Monday, May 23, 2016

Two Different Erratics on Samish Island

I have found that my visual memory from past field work has lingered. I readily observe and notice certain rock features from past field work habits. While walking a gravel/cobble beach on Samish Island in Skagit County, I spotted small boulder among the thousands of other rocks on the beach.

Garnet and kyanite/sillimanite (and maybe andalusite psuedomorphs) schist

I spent a fair bit of time during a past geology work era looking for aluminum silicate schist. The whitish minerals in the rock are likely mostly kyanite with some sillimanite. And as noted might have once had andalusite that was replaced by the kyanite. These three minerals form under different pressure and temperature regimes with anadlusite being the lower temperature and pressure member, kyanite the high pressure member and sillimanite the high temperature member.

Identifying any of the three can give one a rough idea as to the pressure and temperature of the metamorphosed rock. And if you can identify the shape of one mineral such as the small crosses andalusite forms being replaced by say kyanite you can get an idea of the pressure/temperature path of metamorphism. Having the garnets in the mix can further help determine the pressure temperature path as the composition of the garnet shifts based on pressure and temperature as well. The core, central part of the garnet, will have a different composition than the rim. Hunting for this type of rock in the Coast Range of British Columbia or the crystalline core of the North Cascades took up a fair bit of my field time while working in those areas.

The boulder on the beach that I readily spotted would have been delivered via glacial ice from the high ranges of the BC Coast Range and may be from the same schist unit I once collected samples from.

I also spotted a much smaller erratic on Samish Island:
Dunite pebble

This pebble of dunite was not delivered by glacial ice. It had arrived via truck and was part of the gravel road I was walking. A pebble of dunite would not be a surprise given that many of the Skagit County gravel pits extract gravel deposited from an ice age river system that had some source material that came off of the Twin Sisters Range, a block of dunite within the Northwest Cascades. 

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