Medical Lake Bluff, Diving into the Geology

Looking at the beauty of Medical Lake is very relaxing, especially seeing the stoic bluffs or cliffs at the south end of the lake next to Waterfront Park. But have you ever wondered what kind of rocks you’re looking at or how Medical lake formed?

To figure out what kind of rocks made up the cliffs, I took a sample rock off of the top of the cliffs and brought it in to Dr. Chad Pritchard’s office. We used a Bruker Tracer III portable X-ray fluorescence gun to analyze my sample rock. Looking at the Washington Department of Natural Resources interactive geologic map, we know that the rocks are part of the Columbia River Basalt Group. To figure out which specific group these rocks are from, we looked at the calcium and titanium levels from the results of the X-ray and used a special calibration developed by Bruker and EWU (Evarts and Pritchard, 2016). My rock sample from Medical Lake was high in Calcium and also high in Titanium. Comparing it to rocks sampled from other areas around Spokane, we were able to conclude that the rocks making up the Medical Lake cliffs are a part of the Priest Rapids Member of the Wanapum Basalts. By figuring out what group of rocks they are, we know that these cliffs are about 14.8 million years old. (Reidel, 2015)

So how were these cliffs formed? Standing on the cliffs or looking at them from the water, it is obvious that the rocks are very broken up, or fractured. On the other side of the cliffs, across the lake, there are granite rocks. Knowing that the lake separates the two different rock types, granite and basalt, we can hypothesize that the Missoula Floods came through the contact between the two rock types and eroded the cliffs to what they are today. This also created the depression that was filled in by water to make Medical Lake. Looking at the bathymetry map of Medical Lake, it is apparent that the water is the deepest at the base of the cliffs, which may have happened because the floods preferentially eroded the broken up rocks. A little more research and it turns out that the cliffs are probably broken because they are in the Cheney Fracture Zone (Griggs, 1976). The fact that they were fractured before the Ice Age Floods would have made them easier for the turbulent waters to erode them, so the cliffs were probably akin to a small waterfall at some point in time. Medical Lake never ceases to amaze!

Images

Top of cliffs with lots of fractures and red soil

Top of cliffs with lots of fractures and red soil

Creator: Alyssa Fitzgerald View File Details Page

View of the cliffs from Waterfront Park along the southern shores of wonderful Medical lake

View of the cliffs from Waterfront Park along the southern shores of wonderful Medical lake

Creator: Chad Pritchard View File Details Page

Close up of cliffs from across the lake

Close up of cliffs from across the lake

Creator: Flood Explorer Team View File Details Page

Medical Lake Granite from the western shore of Medical Lake

Medical Lake Granite from the western shore of Medical Lake

Creator: Alyssa Fitzgerald View File Details Page

Geochemical Plot to classify the type of basalt at the cliffs along southern Medical Lake

Geochemical Plot to classify the type of basalt at the cliffs along southern Medical Lake

Creator: Alyssa Fitzgerald and Chad Pritchard View File Details Page

Slope Map of area around Medical Lake.  Bathymetry of the lake is in feet, 5 to 10 ft contours

Slope Map of area around Medical Lake. Bathymetry of the lake is in feet, 5 to 10 ft contours

Source: Spokane LiDAR (2015) Puget Sound LiDAR Consortium, Seattle, Washington USGS and Washington Department of Fish and Wildlife - Lake Bathymetry lakebath_arc shapefile | Creator: Chad Pritchard View File Details Page

Hypothetical time-sequence for the development of lakes around steptoes

Hypothetical time-sequence for the development of lakes around steptoes

The top diagram shows a hill prior to the basalt flows that is shown in the second image, approximately 15.5 million years ago (Ma). Prior to the Missoula Floods soil formed over older rocks and the margins of the Columbia River Basalt would be easily eroded by the Missoula Floods ~13,000 to 16,000 years ago (or earlier). Where the basalt meets older soil would therefore be an ideal place to carve out lakes by kolks (shown as underwater tornadoes). Hopefully, these time-sequence snap shots show that the location of Medical and West medical lake are due to a complex interaction of geology:) | Creator: Chad Pritchard with help from EWU OIT's Judy McMillian:) View File Details Page

Cite this Page:

Alyssa Fitzgerald, “Medical Lake Bluff, Diving into the Geology,” Ice Age Floods Explorer, accessed September 25, 2017, http://floodexplorer.org/items/show/51.

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