The Church Mountain Landslide

By Dave Tucker  February 21, 2010

Map of the Church Mountain landslide.

The huge Church Mountain landslide devastated the Nooksack River valley in the area of Glacier, Washington approximately 2400 14C years ago (Carpenter, 1993). It is by far the largest landslide in the Nooksack drainage. This landslide initiated along the summit ridge of Church Mountain; the entire south slope of the mountain collapsed and fell 5300  feet(1615 m) into the valley of the North Fork Nooksack River. The slide then continued for about 6 miles (9.6 km) down the valley. The landslide deposit is over 300 feet (95 m) thick in its upper reaches, and has a calculated volume of 2.83 x 10^8 cubic meters (that’s 283 million cubic meters, or 368 million cubic yards, or 0.06 cubic miles).

Church Mountain landslide hummocks along Mount Baker highway at Mile 32. Click any photo to enlarge.

The Mount Baker Highway (Route 542) runs east across the top of the slide deposit from the Warnick bridge over the Nooksack (a few miles west of the town of Glacier) to beyond the next bridge over the river at the Douglas Fir campground. The deposit features some classic landslide deposit terrain- the road cuts through numerous hummocks in this stretch (see the map and photos). There is very convolute topography on the surface, the result of the rumpled surface of the landslide on the valley floor.  From east of Glacier, the monstrous slide scarp is very apparent on the south face of Church Mountain as you head up the highway. For a photographic view of the south flank of Church Mountain, visit the vignette on the “Geology of the North Cascades” website.

Getting there: Drive the Mount Baker Highway east from Bellingham. Cross the just-refurbished steel bridge at Warnick 0.9 miles east of Milepost 30). As soon as you are across the river, you are on the toe of the landslide. The highway stays on the landslide surface to about Milepost 36.

The Field Trip:

This house sits on a landslide hummock just east of the Glacier Public Service Station.

After the Warnick Bridge, hummocks line the road, especially around Milepost 32. Go slowly and see the mounded topography here. These hummocks may have large rock blocks in their centers, although I haven’t noticed any along this stretch. The Cornell Creek flats beyond Milepost 32 are alluvium on the surface of the landslide, deposited as streams tried to reestablish their drainages across the new surface. Just east of the town of Glacier, both Gallup and Glacier Creeks cut across the landslide surface. Beyond the Glacier Public Service Station (US Forest and National Park service; restrooms and interpretive displays), watch for a log house perched on the summit of a hummock on the north side of the highway. There are plenty of hummocks along the road between Glacier and the bridge over the Nooksack east of Milepost 35. This stretch of the Nooksack is especially fun, and hazardous, for rafters because of the steep gradient and rocks of the landslide deposit. Park at the bridge. If the weather is good, the scarp left by the landslide

The landslide took the entire south face of Church Mountain into the Nooksack Valley. This photo is taken from near the bridge at Horseshoe Bend.

is apparent on the south face of Church Mountain, directly above you. A hike along the Horseshoe Bend trail eastward along the river passes over and through the landslide deposit, as the river finds its way through the boulders. The deposit is a mix of rocks; the south flank of Church Mountain consists of slightly metamorphosed volcanic rocks of the Upper Paleozoic Chilliwack Group that are thrust over younger Nooksack Formation shale and sandstone. A drive up the Deadhorse Road (on the way to the Skyline Divide Trail) will eventually get you above the valley floor for a view north to Church Mountain. This is the same view shown in the Geology of the North Cascades website.

What caused this landslide? There is no smoking gun.  The faulted contact between the Chilliwack and Nooksack rocks near the top of the ridge high above may have been a zone of weakness contributing to the slide. Such huge landslides may be set off by large earthquakes, or simply by our constant companion: gravity. Gravity is always there, all the time, always trying to stabilize steep mountain slopes. Yes, I said ‘stabilize’, because steep valley walls are inherently unstable geomorphic features. They are much ‘happier’ once they become flat- lying shattered rock deposits on the valley floor.

Reference:

Carpenter, M.R., 1993, The Church Mountain sturzstrom (mega-landslide), Glacier, Washington (M.S. thesis): Western Washington University, 71 p. Available from WWU library.