Strewn among the forests of the Puget Lowland are an impressive array of Glacial Erratics. Although technically erratics include small pebbles and even sand grains in till, people tend to get the most excited by really big erratics- those the size of a house or building. Whatever their size, glacial erratics often reveal the direction of the ice flow that ripped them from their source region and deposited them at their resting place. Erratics can also help determine the thickness of the ice sheet that deposited them.
In the Seattle area, it is relatively straightforward to determine the provenance of erratics containing quartzite, as that rock can not be found anywhere in the Olympics, the Cascades, or in the native bedrock of the Puget Lowland. Quartzite, which is durable and resistant to weathering, is commonly found in river valleys of central British Columbia and in the glacial tills of the Puget Lowland. However, granodiorite erratics found in the lowland and on the slopes of the Olympics and Cascades can be more difficult to decipher. The granodiorite in these erratics is chemically similar to rock bodies both in the British Columbia Coast Mountains and in the Cascades.
In the mountains east of Bellingham, erratics found at 5,000 ft of elevation imply that the thickness of the Puget Lobe of the Cordilleran Ice sheet was at least 5,000 ft thick at that location. Maps showing the thickness of the Cordilleran Ice Sheet at various places in Western Washington are drawn, in part, by looking at the depositional altitude of erratics on ridges around the region.
The two most massive erratics in the Puget Lowland are composed of Greenschist metamorphic rock.
One of the giant erratics of Western Washington hides in the forest in Saratoga Woods Park on Whidbey Island. The Waterman erratic is 38ft high and has a circumference of 135 ft. Analysis of the rock shows that it is composed of Greenschist, a rock containing the low grade metamorphic minerals Chlorite and Epidote. The Greenschist chemically matches the rock of Mt. Erie, a prominent Roche Moutonée found 40 miles to the north on Fidalgo Island.
An even larger erratic can be seen in a suburban neighborhood near Everett. The Lake Stevens erratic, 34 ft tall with a circumference of 210 ft, is considered the largest erratic in the Northwest, and possibly the largest erratic in the United States. This erratic is also composed of a Greenschist, although chemical analysis cannot give us a definitive origin for the big rock. There is similar Greenschist on the northern tip of Whidbey Island at Deception Pass, around Mt. Shuksan in the North Cascades, and also up the Fraser River in British Columbia.
Article by Dale Lehman