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!