Glacial Lake Missoula…I never knew!!

Glacial Lake Missoula…I never knew!

Full disclosure. My college education includes a B.S. in Geology (1985) and M.A. in Human Resource Development (1995).  Most of my professional career was as a pilot with several years’ experience in aircraft avionics design and enterprise software development.  What does that all mean?  Nothing really, other than with good directions and $3.00, I might be able to procure a reasonable cup of coffee.  With that disclaimer, I am extremely curious and fascinated by those amazing things around us.

Last year, I decided to knock off 35 years of cobweb growth and start relearning geology. I was particularly interested in the geology of the Western United States.  It is quite fascinating.  Airline travel takes me to some really cool places and the last couple months have taken me to Western Montana several times.  So, it was time to dig in and learn more about the area.  I picked up a book, “Roadside Geology of Montana” by David Alt and Donald W. Hyndman and along with that, started some internet research.  The book did a great job giving a big-picture overview of how the mountains of Western Montana were formed and what forces have impacted their evolution.  One story that really caught my attention was the story about Glacial Lake Missoula.  With my next trip planned to lay over in Missoula I decided to focus in on this lake. What better way to make the time there productive and learn something. 

One of the common things to do on a layover in Missoula is to climb up to the “M” on Mt. Sentinel, attached to the University of Montana. It is great exercise and an easy walk from the hotel. I have climbed this mountain several times completely unaware of the profound geological history and evidence of Lake Missoula in plain sight. I also somehow missed that there are historical markers and boards along the trails that I have walked repeatedly, never stopping to read, explaining the phenomenon.  Hmmm, maybe it is time to pay closer attention. 

If you look closely at Mt. Sentinel, you can see some faint horizontal lines in the side of the mountain.  It’s not quite so obvious in the picture above but in the picture below which is Mt. Jumbo, right across the river to the north, the horizontal lines are unmistakable.

These lines are remnants of an ancient lake shoreline.  We have all seen those before where fluctuating water levels on a lake leave these types of markings on the shoreline.  These markings on Mt. Jumbo rise about 1,000’ above the valley floor in which the town of Missoula sits.  Geologists have determined that this entire valley was filled several times with a lake as deep as 1,000’ in this area.  More significantly, this lake extended well to the east along the Blackfoot River, south along the Bitterroot River and 180 miles to the northwest along the Clark Fork River to Lake Pend Oreille near Sandpoint, ID.  Glacial activity caused ice dams to block river flow near Lake Pend Oreille causing this massive glacial lake to fill with enough water to fill both Lake Erie and Lake Ontario combined. The water depth at the dam was approximately 2,000’ thick.  Since the dam was made of ice, it had a tendency to subsequently melt. As the dam melted, the dam eventually broke and the 2,000’ wall of water rapidly and devastatingly flooded eastern Washington State and down the Columbia River Valley into Western Oregon…carving, in short order, many of the valleys that exist today.  The amazing thing is that this glacial dam not only formed and broke once, but somewhere on the order of 40 plus or minus times, most recently 15,000 to 11,000 years ago. 

As I hiked up the mountain, I wondered how high did Lake Missoula reach?  I hiked a few hundred feet above the “M” to take in the view.  I could see I was getting close to the highest lines on Mt. Jumbo across the valley but not confident I was near the highest point the lake reached.  Regardless, the view was spectacular on this nice spring day!

After enjoying the view, I started back down, my question about how high the lake level reached was answered.  Had I been on this mountain 15,000 years ago, I likely could have skipped a rock on the lake right from where I was sitting.

The last sentence of the trail marker sign speaks volumes, “Lake Missoula’s tremendous flooding represents one of the most significant geologic events in the history of the world.”

Pretty amazing!