DescriptionThe Fish Lake Trail starts near Sunset Boulevard on the west side of Spokane. On this trail, it is easy to see many different geological features. One of the features is an outcrop of the Latah Formation. This can be seen about one mile from the beginning of the trail on its western side. The Latah Formation crops out as a claystone that is light in color ranging from tan to a yellow often with red from oxidizing mineral layers. This particular clay is up on the ridge behind trees and might not be immediately noticed. All around the clay is basalt the is cracked and looks to be rounded like the leading edge of a lava flow.
The Latah Formation was formed during the mid-Miocene. It is made up of silt sediments and fine grained sands. This is known from age dating floral fossils found within the clay. It was in the Miocene that there were many bodies of water in this area, including lakes and swamps (Pardee and Bryan, 1926). During the Miocene, the Columbia Flood Basalt had already been flowing and formation like the Grande Ronde Basalt were forming to the west. The Grande Ronde, which is 16.5 to 15.6 m.y. old, most likely blocked flows from the lakes and swamps allowing for a large amount of sediments to gather and over millions of years they lithified (the process in which sediments become stone) into the clay that is seen today(e.g. Reidel, 2015).
Interestingly, the basalt that is along the trail and interbedded with the Latah Formation is basalt from the Priest Rapid Member group, the Wanapum Basalt formation. In most places this basalt overlays the clay but for some reason at this location it is on either side and possibly below. It is supposed that the basalt as it formed about 15 million years ago either pushed the clays aside as the new lava flows came in or even that the lave flows being more dense pushed down and in between the contact layer of the Latah Formation and the underlying rock. On certain parts of the trail Wanapum pillow basalt can be found indicating that there was still water there at the time of this flows. So it might even be that the clay was not yet hard and the basalt literally flowed through mud and surrounded it. Then when the Missoula Floods came through any pillow basalts that were above the clay was broken up and washed away. Either way this is a fascinating feature to behold.