I recently took advantage of the opportunity to hike across a portion the interior of Blakley Island in the eastern San Juans. I had a project site on the south part of the island and hiked to the site from where I landed my kayak on the northwest part of the island. All and all a fun field trip.
The interior is mostly forested except for the western side which has an airport and a homes. The interior is mapped as being underlain by greenstone (Whetten, 1976) and Lappen (2000) designates the interior rocks as part of the Fidalgo Ophiolite. An ophiolite is a block of ocean crust. The Fildago Ophiolite was recognized by Brown as well as Vance and is a block of ocean crust that was accreted to North America and then further displaced to its present location on Fildago island and the southwest part of the San Juans.
The section of the ophiolite outcropping in the interior of Blakely is a section of intrusive rocks that appear to be the root of a former island arc. What struck me about this rocks is how unsheared and lacking in foliation or lineations the samples I grabbed were. The north shore is mapped as tectonic zone, but as I have been discovering of late the major tectonic zones and shearing in the San Juans are surprisingly narrow.
Glacial striations on outcrop near the western summit of the island
Although the interior is forested, soils are very thin and fresh rock samples were relatively easy to find
Multiple intrusive injections and some mixing