sills in the cliffs around the lake are associated with an even larger formation called the Lost Lake Laccolith - a mass of igneous rock hundreds of feet thick that formed as magma forced its way between layers of sandstone and pooled below the surface.

Fast-forward 50 million years: The Laurentide Ice Sheet grew southward into this area (13,000 -15,000 years ago), pinching up against the north slope of the Highwood Mountains, blocking the flow of the ancient Missouri River and causing the formation of Glacial Lake Great Falls. The lake grew deeper and eventually broke through the blockage near the town of Highwood -- an outburst flood occurred. As the glacier advanced and retreated, the lake drained and reformed many times. The path and intensity of the overflow varied, but the most distinct channel remaining today is called the Shonkin Sag. Today the Sag is a wide, deep valley, with scattered brackish lakes, that winds its way along the north side of the Highwoods. Although not part of the Sag, Lost Lake was formed by a tremendous stream of water that included 250 ft (76 m) waterfalls on its west end, proving that floodwaters from Lake Great Falls occasionally cut across there. Photo taken on October 27, 2016.

Photo Details: Camera Maker: Panasonic; Camera Model: DMC-LX7; Focal Length: 4.7mm (35mm equivalent: 24mm); Aperture: ƒ/2.8; Exposure Time: 0.0008 s (1/1300); ISO equiv: 80; Software: iPhoto 9.6.1.