Sauk River Geology

             The peaks and valleys of the Cascades Range are carved from a complex geologic patchwork, with volcanic intrusions and massive sedimentary blocks pressed up against the range’s metamorphic spine on every side. The Sauk River, which drains Glacier Peak and the surrounding area, provides a perfect snapshot of this turbulent history.

              Towering over 10,500 feet, the summit of Glacier Peak is the highest in the Sauk-Suiattle watershed. The combined watershed of these two rivers drains 96 glaciers. In the upper drainages traces of older alpine glaciers can be seen in deposits of glacial till--the haphazard piles of rocks left by retreating ice--and glacial outwash plains. Most of these deposits are about 20,000 years old. West of the river the geologic footprint of a larger glacier can be seen; most of the lower valleys in this region are filled with sediment dropped by the Cordilleran Ice Sheet 15,000 years ago. The Cordilleran Ice Sheet was an enormous continental glacier that stretched from southern Alaska all the way through British Columbia to northwest Washington. The projected ice surface altitudes of this ice sheet suggest that nearly the entire crest of this part of the Cascades was covered in ice!     

              The section of Sauk River area rocks you’ll see from our rafts is a unit called the Darrington Phyllite, whose original source rocks were likely formed in the Jurassic period. These rocks have gone through multiple deformations that destroyed most of the original sedimentary structures; the original minerals were also replaced during metamorphosis. Keep an eye out for traces of the original sedimentary bedding (layers) in any boulders you find along the way!

             Beneath these geologically young deposits the rocks of the Sauk River area are divided into two distinct halves by the Straight Creek Fault, which separates the western area of low-grade metamorphic rocks and the eastern area of medium- and high-grade metamorphic rocks. The eastern half of the Sauk River region contains three distinct “terranes”, or geologic regions bounded by faults: the Swakane terrane, the Chelan Mountains terrane, and the Nason terrane. The Swakane terrane is composed entirely of biotite gneiss, which probably started as sandstone before being subjected to enough heat and pressure for metamorphosis. Over the Swakane terrane lays part of the Chelan Mountains terrane, a rock unit called the Napeequa Schist. This unit is made up of quartzite, amphibolite and hornblende schists that were once part of an ocean floor. The last unit, the Nason terrane, is also former seafloor, likely from the Jurassic.

              West of the Straight Creek Fault most of the rocks are also ocean-sourced, though they were metamorphosed at lower temperatures and pressures than those east of the fault. This area is cut by a series of tectonic faults known as the Darrington-Devils Mountain Fault Zone, complicating the local geology. A set of low-grade metamorphic rocks known as the Northwest Cascade System border this fault zone on one side, while on the other side of the fault system two mélange belts present a jumble of different rock types. Parts of these mélanges are sedimentary rocks instead of metamorphic, and contain a few different kinds of fossils; however, most of the fossils are very small and deformed, and were described by the USGS geologists mapping the area as “usually uninspiring”.

              Any journey through the Sauk River region presents an intriguing slice of these mountains’ complicated geologic history, from the enormous glaciers of the recent past to the ancient ocean floor that now forms much of their rocky peaks. These rocks have been folded, buried and reformed, torn by tectonic faults, scraped clean and then covered in sediment by ice. Each one of them shows the traces of this past, in minerals and bedforms and tiny fossils; a story, ready and waiting for those who learn how to read it.


Geologic Definitions:

Metamorphic—a rock that has been changed from its original structure and composition through heat and pressure. Common examples include gneiss and schist.

Igneous—a rock of volcanic origin. Common examples: granite and basalt.

Sedimentary—a rock formed from compressed deposited sediments. Common examples include sandstone and shale.

Sources: GEOLOGIC MAP OF THE SAUK RIVER 30- BY 60-MINUTE QUADRANGLE, WASHINGTON By R.W. Tabor, D.B. Booth, J.A. Vance, and A.B. Ford, part of U.S. Geological Survey Geologic Investigation Series I-2592

Skagit WSR - Skagit Wild and Scenic Facts Mt. Baker-Snoqualmie National Forest - Resource Management, (accessed May 2017).