5 June 2020
Friday fold: Harbledown Island
Posted by Callan Bentley
Reader Christian Gronau writes with this Friday fold contribution:
Greetings from Cortes Island, BC – at the opposite end of the Strait vis-a-vis Lopez Island.
Your Mountain Beltway blog is always of interest, and I have been following it for several years by now. Thank you for putting the effort into this worthwhile website.
Quite regularly your posts elicit “echoes” and make me go back to some of my own geological notes and photographs. The Lopez Island tension gashes are exquisite in their elegance and classic simplicity.
A little further north again from Cortes Island lies Harbledown Island, eponymous of the Harbledown Formation (Early Jurassic sediments, often fossiliferous). The west of the island has a bay called Parson Bay, eponymous of the Parson Bay Formation (Late Triassic, also fossiliferous) – making this geographical area one of rare distinction as well as historical complexity (early 20th century stratigraphic work).
The attached photograph shows part of an extensive stress zone in Parson Bay limy shales, looking remarkably similar to the image of your last “Friday Fold,” though a bit more complex and, perhaps, confusing. I thought you might be interested in seeing it:
That’s a lovely outcrop photo, Christian! It has many of my favorite features: a multi-chapter story (i.e., it’s a palimpsest rock), good structural geology, a well-placed sense of scale, and a lack of vegetation, barnacles, and/or anthropogenic detritus. A very pleasing sight to contemplate at the end of a rough week.
I just noticed a special feature of this wonderful outcrop. The separation of the sandstone layers—right-side down with respect to the left—is exactly compatible with the sense-of-shear inferred from the extension fractures.