Chasing Erratics on the Great (Kitsap) Peninsula.

Yesterday I decided to go on a little adventure and investigate some of the known glacial erratics that I had heard of on the Great (Kitsap) Peninsula, specifically in the Illahee Area.

A glacial erratic is a piece of rock that differs from the size and type of native rock in the area on which it rests. They are carried by glacial ice, often hundreds of miles. Erratics can be as small as pebbles or larger than a building.

Erratics are common on the Great (Kitsap) Peninsula because this area has been subject to half a dozen or more glaciations over the past 2 million years, with the last one ending 16,000 years ago. During these glaciations, a large tongue of ice known as the Puget Lobe advanced south to fill the Puget Lowland as far as Olympia. Over the Great Peninsula, it was probably well over 2,000 feet thick! More than enough to flow entirely over the top of Green Mountain and the rest of the Blue Hills.

The extent of the Puget Lobe of the Canadian ice sheet. Photo courtesy of Washington DNR. 

Since this glacier came from the north, originating in southern Canada, it was plucking pieces of rocks off the mountains there. The rock types found in the Canadian Rockies and coast ranges are much different than those found here on the Great and Olympic Peninsulas, mostly granites and metamorphic rocks. Our hard bedrock here is either igneous rock of the Blue Hills or sedimentary rock of the Blakeley Formation. So if you see a granitic rock somewhere on the Peninsula, whether it be your yard, a beach, or a park, you can assume it's a glacial erratic.

We likely have hundreds if not thousands of significant glacial erratics on our peninsula. Most of them are probably buried beneath the surface, trapped in the thick glacial formations. This point was illustrated to me quite nicely when I was watching a well being drilled near Central Valley, it encountered a glacial erratic along the way down, and suddenly shards and flakes of beautiful granite came flying out!

However, there are some sizable erratics on the surface that you can find. Yesterday I found 3, which are listed below.

• Ther is a glacial erratic in the Almira Drive parking lot for the Illahee Preserve in East Bremerton. This is a metamorphic rock showing smooth surfaces probably polished by glacial ice as it was transported. 

The Illahee Erratic, with a telephone pole for scale. 

• There is a large glacial erratic on the east side of Illahee road just as the road begins to drop towards the coast traveling northbound. This is one of the largest erratics I have seen on the Peninsula. And interestingly. It is not granitic or metamorphic, it appears to be a large chunk of basalt, which means it may have been plucked from the Olympic Mountains.

• On the corner of Trenton Ave and Fernwood Street, in a lucky person's front yard, is a huge glacial erratic of Blue-Gray volcanic rock suspiciously similar looking to the rocks found in the Blue Hills. Perhaps it was plucked off of Green Mountain or one of the surrounding summits? The best viewing for this erratic is in the pullout on the west side of Trenton Ave, please respect the landowner's property by not attempting to go into their yard. 

These are just a few of the many significant erratics that are scattered around the peninsula, as I stumble across more, if they are substantial enough, I will do a write up for them too. I know there are several in North Kitsap, so perhaps I'll go explore up there sometime in the near future. Summer is just around the corner, and geologic adventures are everywhere in Washington State and the Great Peninsula!