Glacial Lake Missoula

Was out stacking up small dead trees trying to clean up the northwest corner of our property when I noticed how sandy the area was.  In fact, this used to be a beach.  Imagine that – Connie and I own land that was lake front property.  Of course that was 15,000 years ago, or 12,000 years ago according to some geologists.

I thought I would share a little geology of our area in my blog.  During the last ice age, a glacier from the British Columbia area moved south into Idaho down the Purcell Valley.  The glacier was about 20 miles wide when it crossed into the Clark Fork river valley around where Pend Oreille Lake is today, it closed off the Clark Fork River and Glacial Lake Missoula was formed.  This lake was 2000 feet deep at the dam and was about the size of Lake Ontario and Lake Erie combined containing 500 cubic miles of water.  At its highest water level, the waves were lapping at 4350 in elevation or 300 feet above where our house/barn sits now. 

Ice dams tend to float before the water will go over them.  When that happens the glacier breaks up and the dam fails “catastrophically.”  Glacial Lake Missoula drained in a matter of days.  Some say it was the greatest flood in geological record.  Now that is impressive!  Then guess what – the glacier continued south and blocked it again.  This happened not once, not twice, but many times.  Some say 36 times and some say 41 times.  Geological evidence shows 41 layers of sediment near Spokane and at least 36 fillings in western Montana.  Some fillings may not have backed all the way up the valleys here which explains the difference in number of the sediment layers.   So it at least caused 36 pretty major floods in eastern Washington State according to the book referenced below. 

The following is from the book “ROADSIDE GEOLOGY OF MONTANA” by David Alt and Donald Hyndman, page 53.  Glacial lakes keep their archives in thin layers of light and dark sediment called varves.  Glacial meltwater is typically milky with finely ground rock flour pulverized as the rocks embedded in the moving glacier grind each other.  Rock flour accumulates on the floors of glacial lakes during the summer when large quantities of ice melt.  Meanwhile, algae and microscopic animals flourish in the sunlit surface waters of the lake.  The coming of winter ends the melting, thus cutting off the supply of rock flour, and the algae and animals that thrived during the long summer days die with the freeze.  Their remains settle to the lake floor during the winter to become a layer of dark sediment.  Each pair of light summer and dark winter layers records the seasons of one year of the glacial lake’s existence.

The earliest filling of the lake lasted 58 years with subsequent fillings lasting shorter periods of time.  It existed almost 1000 years.  So now you know why we have what we call the “sand pit” where we get our sand for our road.  It must have been part of the beach a “few” years ago.  And our prevailing wind now is from the southwest and if it was the same then, we would have had lots of waves working on our southwest facing Precambrian rock outcroppings.  And just a few hundred years would make up a little sand.