‘Mounds’ between Sedro Woolley and Burlington

Hello Dave,
I’ve always wondered about the large “mounds”/irregularities to the East of I-5 between the Skagit and Samish rivers… They’re pretty hard to miss. How did they form, and what exactly are they?

Carston, May 29, 2013

Burlington Hill looking south from Cook Road. Click to enlarge any image.

Burlington Hill looking south from Cook Road. Click to enlarge any image.

In response to Carston,  my geologist friend Doug McKeever and I finally got around to making the jaunt south to look at these conspicuous wooded hills that rise out of the Skagit River flood plain. The greenschist and phyllite is quite interesting and very easy to get to. The driving trip is a worthwhile short detour of just a few miles when you are passing by on I-5. On a nice day the view from one of the hills over the flats is wonderful. The field trip is posted here.

Silvery phyllite and greenschist.

Silvery phyllite and greenschist on Burltingon Hill.

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2 Responses

  1. Your latest post couldn’t have been more timely for me, as I had just returned from jogging up and over Burlington Hill when I read your post. I live between Burlington and Sterling hills, and have explored both of them quite a bit. Thank you for providing more information about the origins of these and other hills in the alluvial plains of the Skagit Valley.

    I have always assumed that the rock quarry you waltzed into was a borrow-pit for the construction of the access roads and spurs on the east side of the hill. Those roads, as well as the north access road, were constructed 15 to 20 years ago(?) to allow the residential development now present on the eastern slopes of the hill. Several years ago, there was some significant subsidence on the eastern access road, and I believe the developer reopened the borrow pit to quarry additional fill for the road repair.

    Regards,

    Eric Bosell

    • Eric,
      The big quarry is phyllite, not a very good road building material. It is highly foliated and breaks into small bits. You said you have explored both hills. Is there any public access to Sterling Hill? I’d really love to lay hands on an outcrop.
      Dave

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