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.
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
Vancouver Island erratics
Harpoon erratic, Harling Point, Oak Bay
Erratics in the San Juan Islands
Jackass conglomerate erratics on Crescent Beach, (near Eastsound) Orcas Island.
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)