Sandstone Concretions, Red Rock Coulee
Red sandstone concretions at Red Rock Coulee Natural Area, Alberta.
Red Rock Coulee is a patch of dry mixed grassland at the head of a coulee system cut by retreating glaciers. The site is fenced and surrounded by agricultural land on all sides. The coulee system drains the west slope of Bull Head Butte, the highest topographic feature in the area.
In the upper parts of the coulee, erosion has exposed a number of large, spherical sandstone concretions, which give the area its name. These round boulders, some more than two meters in diameter and tinted red with iron oxide, stand out in stark contrast to the light gray clays and shales that make up the surrounding bedrock. Some of the boulders have completely emerged and appear ready to roll away, while others remain only partly exposed.
Farther down the coulee, the shales and sandstones have been weathered by wind and water to form miniature hoodoos and badlands. The exposed rock here is some 75 million years old, and was exposed when the last glaciers retreated from this area 13,000 years ago.