Beaver Lake Geology
Written by D. Bruce Morgan
Many of you who live in the Beaver Lake watershed or who have enjoyed spending time at the lake may have questions about how and when Beaver Lake came to be. Well the story is a long one starting about 20,000 years ago when the latest period of glaciation in the northwest had its beginnings. A vast ice sheet originating in British Columbia had begun growing in response to cooler global temperature and had begun migrating south becoming the Vashon glacier. By 15,000 years ago the front edge of the ice sheet had arrived in what would later become modern day Seattle. As the ice sheet flowed south it ground up vast amounts of rock underneath leaving thick glacial till deposits and carved some of the features that we would recognize today.
At the peak of glaciation, the ice sheet extended to a point south of Olympia and the region surrounding Seattle was buried by approximately three thousand feet (over one-half mile) of solid ice. The southerly progression of this ice sheet and sub glacial melt-water had carved a series of depressions generally oriented in a north-south direction that would later become Puget Sound, Lake Washington, and Lake Sammamish. Other areas of the landscape at higher elevations were scraped and carved by the glaciers into elongated cigar-shaped hills (drumlins) oriented in a north-south direction. At the same time the crushing weight of over one-half mile of ice had compacted the finely ground rocks beneath into the dense hard soil that today is referred to as hardpan, “natures concrete”, or more accurately as Vashon lodgment till.
As the weather changed and temperatures warmed the glacier front began retreating. Within 2,500 years of when the ice sheet first made its arrival in the Seattle area, it had already retreated to the north where it would continue to shrink until it resembled the conditions that we would recognize today. One of things that many people do not realize about glaciers is that they discharge huge amounts of water. They are also carry huge amounts of rock debris accumulated along the entire length of the ice sheet back to the head of the glacier. As the glacier retreats rock debris is dropped along the front of the glacier, and finer sands and gravel are deposited by the melt-water emerging from the base of the glacier. Unlike the till that is overridden by the advancing glacier, these sand and gravel deposits are not compacted by a great weight, and these recessional deposits are loose and permeable and often make great aquifer beds.
By now you may be wondering how this is related to formation of small lakes? Well, one of the interesting things that can occur when a glacier retreats is that large chunks of ice may break off from the retreating face and become stranded and partly buried by the sand and gravel deposits flowing out from the face of the retreating glacier. (The process is somewhat similar to ice sheets that calve off pieces into the ocean which become icebergs.) Over a period of years the ice melts away and along with rain and ground water fills the depression defined by the block of ice, and there we have our lake. The geologic term for this is “Kettle Lake”. Other examples of Kettle Lakes include nearby Pine Lake, Green Lake in Seattle, and Steilacoom and Gravelly Lakes near Tacoma.
Two other interesting things are worthy of note. One is that the permeable sands that ring the lake are an excellent conduits of groundwater to recharge the lake. While Beaver lake and Little Beaver are each fed by small streams, it is not hard to find up-wellings of cold water from the bottom that can be quite a surprise to the swimmer who happens to swim across one. In the old days before there was a local water district, homeowners would pump their water directly form the lake and would attempt to locate their pump intake over one of these cold springs. (I can state from experience that was not always a completely successful strategy.) This also illustrates why pollutant sources discharged in the watershed will often readily find their way to the lake.
Another interesting thing to observe around Beaver Lake is the larger boulders that the glacier left behind. The geologic term for these is “erratic”. The name refers to the fact that these features are not representative of local rocks and have been transported a great distance to reach us here. A very large erratic is located near the NE corner of Beaver Lake in the lake, and projects about five feet above the surface and about ten feet from the shoreline. It was originally free standing in the lake but in the 1960’s the closest landowner built a causeway to the rock from his property. Very close by on each side of E Beaver Lake Dr SE near SE 18th PL there are three more large erratics. A quick calculation suggests that the largest of the three may have a weight approaching 100 tons.
Last Updated on Monday, 23 November 2009 12:18