A geological exploration of a deep-seated bedrock landslide
By Dave Tucker. January 10, 2010
By Dave Tucker. January 10, 2010
A field trip to the January 2009 bedrock landslide in Racehorse Creek makes for a fun and very worthwhile 1/2 day geo- hike from Bellingham. In addition to visiting the large landslide, there are lots of fossil plants in the blocks of Chuckanut, and fine views over the Nooksack valley. The main viewpoint you will need to get to is at about 1800 feet elevation, so if snow is low, don’t go.
On January 5, 2009, a ‘pineapple express’ storm smashed into the Pacific Northwest from the subtropical Pacific, and began delivering a deluge of rain and warm temperatures to the deeply snow-covered foothills of Whatcom County, Washington. The resulting accumulation of water in saturated snow and heavy runoff resulted in hundreds of debris flows and slope failures across the region. A few of these damaged or destroyed houses in Whatcom County. The largest landslide occurred on the afternoon of Jaunuary 7th in the Racehorse Creek drainage, a west-running tributary of the North Fork Nooksack River about 4 miles south of the community of Kendall. Unlike virtually all other landslides associated with this weather event, the Racehorse Creek landslide involved deep bedrock failure, releasing around 500,000 cubic meters of rock. The slide left behind a 90-foot)-high vertical wall crosscutting bedding planes in Chuckanut Formation sandstone and shale. Debris from the rockslide tumbled 800 feet down the south valley wall of Racehorse Creek, and temporarily dammed the stream. Two debris flow slurries were spawned at the base of the landslide; these entered Racehorse Creek below the debris dam. A flood of water, ice, rocks and logs scoured the
channel of Racehorse Creek, an important salmon spawning stream, stripping vegetation from stream banks and eroding bedrock walls to several meters above normal high water levels. High water nearly destroyed the state-owned bridge crossing Racehorse Creek. Resident Barry Hutchinson, who lives in the Racehorse Creek valley below the bridge reported a series of very loud sounds beginning at 3:20 p.m. on Wednesday, 7 January. The initial sounds were loudest and prolonged, and “sounded like a 747 at full throttle and tethered very close”. Approximately 20 minutes later he heard a wave of water, ice and wood debris impact the bridge not far from his house. “Blocks of ice the size of VWs” were tossed up on the bridge. Here is an article that appeared in the Bellingham Herald. There are a number of photos on a Washington State Department of Natural Resources website about the Racehorse landslide. and many other slides from the period during the pineapple express storm. The website is worth a look-see.
Field trip guide
You may want to orient yourself using the maps and geological descriptions on a poster that was prepared for the 2009 Geological Society of America meeting in Portland Oregon, the result of research into the dynamics of this fascinating landslide. Included are meteorological charts showing the weather conditions leading up to the slide, aerial photos, geologic maps, and an attempt to interpret the events of the complex landslide and attendant debris flows. Click to see the poster pdf file: Crider_et_al_GSA 09 Racehorse poster. Note that the maps are drawn with south at the top, to correspond to the pair of oblique aerial photos on the poster. The poster abstract is at the bottom of this page.
How to get there: Drive the Mount Baker Highway east from I-5 in Bellingham 17 miles to the junction with Mosquito Lake Road. Turn south (right). Cross the North Fork Nooksack, and turn left on the North Fork Road. Set your odometer to 0.0 here. This paved road eventually becomes gravel. You will notice some big blocks and hummocky terrain along the road at about 2.2 miles, the hall marks of rockslides. These are the deposit of a large prehistoric landslide that reached the Nooksack, dwarfing the 2009 slide. The hillsides above have slid multiple times since the end of the last glacial period, and the mountain itself is called ‘Slide Mountain’. Follow the mainline to a major junction immediately before the bridge across Racehorse Creek, 4.1 miles, UTM E563652 N5414815. NOTE- UTM coordinates given are zone 10, NAD 27 datum to correspond with USGS topographic maps. Turn right on this gravel logging road, and begin climbing up the hill slope. In about 0.2 mile, you may note a trailhead on the left, with parking on the right. This goes through the woods a couple of hundred yards to the rubble-choked course of Racehorse Creek. A quick trip out this trail will show you the effect of the January 2009 debris flows on this stream. Prior to the debris flows, Racehorse Creek was closely hemmed in with trees and brush. Now it is a wide, vegetation free gravel bar. You can walk up stream a few hundred yards to the lovely multi-tiered water fall. Believe it or not, kayakers go over the falls! But, we digress. Continue up the road beyond the trailhead. You will pass a few side roads, but stay on the main line. Turn sharp left 5.1 miles from Mosquito Lake Road, UTM E563745 N5413464, and enter clear cuts. The road switchbacks up. Pass a spur to the left- this was cut and destroyed by the debris flows. The road is ditched (in 2011) at about 5.8 miles; park here (plenty of parking and turn around space). There is a rough path along the road on the left side of these ditches. Continue up along the abandoned road to the next switchback, which curves to the right, next to a pile of logs. Leave the roadbed here ( UTM E564451 N5413353) on a boot track through the bushes, elevation is approximately 1450′. The entire field trip is on Washington State Department of Natural Resources timberlands.
From the point where you leave the road, look northeast up the clearcut and landslide-battered hillside to the skyline, around 350 vertical feet above. You will be heading up the slope to that ridge. Leave the road on a boot-beaten track and walk 100 yards across the late 1990s clearcut, aiming for a lonesome, prominent, tall fir tree with a bushy top (see photo above and at left). This is at the edge of the lower debris flow zone. Muddy debris flows swept past this tree, taking out a spur of the road you drove up, bifurcating, and reaching Racehorse Creek. The big tree survived the flood of mud; look up its trunk to see gouges in the bark left by flying rocks or logs. You may find a faint boot track heading up the slope from this tree, but it is of little importance if you don’t. You will now have to scramble around 300 vertical feet up a steepish slope that is littered with blocks from the landslide. These are Chuckanut Formation sandstone and mudstone, and are rich in plant fossils. Along the way, you could get totally absorbed looking for fossils of delicate ferns and deciduous tree leaves, or you might remember the ostensible reason you came and continue your trudge up the hill. Hike up the sometimes muddy but always rocky hillside, taking the path of least resistance, and staying in the scoured area of the debris flow fairly close to the vegetated edge of the landslide debris, all the way to the skyline ridge. You will need to crawl over or under fallen trees in a few places before you get there.
At the skyline, (somewhere near UTM E564754 N5413250, elevation ~ 1800′) the main portion of the landslide is before you. The gulf of the Racehorse Creek valley is to your left, and the creek itself is out of sight 800 feet below- the bulk of the rock slide went down that way. Look east across a jumble of vegetated debris on the bottom of an elongated depression. This is the clearcut forest floor, which includes intact section of rock, the soil mantling the bedrock blocks, living brush, stumps, and even a few standing trees. All this was translated with only minor disaggregation down the slippery dark rock slope on the other side; this rock slope is the sliding plane of the Racehorse landslide, which initiated along the crest at the top of the boulder-strewn slope. The slabby rock glistens in the right light, or when wet. The rock of this slope is a carbonaceous shale Chuckanut interbed, and the slope corresponds to the dip slope of strata here. The organic mat that eventually became this shale probably accumulated in a swamp or lake in a subtropical flood plain during Chuckanut times (Eocene). To the right is the high cliff that remained after the rock slide- it is as much as 90 feet high. The uppermost southwest corner of the slide is in the woods at the top end of this scarp. If you look again at the ‘black slab’ you may see some grooves worn into it by the passage of the sliding rock. These are shown mapped on the poster figure labeled ‘Map of the source area’. The grooves run more or less straight down the dip slope at the upper right, but grooves further to the left and lower run at a right angle to the slope, out toward Racehorse Creek. This pattern shows that initial movement was down slope, which is predictable enough, but then the debris changed direction and moved northeast over the steep valley wall to the creek.
A walk along the ridge crest toward Racehorse Creek, scrambling over downed trees and over landslide blocks, will allow you to see the jumble of rock on the slope leading down to the creek. It is pretty impressive. You can bushwhack upstream from the water falls (staying on the eastern bank and much higher in the forest) to get to the bottom of this slope. This is one of those “trips for demented folks” that I mention on the “About this website” page. It is not for the uninitiated in the ways of northwest off trail forest travel.
You may feel like scrambling off the steep slope from this rubbly ridge to the translated forest floor below. If you go out on that surface, be very careful, as there are some deep fractures between the duff-covered blocks. It is fascinating to see these, however, and wonder at how little the forest floor and its underlying rock were disrupted during movement. If you make it across the translated forest floor to the black shale, note that the dip slope shale is rich in fossil leaves. Alas, the rock is so friable it is almost impossible to collect anything intact or more than a few cm across. Should you try, you’ll probably end up with rock chips. This slope is just possible to ascend if it is bone dry- otherwise, forget it.
At the top of the black dip slope is another subtle ridge crest. This is more or less the margin of the clearcut. It is easy to suppose that the clearcut is related to the slide, but beware. Note that the rock of the dip slope rises to within a couple of feet of the surface at the ridge crest at the top. Regardless of the amount of forest cover, it is likely that water from the heavy rain and rapidly melting snow could percolate to this impermeable, low-friction, smooth, slippery, downslope dipping rock layer. Also remember that there have been numerous prehistoric deep seated landslides on this dip face further west on the mountain- it isn’t named ‘Slide Mountain’ for nothin’! There is a logging road very near the brink of the upper part of the high rock cliff. However, there is no gullying leading from this road or its adjacent drainage ditches to indicate that water from this road may have poured through the uncut trees at the upper right corner of the slide to lubricate the slide surface.
GSA POSTER ABSTRACT
THE 2009 RACEHORSE CREEK LANDSLIDE: FORENSIC DYNAMICS OF A LARGE, COMPLEX CATASTROPHIC MASS MOVEMENT
by Juliet G. Crider, David S. TUCKER, Douglas H. CLARK, and Scott R. LINNEMAN, Geology Department, Western Washington University, Bellingham
The January 2009 Racehorse Creek landslide in Whatcom County, NW Washington is a composite, storm-triggered rock slide–debris flow, which released ~5×105 m3 of rock, much of which was delivered to Racehorse Creek. An “ear-witness” report indicates the failure occurred the afternoon of 7 January.
The landslide exposed a ~2 x104 m2 carbonaceous bedding plane (the main rupture surface, dipping 28° NW), within the Eocene Chuckanut Formation. We estimate the friction angle of this surface to be <27°. Simple limit equilibrium calculations suggest that this surface was unstable before the triggering rainstorms.
The kinematics of the landslide is complex, as evidenced by the spatial variation in striation trends on the failure surface and by the unusual distribution of deposits. The principal slip direction is NE, nearly perpendicular to the dip of bedding, and towards Racehorse Creek. The debris deposit in this direction comprises three lobes of large (≥ 4 m3) blocky debris. Absence of run-up on the opposite side of the canyon limits the velocity of the debris flow. A large secondary debris lobe flowed NW, bifurcating twice down slope. A third deposit at the base of the exposed slip surface is a ~5000 m2 translated block supporting disrupted forest floor.
The crown of the slide is at a local ridgeline; several en echelon, ~0.75 m-wide cracks in the crown show no significant change from May-August. Lateral release was enabled by bedding- and strike-perpendicular joints, creating a 27-m-high vertical wall at the S boundary. At the N edge of the exposed dip slope, a parallel weathered joint surface is exposed above the canyon.
Together, these observations suggest several components of failure for this landslide: Rainfall induced failure on existing joints at the edge of Racehorse Creek canyon. Retrogressive failure removed bedrock buttressing the main failure plane, releasing the main failure, which broke up and flowed/tumbled northward and down 240 m into the canyon. As the main source zone was evacuated, the higher zone slid down-dip on the low-friction bedding surface, transporting in situ stumps and forest floor into the original source area. From the highest elevation, debris gained enough momentum during translation to overtop a local ridge and form the NW deposits.