Milepost “Central Access Route”:

We’re here!  We have finally arrived on the “Central Access Route” approach to the Alaska Highway.  However, we didn’t get on it at Mile “0” at Ellensburg, but at Omak via WA 155 after visiting Grand Coulee Dam.  So, I can’t really comment much on the geology farther south, but even beginning at Milepost E157.6 and following the access route only to Kamloops at Milepost E386 provides an amazing journey into an “exotic terrane.”

We drove across a granite batholith called the Okanogan Dome from Grand Coulee Dam,  The granite dome is a melted portion of a “subcontinent” (larger than an island arc) that migrated by continental drift and hit the North American Plate just before it melted 50 million years ago.  The photo below shows an pale gray outcrop of the granite of the dome.

E157.6: At Omak, we entered the exotic terrane called the Okanogan Trench, the old subduction zone at the edge of the North American Plate plate (see my June 10 post on Amchitka Island and the Aleutian Trench). 

The photo below was taken from WA 155 just east of Omak, looking west across the Okanogan Valley.  I am standing on the rocks of the Okanogan Dome, looking down into the trench and across to the mountains of the Cascade Subcontinent on the west side of the valley.  Millions of years ago, these three zones of the Okanogan Trench were separated by thousands of miles of Pacific Ocean.

 

E159 - E180: The Okanogan Dome is surrounded by one-time sedimentary rocks of the subcontinent that were metamorphosed by heat from the melted granite below them.  As the dome rose, the metamorphic rocks were tilted upward around it.  In the photo below, the angled rocks are located east of US 97 and dipping (sloping) westward into the Okanogan Trench.

E171 – E181 (north of Riverside, south of Tonasket): Look west of US 97 at the base of the hills.  Do you see a valley (depression) that is lower than the road and looks like it should carry a stream, but there is no stream?  There are a couple of small lakes at the bottom of the deeper parts of the depression—Booher Lake and Crumbacker Lake—but nothing that could have carved such a deep valley.  That valley is an old river channel, now dry, that was carved by glacial meltwater at the end of the last ice age about 14,000 years ago (see my May 27 post).  This channel will probably remain dry until the next ice age!

The photo below shows the dry channel west of US 97/WA 20 and the present course of the Okanogan River east of the highway.  It also shows the short distance across the Okanogan Trench on the U.S. side of the border.

 

E197 - E315: A series of natural glacial lakes filling the bottom of the Okanogan Trench begins at Oroville, WA and ends at Vernon, BC, a road distance of 120 miles.  The lakes, from south to north, are Osoyoos Lake, Vaseux Lake, Skaha Lake, the big one—Okanagan Lake, and Kalamalka Lake.  These remnants of the last ice age provide substantial quantities of irrgation water for the intensive fruit and wine growing industry of the Okanagan Valley.

E231.2: Photo taken at Christie Memorial Provincial Park day-use area on in Okanagan Falls showing Hwy. 97 climbing into the bedrock on the west side of Skaha Lake.  This dark gray, white-banded gneiss is part of a subcontinent that “docked” with the NA Plate about 50 million years ago.

E237.9: Other remnants of the ice age are less obvious than the huge lakes of the Okanagan Valley.  From the overlook of Skaha Lake just before entering the city of Penticton, there is a view of two of those remnants.  In the photo below is a grass-covered, light-colored, nearly vertical bluff rising from the lake.  This bluff is composed of silt and clay deposited in Glacial Lake Okanagan, which once filled the entire valley with significantly more water than exists today in all five lakes. 

Above and to the left of the bluff are several dark gray, rounded rock outcrops.  And, if you look to the right above those three outcrops, you can see a number of other smaller, similarly shaped outcrops.  These rounded features are called “roche mountonnees,” a French term meaning “sheep rocks.”  On all of these outcrops, there is a smooth, gently sloping side on the left and a steeper, more angular side to the right.  These features can be used to determine the direction of movement of a glacier, with the smooth side ground down more by the glacier moving across it from left to right.

 

E290 - E320: Highway 97 barely fits into the bench along the east side of Okanagan Lake.  The slopes above the road are steep because the road is built almost on top of the Okagangan Valley Fault, a system of normal faults that run the entire length of the Okanagan Valley.  Near Vaseux Lake in the southern end of the valley, the faults dip (slopes) only about 10 degrees to the west.  North of Kelowna, however, the faults become much steeper before veering off to the northeast north of Vernon.  The photo shows the northern end of the fault along Kalamalka Lake just before it leaves Highway 97.

E323 - E360: Highway 97 follows a valley that formed along another fault system called the Bolean Creek Fault.  This fault divides two very different geologic provinces—with metamorphic rocks to the northeast of the highway and sedimentary and volcanic rocks to the southwest.  Interestingly, both rock types weather into rounded hills and long ridges and both are nearly completely covered by conifer forest, so little geology is visible to a 55-mph traveler through this area.  The valley, however, is also a geologic feature because it follows the weak zones created by the fault.  Movement of faults form "shear zones" of broken-up rock rubble that are more easily eroded by streams than the surrounding rocks.  So, the streams and rivers follow those shear zones, sometimes in a very straight line, and geologists use stream patterns to locate those faults.  They also use stream patterns to identify certain rock types and geologic structures.

E370 - E386: Highway 97 joins Trans-Canada Highway 1 at E369.5 and follows the South Thompson River Valley into Kamloops.  The most noticeable geologic feature of this portion of the highway is a light-colored bench running the entire distance (photo below).  This is “lucustrine silt,” the rock flour created by the glaciers in the mountains to the north and east washed into Glacial Lake Thompson.  The ice dam that created this lake was located in the area of today’s Kamloops Lake on the Thompson River east of Kamloops.  The silt settled to the bottom of this huge lake after each year’s spring runoff, forming layers called “varves.”  The thickness of this silt is estimated at nearly 1,000 feet and, some geologists believe, may have been deposited in no more than 100 years!