The “Unusual Currents” of JT Pardee

Looking east-southeast from interpretive sign on Road 382.

This is Camas Prairie in northwest Montana, home to some very curious ridges and swales (mid-ground in photo).  The crests are as much as 45 feet high, but this is hard to see at ground level even though the vegetation -- pale on ridges and dark in swales -- helps highlight the shapes.  In contrast, these landforms are quite obvious from the air.

Ripples on the prairie.  From Google Earth; click photo for better view.

From the air, the ridges and swales look a bit like ripples that develop in beach sand washed by waves or on sandbars in rivers.  Might this be a clue?

Source.

Possibly, except there’s a problem.  The Camas Prairie ridges aren’t made of sand.  They contain gravel, cobbles and even boulders, which are much too large to be arranged into current ripples ... or are they?

More curious ripples on the prairie, looking southwest.

It was Joseph Thomas Pardee who made the great conceptual leap ... and without the aid of aerial photos!  He concluded that these ridges are indeed ripples, produced by "unusual currents" in massive floods when huge volumes of water traveled at cataclysmic speeds (Pardee 1942).  The source was Glacial Lake Missoula, which we had visited the day before.

Old shorelines of Glacial Lake Missoula visible on right side of “L” hill.  Click photo to view.

Lake Missoula formed when a lobe of the Cordilleran ice sheet dammed the Clark Fork River near Sand Point, Idaho, forming a huge lake (map below).  When the dam gave way around 15,000 years ago (largest of multiple flood events) the lake drained at tremendous speeds, enough to create giant current ripples composed of very coarse materials.  The height and spacing of the ripples have been used to estimate rate of flow, perhaps as much as 60 to 80 miles per hour.

The shapes of the ridges also attest to the speed of the flood.  Many are antidunes, with steeper sides facing upstream, indicating that water movement was quite rapid.  The Camas Prairie basin drained north across Markle Pass, and eventually into the main channel of the Clark Fork River.

Before the big flood, Camas Prairie sat under 1000 ft of very cold water.  Map of Glacial Lake Missoula from Montana Natural History Center. 

Another great geo-stop, courtesy Montana Department of Transportation.

This is the second in a series of posts about recent travels in the land of Pleistocene floods -- a terrific vacation destination!
Sources
Allen, JE, Burns, M, and Burns, S.  2009.  Cataclysms on the Columbia.  Portland State University, Ooligan Press.Pardee, JT.  1942.  Unusual currents in Lake Missoula, Montana.  Geological Society of America Bulletin 53:1569-1599.