July 19, 2009

Upper Missouri River Breaks National Monument

The geology on this trip was spectacular. Millions of years ago this area was covered by an interior seaway that connected the Gulf of Mexico with the Arctic Ocean cutting North America in half. Thousands of feet of sand, silt, clay and lime were deposited on the bottom of the seaway by rivers which drained to the sea. Eventually the layers of sediment hardened into the layered rocks of today. About 70 million years ago, central Montana was uplifted and the sea retreated.

Around 55 million years ago, hot magma welled up from great depths and pushed its way into cracks in the sedimentary rocks. Some of it reached the surface and produced volcanos, but great quantities of the magma cooled underground in the network of cracks, and formed veins of igneous rocks.

Thru the years the Upper Missouri River has cut down through the entire rock package exposing the 300 foot high white sandstone cliffs laced with the thick veins of dark brown igneous rock.