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Grinnell Glacier Hike

By: Emily Taylor

Grinnell Glacier

Purcell Sill

Up the mountain and near the stromatolites is where we saw the Purcell Sill up close. The Purcell Sill is the thick, dark layered rock with the thin white rock strips parallel to it in both pictures. The Purcell Sill is an igneous dike intrusion of diorite found in the Helena Formation and dates back to 725-775 million years old.

How was it formed?

This sill was formed when molten rock intruded the Helena formation, then cooled and crystallized. There's evidence of contact metamorphism near the sill, the white lines paralleling the dark sill, where the heat from the hot molten igneous rock altered the sedimentary rocks above and below.

Stromatolites

Video of the trail

Up at the top near Grinnell Glacier was the bed of massive fossilized bacterial mats, known as stromatolites. The waters of the belt sea at the time relatively shallow and warm, making for the perfect environment for limestone secreting cyanobacteria to grow and live. stromatolites precipitate layers of calcium carbonate (limestone), making them thicker and taller, allowing them to get the needed sunlight for survival.

We took strike and dip while we were there, using the right hand rule and found that one the areas had a strike of 171 and a dip of 22

When the sunlight hits the cyanobacteria, it consumes carbon dioxide from the seawater and in turn releases oxygen into the atmosphere. The stromatolites were deposited during the middle to late Proterozoic.

Paternoster Lakes

One of the features along the trail during the 10 mile hike, we saw were paternoster lakes, these are a chain of lakes which have a stair stepping look to them due to the lakes being at different levels, in a glacial valley, connected by a stream but are dammed off by end morraines.

Swiftcurrent Lake

Lake Josephine

Lake Grinnell

These lakes are called Paternoster lakes because they resemble rosary beads.

Topographic view of trail

Location

Resources

http://www.nps.gov/glac/learn/nature/fossils.htm

http://www.nps.gov/glac/learn/education/geology.htm

http://www.nps.gov/glac/learn/nature/geologicformations.htm

https://books.google.com/books?id=Xr7v7GNmHs0C&pg=PA367&lpg=PA367&dq=age+of+appekunny+and+grinnell+formation&source=bl&ots=Vcs81XMQRK&sig=Y__x2FmBwRAB6-f4XpywpKdExhI&hl=en&sa=X&ved=0CDUQ6AEwBWoVChMIyNjVsv6LxwIViVY-Ch3BMgm0#v=onepage&q=age%20of%20appekunny%20and%20grinnell%20formation&f=false

Group gathering before the hike.

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