Snoqualmie Pass in the Cascade Range

this is Snoqualmie Pass easily the most heavily traveled mountain pass in all the Pacific Northwest I 90 crosses the Cascade Range here let's take a little trip from North bend up and over Snoqualmie Pass to learn old stories from the bedrock the landforms and even days of early human travel up and over the range remember from our last episode of i-90 rocks heading east out of North Bend means climbing grouse Ridge a complex of glacial till glacial outwash and glacial erratics from the Puget lobe the thick ice sheet from Canada that covered Seattle but I ninety East here uses a valley sculpted by glacial ice that flowed down from the mountains alpine ice not an ice sheet the freeway goes up to the South Fork of the Snoqualmie River and then down the Yakima River on the other side of the pass both valleys were under alpine ice many times during the Ice Age but in the North Bend area the mountain glacier was already melting back into the mountains when the Puget lobe the ice sheet from Canada arrived for the last time sixteen thousand nine hundred years ago as you drive the thirty miles from North Bend to Snoqualmie Pass alpine glacial deposits are mostly concealed by a dense forest cover but in the high country glacial erosion afia's abound McClellan Butte a glacial horn was shaved by ice and huckleberry mountain and ChiCom in peak their beauty is a direct result of glacial erosion and u-shaped valleys a classic signature of alpine ice are on display but something's missing up here at the pass where are the cirques so this is the usual scene in a mountain range with alpine glaciers ice flowing in both directions away from the crest of the mountains ice is carrying rocks plucking rocks away from the bedrock and as the glaciers continue to erode they leave steep walls so when the ice melts back we have a search on one side and a cirque on the other big bowl shaped cuts that the glaciers eroded away from the bedrock to steep walls on the backs of both Sirk's is the usual scene so to build a road up and over the past usually you have to get up and over these back-to-back circles Snoqualmie Pass is different so Kwame Pass is lower because more than a thousand feet of glacial ice crossed the divide in this part of the Cascade Range there's no steep wall there's no big circ right at the divide right it's no qualms you pass a thousand feet of ice crossing from west to east here's the map that will help us two glaciers flowed toward the pass from the north glaciers formed in the valleys of what is now source Lake and Commonwealth Creek the glaciers merged at the base of guy peak then now bigger glacier flowed down to Snoqualmie Pass and then right at the ski area the ice split again one tongue of ice dropped to the west down to North Bend the other ice tongue flowed right over Travelers Rest at the pass and headed down the Yakima River Valley to the east the ice disobeyed the drainage divide and did its own thing impressive all of that ice is gone now the Yakima River today takes the scenic route to the Pacific Ocean east to Ellensburg south to Pasco and then finally west the Columbia River Gorge to Portland and then on to the ocean but the Cascades have a much longer history than the Ice Age world-famous cone-shaped volcanoes fueled by ocean floor subduction has been steady here for tens of millions of years but Mount Rainier is only half a million years old and Mount st. Helens much younger than that state of Washington Cascade Range it's a volcanic mountain range with 40 million years of history of volcanic eruptions these are the five cones we have today Mount Baker Glacier Peak Mount Rainier Mount st. Helens Mount Adams and here Snoqualmie Pass in i-90 cutting right through that scene there's a 40 million year history but each of these cones have been standing for less than two million years Cowen's have a two million year lifespan so if we go back over the last 40 million years there are other locations where cones used to stand they've been carefully mapped out dozens of old places where cones used to stand they've been eroded away completely and these five are the ones we have to enjoy during our time on the planet so I ninety travelers crossed through the Cascades without ever seeing a volcano but looking for coarse-grained granite has its advantages so how do we know where these cones used to stand if they're completely gone how do we know where they used to be the answer is the magma chamber magma chambers feed active cones it's the plumbing supply the magma is feeding the volcanic eruptions but if you remove the heat the magma turns to stone and when we around the cone the magma chamber rock is right at the surface often accompanied with uplifts you can have a mountain range composed of this rock that formed down below in a magma chamber that's the scene in many places where the cones used to stand I ninety travelers drive right through one of those old magma chambers the Snoqualmie batholith much of the bedrock west of the summit is granite that was once molten supplying Magma's to volcanoes 25 to 17 million years ago a huge batch of magma chamber rock that's a batholith the beautiful granite has coarse mineral grains of feldspar mica hornblende and quartz the minerals are big due to the slow cooling of the magma deep underground and there are more exposed granite batholiths north of the Snoqualmie Pass area all the way to Canada huge magma chambers compared to the plumbing systems beneath our majestic cones today east of the summit rock blasting has been an annual summer tradition to straighten out some troublesome I 90 curves digging and blasting has been a full-time job to make room for additional lanes straighter stretches and safer travel through this zone that is prone to Rock Falls and snow avalanches what are they blasting here more granite of the Snoqualmie batholith nope these are older andesite lavas and dents welded Tufts of the Johanna Prakash formation volcanic layers that are typical south of the pass clear down to Oregon volcanic deposits from explosive volcanoes that died long ago these aren't magma chamber rocks these rocks record volcanic eruptions between 33 and 27 million years ago the deposits outlived the volcanoes that made them here along i-90 dangerous Rock Falls have been a regular occurrence the bedding planes and fracture sets in the bedrock are tilting toward the highway highway engineers and geologists have been working together to improve safety along this stretch of i-94 being removed and to secure the remaining bedrock deep anchors are drilled into more stable rock behind the fractured faces wow what a challenging place to work a narrow path between Lake Ketchel as' and the rock wall thousands of vehicles each day driving right through your work site there's a long history here of trying to squeeze a road into this narrow path between Lake Ketchel as' and the rocky ledge along the shore and before that water navigation across Ketchel as' was required and before that no vehicles of any sort through here for centuries Native Americans used a narrow footpath to cross the Cascades at Snoqualmie Pass much of i-90 follows the old trail used by the Snoqualmie people and the Yakima's the Indians told of deep snows tens of feet deep at the pass a product of the rainshadow effect west of the pass the forests were so dense that sunlight seldom touched the earth huge trees towered hundreds of feet above the forest floor locking their branches together overhead and shutting out the sunlight early white explorers in the 1850s searching for a possible wagon road and maybe even a railroad looked hard for a suitable place to cross the mountains a decade later wagons rumbled over the old Indian trail through the pass on the west side sections of the valley floors were bottomless quagmire impossible to cross without corduroy split wooden planks laid over the muddy bottoms and then in the first two decades of the 20th century a new era came to Snoqualmie Pass the very first automobiles somehow got through on the rough primitive wagon road the Milwaukee railroad completed a line over the pass and then an impressive new two-mile tunnel through the ridge opened in 1914 which cut off four miles of track and a steep climb for the trains reliable year-round travel over the Cascades was now a reality by rail thousands of people began enjoying the mountain grandeur in the summer by taking the train from Seattle and picnicking at Lake Ketchel as' and the construction of the railroad quickened the interest in an improved Road over the summit especially now that the horseless carriage was now on the scene the sunset highway Washington's first passable automobile rode between Western and Eastern Washington was opened but the early cars weren't exactly reliable Inns were situated every 10 miles or so and keeping the road open in winter was a major challenge the first year the pass was open all winter 1932 plans for more lanes of traffic were begun in the late 1960s but environmental concerns resulted in changes to construction techniques the Franklin Falls Denny Creek viaduct used a moveable scaffold system without ground support to preserve the forest and mountain landscape as much as possible the new elevated stretch of i-90 was completed above the Falls in 1981 think of all the travelers through time that have passed below guy peak a Sentinel that watches over Snoqualmie Pass it's not a volcano it's not a lava flow or part of a granitic batholith it's not even igneous rock guy peak is made of sandstone that predates the formation of the mountains themselves so the Cascade Range how long has it been around 40 million years but there are are rocks that are older than 40 million years sitting on top of old bedrock there's a thick deposit of sandstones and shales that were deposited between 55 and 40 million years ago when Washington was a flat place there were no major mountains these are thick sands shales Clay's coal beds that stretched across from west to east this is the whole state of Washington here but when the Cascades began Magma's from below work their way up through the middle of this picture and volcanic activity started so we have those sedimentary rocks to the east and west of the Cascades and occasionally you can find little remnants of these sand stones within the Cascades that's guy peak part of this sedimentary cover that predates the development of the Cascade Range the sandstone of guy peak is clearly much higher than it was when Washington did not have the Cascades there's been serious tectonic uplift here the Juan de Fuca plate has been slowly bashing in the leading edge of the North American plate a collision which has forced more than 2,000 feet of uplift in the last few million years and more than old sand stones and magma chamber rocks have been revealed by the uplift down by North Bend mount sized oceanic meta gabbro bedrock requires tens of miles of movement inland and thousands of feet up Franklin Falls west of the summit exist due to stubborn metamorphosed lava rock also with an oceanic history that has since been uplifted Denny mountain is a huge block of marble another metamorphic rock hanging out high in the Cascades so many misplaced blocks of exotic bedrock up here that have stories of their own and the past lies at a newly discovered boundary Central Washington University operates a dense network of high-precision instruments that have been drilled into bedrock all across the northwest the Pacific Northwest geodetic array panga closely monitors tiny movements in the crust together the GPS stations have revealed a new way to look at the Snoqualmie Pass region so all those GPS stations of the panga network are across this map and these are the results there's a beautiful graceful clockwise rotation of the crust in the Pacific Northwest these yellow arrows the longer the arrow the faster the motion everybody is rotating around Pendleton Oregon for some reason and it's like the old game of crack the whip you come around the outside you're moving faster than the guys in the middle but not everybody is rotating this northeastern part of Washington is stable it's part of the Canadian buttress that's not playing this rotation game so right here at Snoqualmie Pass and the Hyack station the gps station from panga that's showing still a little bit of motion to the northeast we're crunching the crust rotating block into non rotating block tons of faults tons of folds that explains most of the topography in Central Washington plus the newly discovered clockwise rotation of the North West offers an elegant solution to an old Cascades volcanism question cone shape volcanoes in the Cascades why do they exist there's a subducting Juan de Fuca plate and magma is rising to the surface on a map the cones are in a beautiful line stretching through the Pacific Northwest that's today but 16 million years ago and this has been known for a long time now the line of cones was different and the line switches to the east of the Cascades right here at Snoqualmie Pass nobody could explain this pattern until recently until when until we discovered this clockwise rotation of the Pacific Northwest we're rotating the crust over magma coming up from the subduction zone new instruments solving old questions young volcanoes replacing older ones an Indian foot trail a wagon road the sunset highway and now interstate 90 amazing changes in just a few decades today crops grown in Eastern Washington's Columbia Basin irrigation project are trucked every day to Seattle's ports using Snoqualmie Pass and the mountains to Sound Greenway first envisioned in 1990 was founded to work toward a shared vision of keeping some of these natural lands intact along the i-90 corridor through the Cascades well we're up and over the pass looking back it Snoqualmie Pass there's the road construction site the u-shaped valley remember that Yakima Valley glacier flowing east that's where we head next how far east did that glacier try does catch Allah slake have anything to do with that glacial activity onward to Cle Elum and the coal mines east of here thanks for watching

Channel: hugefloods

Views: 255,039

Rating: 4.9212446 out of 5

Keywords: Snoqualmie Pass, Snoqualmie Summit, I-90, Hyak, Cascade Range, Volcanoes, Rainier, Geology, History, Hiking, Yakima River, Snoqualmie River, Keechelus Lake, WSDOT, Construction, Sunset Highway, PANGA, Ski, Mountains, Cascades, Pacific Northwest, Interstate 90, Snoqualmie People, Yakama People

Id: fuK2kGeV15Q

Length: 18min 55sec (1135 seconds)

Published: Tue May 26 2015

Reddit Comments

[u/my_lucid_nightmare]

/u/GeologyNick is quite possibly the best thing to happen to this sub in a while.

[u/Lavenders_Blue]

Wow! I moved from Southern California to Washington about 3 years ago (my Husband grew up here) and have loved exploring this beautiful state!
This video is awesome! I love the way you explain everything in a way that I can understand, without much geology knowledge.
My husband works at the ski area in Snoqualmie Pass, so I shared this video with him on Facebook and tagged a bunch of other guys from Summit. I can't wait to watch the rest of this series! Have you made anything similar to this on US 2? I love that highway, it's so beautiful!

[u/wpnw]

The existence of the crustal rotation phenomena is fascinating, never even heard of that idea (around here at least) before. Thanks for going in to that a bit.

Fairly specific and random question for you, at the 10:45 mark you used a photograph of the upper section of Weeks Falls on the South Fork - is that per chance sourced from a publicly accessible digital collection somewhere (UW has a pretty extensive collection of historic photographs online, for example)? I'd love to be able to dig through it if so, if you could point me in the right direction if possible.

[u/dlannoye]

Really enjoyed these videos. I am looking forward to driving up the pass again looking for these features.

[u/seahanks]

Just went out to Twin Falls today from Seattle and was talking about this series. I really enjoyed looking at the different types of rock that was exposed while out there. Great work!

[u/koshkat]

Really interesting video! Thank you for sharing your knowledge.

[u/Gerhardwa]

Fantastic video! Your geology 101 class is still one of my favorites.

[u/hunchxpunch]

Wow, great effort and series. Thanks for sharing.

One critique, consider a mic and windscreen for your outdoor filming.

[u/Calypte]

I love these videos. I just picked up Roadside Geology of Washington. Thanks /u/GeologyNick!