Glacial erratic geology field trips

A glacial ‘erratic’  is a rock fragment carried by glacial ice and transported some distance from the outcrop from which it was derived, and generally resting on bedrock of a different type* [although glacial or alluvial sediment may directly underlie the erratic. Although technically the dimensions of erratics include pebbles, these trips visit BIG erratics in northwest Washington and southwest British Columbia.

Ways that erratics may be incorporated into glacial ice. Click to enlarge.

How do really big rocks, such as the Waterman and Lake Stevens erratics, get carried by glaciers? The conventional wisdom is that glaciers do not pick up really huge rocks they override. They may push big monsters along, but it is likely that the ice mostly flows around these big ones by way of “pressure melting”- the ice simply melts as it is forced against these big obstacles, and reforms downstream. More likely, these big blocks fall onto the ice surface in the mountains, and are carried on the surface, or near the surface fi buried by seasonal snow, of the ice. Or perhaps large blocks are plucked by glacial pressure from high points the ice overrides, and then carried within the glacier. Once the climate is too warm to sustain further glacier growth, the glacier ice begins to melt until eventually the erratic rocks are left sitting on the surface where the ice once stood. Much smaller boulders are easily enough shoved along by the base of the glacier, as they don’t have enough mass to interfere with the forward pressure of glacier movement.

Trips to these erratics are categorized geographically. Use the ‘categories’ menu to find the area and then the erratic you’d like to visit.

*Definition is from Dictionary of Geological Terms, American Geological Institute (Doubleday).

Field trips here visit

Lower Mainland of BC erratics

Shasta Court, Coquitlam

The Aldergrove erratic

Vancouver Island erratics

Harpoon erratic, Harling Point, Oak Bay

Bellingham erratics

Whatcom County, including these excavated ones in Ferndale , this one in Blaine, and the Saar Creek erratic east of Sumas.

Cascade Range erratics

Erratics in the San Juan Islands

Jackass conglomerate erratics on Crescent Beach, (near Eastsound) Orcas Island.

Everett-Seattle-Tacoma area erratics:

Lake Stevens Erratic– the largest in the USA?

True Granite Erratic– Lake Stevens

Wedgwood Erratic , Seattle

Beach erratics at Discovery Park, Seattle

Ravenna Park erratic, Seattle

Leschi Park fossiliferous erratic , Seattle

Big Rock , Duvall

Edmonds Way erratic, Edmonds

Fantastic Erratic, (hike in Cougar Mountain Park, Issaquah)

Martha Lake erratic, Martha Lake County Park

Des Moines Beach Park erratic, Des Moines

This en echelon dikes in the Highline Community College erratic, Des Moines

Whidbey Island erratics

Big Rock erratic, Coupeville

Waterman Erratic, (hike near Langley)

Olympic Peninsula Erratics:

White Rock (Hood Head)