North Cascades glacial erratics

These erratics are chosen because they help constrain Pleistocene ice thickness, or are particularly exotic rocks.

Entries in this chapter:

Chilliwack Group erratics on Table Mountain, above Heather Meadows.

By Dave Tucker. December 7, 2009


Several greenish erratics with  brown to gray angular clasts sit on the summit of Table Mountain, a stack of andesite lava flows above Bagley Lake in Heather Meadows. The erratics are clearly different from the surrounding sea of shattered andesite lava covering the plateau of Table.

Green erratic of Permian breccia on the crest of Table Mountain. A glacier carried it from Mount Herman, in the background, 2 km to the north.

The erratics are Chilliwack Group breccia, which underlies the high peak of Mount Herman rising just across the deep gulf of Bagley Lakes to the north. Here is an interesting juxtaposition, for although both Table Mountain and Chilliwack breccia rocks are andesitic, they have very different origins. Table Mountain lavas are young and subaerial, erupted around 300,000 years ago from a vent to the west on Ptarmigan Ridge; the Chilliwack rocks are ancient and submarine, the remnant of a Permian island chain of volcanoes that shed eruptive debris into an ancient ocean. These rocks were later accreted to the margin of North America during subduction.

Significance of this site: these erratics demonstrate that 1) glacial ice of the last glaciation (Vashon) came south and was thick enough to deposit erratics at 5550 feet (1590+ meters) elevation; 2) this ice had to cross Excelsior Ridge, about this same elevation or higher, between Table Mountain and the Fraser River.

To get there:

Hike the Table Mountain Trail, which begins at 5040 feet (1536 m) at the end of the Mount Baker Highway (WA Highway 542). The erratics are found on the flat plateau of Table Mountain, near the north rim, after you have climbed the steep switchbacks from the parking area, not far west of the eastern summit at 5553 feet (1690 m). They are about 1/2 mile from the parking lot, after a gain of 500 feet.

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