Ross Lake, a hydroelectric reservoir and principal attraction of the Ross Lake National Recreation Area, is 22 miles long and ends just beyond the U.S. border. It's also the dividing line between two very different geologic zones, thanks to the Ross Lake Fault, which runs along the east side of the lake, parallel to the East Bank Trail. On the west side of the lake are the hard-rock metamorphic terranes of the North Cascades. On the east side are the pastoral, sedimentary terranes of the Pasayten Wilderness. (A terrane is a rock formation, a crustal block, that has a geologic history different from surrounding areas and is usually bounded by faults.)
West Side Story: Metamorphosis
The hard rock of the Metamorphic Core Domain creates the rugged alpine vistas of the North Cascades. On the west side of Ross Lake, the scenery is dominated by high ridges carved from the Skagit Gneiss Complex, leading up to Mount Prophet (7,660 feet). Gneiss (pronounced "nice") is a coarse-grained metamorphic rock similar to granite, with alternating light and dark bands that cause it to break rather easily into course slabs. The light-colored bands are usually composed of quartz and feldspar; the dark bands are dominated by mica.
As the word "complex" indicates, Skagit gneiss is not a simple rock. It contains remnants of sedimentary seafloors, volcanic eruptions, and continental crust, which were sheered, crushed, melted, and recrystallized together deep inside the earth to produce the rocks you see today. The process began 220 million years ago, when volcanic islands of basalt and andesite rose from an ocean floor. Then, 90 million years ago, tectonic forces shoved the crustal block containing the volcanic and sedimentary rocks 100,000 feet below the crust, where tremendous heat and pressure metamorphosed them, alternately melting and recrystallizing the minerals over the millennia. Skagit gneiss even contains rare outcroppings of marble, which may be the metamorphosed fossil remnants of calcareous algae mats from that ancient Skagit sea.
East Side Story: Sedimentation
East of Ross Lake Fault, the geology is less dramatic. The Methow Domain begins here and extends eastward, serving up the rounded hillsides and broad valleys of the Pasayten Wilderness and Methow Valley. Softer, more erodable sedimentary bedrock is the primary reason for this scenic transformation. A gentler climate also plays a role; rain clouds drop most of their moisture over the Cascades.
The Methow Domain began as an ocean floor. Formed between 350 and 220 million years ago, it consists mostly of layered sedimentary rock: marine sandstone and shales (mud), along with ocean-floor volcanic basalt and lots of chert. Compared to the jumbled, metamorphosed rocks in the Metamorphic Core Domain, the rock beds of the Methow Domain are quite orderly. Over broad areas, the sedimentary layers are stacked neatly on top of each other, in the order they were deposited. Although fossils and sedimentary structures indicate the rocks have been strongly folded, and in some areas faulted, the sequence in general represents deposits from an ocean basin that gradually filled up.
At Ross Lake, the Methow Domain's most prominent feature is Jack Mountain (9,066 feet), which is carved from basalt in the Hozomeen Terrane. The lower slopes of Jack Mountain are underlain by mica schist and other rocks, which are separated from the overlying Hozomeen Terrane by a thrust fault.
Literary InSPIREation
Looking far up the lake, you can pick out the distant spires of Hozomeen Mountain and Desolation Peak, a source of literary inspiration for Beat Era writer Jack Kerouac.
Poet-author Kerouac's spiritual quest and desire for simplicity and solitude led him to work as a fire watcher at Desolation Lookout during the summer of 1956. The experience proved pivotal, contributing much to his books The Dharma Bums, Desolation Angels, Lonesome Traveler, and Book of Blues.
Read the informative plaque, and if you have binoculars, see if you can pick out the (still active) fire lookout atop Desolation Peak.
Logging Requirements
To claim a find for this earthcache, message me the correct answers to the following questions. Finds posted without the answers being sent simultaneously, or with incorrect answers, will be deleted per earthcache requirements.
- Desolation Peak is part of what geologic domain? (Hint: Compare the diagram on the plaque to the one on this page. Which side of the fault is the mountain on?)
- Which mountain would you expect to erode away first, Mount Prophet or Jack Mountain? Why? (Hint: Which domain is each mountain in? What does this page say about the rock types?)
We'd also love to see a photo of you at the viewpoint.
Winter Access
The North Cascades Highway (SR 20) closes each winter after the first major snows, usually from early December until late April. The closure is from milepost 134 (west) to milepost 171 (east). This cache is inside the closure area on the west side, slightly more than a mile east of the MP 134 gate. However, while it is inaccessible by car during the winter, you can get to the location on XC skis or snowshoes, according to North Cascades NP, so I will be leaving it active year-round. You can check the status of SR20 here.
Sources
Geology of the North Cascades: A Mountain Mosaic by R.W. Tabor and Ralph Haugerud (Mountaineers Books, 1999).
Hiking Washington's Geology by Scott Babcock and Bob Carson (Mountaineers Books, 2000).
To learn more about geology within the North Cascades National Park Service Complex, visit Natural Features & Ecosystems on the park's website.
Note: This earthcache was approved by the Superintendent of the North Cascades National Park Service Complex and the Chief Ranger of Ross Lake NRA.