Sitka Spruce Trail — Wednesday, Mar. 25, 2020

Nerd Alert. This report is all about Geology.

The Sitka Spruce trail is in the Mid Fork Snoqualmie River valley.  It is an unmarked and muddy path that takes you through ground zero of the Mid Fork Claylands.  Not a great trail per se but the complex geology that gave rise to the Claylands is facinating.

First you have to go back to the last ice age (13,000-18,000 years ago).  All of Canada was covered in a massive ice sheet that was 2 miles thick.  The Puget Lobe of the Cordilleran Ice Sheet was an extension off of the southern margin of this greater ice mass.  The Puget Lobe filled the entire Puget sound area from North Bend to the Olympics with Ice that was 1/2 mile thick (1 mile thick at the Canadian border). At its maximum, the Puget Lobe extended all the way down to Olympia.  All of the land features in Puget Sound were carved out by this massive glacier.

The Puget Lobe pushed through North Bend and in the process shaved off the steep face of Mt Si.  It continued up to the mouth of the Mid Fork Snoqualmie river and deposited massive amounts of rock debris in the form of a lateral moraine.  This big flat topped ridge can be beautifully appreciated from Rattlesnake Ledge. This resulted in a very unusual situation where the valley was dammed up from below and created a large lake in the Mid Fork Valley.  Over hundreds of years, silt eroded down from the surrounding mountains and settled in the lake bottom, creating thick layers of grey mucky clay called "Varves".  Eventually the lake spilled over the top of the moraine and then carved a V-shaped gap in the moraine (near Mailbox TH) that allowed the ancient lake to drain out.

The clay has a profound effect on the types of plants that can grow in this portion of the Mid Fork.  Only trees that can tolerate the constant wetness of clay soil can survive.  Sitka Spruce which is relatively uncommon in the Cascades thrives here. Western Hemlock, Big Leaf Maple and Cedar also grow well here.  Douglas Fir, which needs well drained soil is a rare as hens teeth in the Claylands.  Just go a few hundred feet above the valley floor and above the clay and Doug Fir becomes the dominant species as it is in most of the Cascade lowlands.

Awesome Video on Geology of the Puget lobe. I would watch the whole thing but the part pertinent to the Mid Fork is 8:30-11:00:  https://youtu.be/ip8nTokv_k0

Greenway Article on the Claylands:   https://mtsgreenway.org/blog/goopy-gems-the-claylands-of-the-middle-fork-snoqualmie/

More Information on this Trip Report

• https://mtsgreenway.org/blog/goopy-gems-the-claylands-of-the-middle-fork-snoqualmie/