Friday, March 2, 2012

Erratic Conglomerate

Glacial erratic boulders are a common site along most of Washington State beaches. Only the south half of the outer coast lacks glacial erratics. A couple of weeks ago I came across this erratic along a beach on the south shore of the Strait of Juan de Fuca.

Jackass Conglomerate

This particular erratic is a rather distinctive metamorphosed conglomerate. Full of angular rocks, but recrystallized such that it fractures through the cobbles versus around them. It is one of the few erratics that one can be fairly sure of the source. The source is the Jackass Mountain Conglomerate - a formation in British Columbia up the Fraser Valley. I had sort of had my eye for this type of erratic as David Tucker had called out for Jackass Mountain erratic locations appeal-to-citizen-scientists-do-you-know-of-any-jackass-conglomerate-erratics. The idea is that the presence of this conglomerate in glacial sediments means that source of ice had been from the interior of British Columbia versus some other source. Apparently there was a hypothesis about ice flow and source that involved using Jackass Conglomerate as a marker.

Glacial erratics embedded in glacial drift on shoreline bluff, Strait of Juan de Fuca

The particular Jackass Conglomerate boulder I saw was likely derived from glacial sediments associated with the Possession Drift. The Possession Drift is not from the last glacial period but an earlier glacial period somewhere on the order of 40,000 years ago. If so, that means that some of the ice from the Possession Drift glacial period was from the interior of British Columbia.  

The Jackass Conglomerate has some local fame in Bellingham. Doug Clark at Western Washington came across some old photos in a box at the department.

Donovan Rock, Bellingham, WA. (Western Washington University, via Doug Clark)

Donovan Rock is named for Donovan Avenue which had to split around the rock (Looking Back)

Donovan Rock as can be seen above diverted the street route a bit on Donovan Avenue. However, the rock also happened to be located smack dab in the middle of the proposed Interstate 5 route. The rock was blasted into manageable pieces and and pushed aside during freeway construction. The broken fragments are still located at the end of Donovan Avenue just west of the freeway. Some of the fragments were transported to a new location by anthropogenic forces HERE.   

1 comment:

Dave Wenning said...

Jackass Conglomerate, for a minute I thought you were talking about the US Congress. Actually, this is amazing information. I wish I knew more geology than I do. I believe I have seen rocks like this on my beach on South Fidalgo Island. Now I will go looking for them specifically. Thanks.