(C) Daily Kos
This story was originally published by Daily Kos and is unaltered.
. . . . . . . . . .



The Daily Bucket: Ice Age Revealed at Barnum Point [1]

['This Content Is Not Subject To Review Daily Kos Staff Prior To Publication.', 'Backgroundurl Avatar_Large', 'Nickname', 'Joined', 'Created_At', 'Story Count', 'N_Stories', 'Comment Count', 'N_Comments', 'Popular Tags']

Date: 2023-05-24

Map showing Camano Island. The red dot marks Barnum Point.

Our routine for a day out starts with a hearty breakfast. This day was no exception as we paused in Silvana for breakfast at Willow & Jim’s Country Café.

Willow & Jim’s Country Café

Willow & Jim’s Country Café

Fortified we drove over to Camano Island and started our walk at Barnum Point County Park. Here’s the trail map displayed at the gravel-paved parking lot:

From the parking lot (You are Here), we took the Beach Trail (yellow) to the beach, then walked the entire beach all the way around the point to where we picked up the Barnum Trail (green). We then took the short Caroline’s Trail (purple) and then all the way along the Bluff Trail (Blue) where reconnected back to the Beach Trail.

The Beach Trail courses through a mature forest of mainly Douglas fir ending on the beach. The ability to beach walk along Puget Sound shorelines backed by bluffs is subject to the whim of tides. We were in luck that it was an outgoing tide. There were still driftwood logs to negotiate. The beach is cobble shingle beach making footing less than firm.

Typical shingle beach of cobbles.

Navigating driftwood. The bluff face marks the history of ice sheet advance. Note that the sand at the lower sandy part of the bluff is at the angle of repose.

Well, let’s look at the bluff and see what it tells us about the ice age, specifically the last advance of the Puget Lobe of the Cordilleran ice sheet. This last advance of the Puget lobe is known as the Vashon stade.

Here’s a straight up look at the bluff face. Classic glacial sequence in this region displayed in the bluff with advance outwash sand mantled at the top by till from the ice sheet advance of the Vashon stade, i.e. the Puget Lobe. Outwash sand dated to estimated 18 to 20 ka. Till from about 18 ka to about 13.5 ka. (One-thousand years is represented by the abbreviation “ka,” which means “kilo annum.”)

This is a detail view of the advance outwash sand. The sands and gravels were deposited in outwash channels that issued from the terminus of the glacier as it advanced southward into the Puget Lowland approximately 15,000 years ago.

Once the ice sheet reached this area, the ice at its peak was about 4,000 feet thick. That is a considerable mass of ice pressing down on the surface, not only pressing down but moving at a glacial pace (135 meters / 443 feet per year). The weight and movement created the till layer (Vashon till) that you see at the top of the bluff. Till is a non-sorted, non-stratified material that was deposited or “smeared” beneath the sole of the glacial ice. It is dense to very dense and contains a mixture of clay, silt, sand, and gravel. It resembles concrete and is almost as hard.

Detail of the till farther down the bluff where the outwash sand is not exposed below the till. Note the soil formation on the till.

Till looks a lot like concrete.

The following map is a cartoon rendering of the geology at Barnum Point. You can view the whole map and descriptions here:

Geologic map of the Juniper Beach 7.5-minute quadrangle, Island and Snohomish Counties, Washington

Qgasv — is the Advance Outwash Sand we see in the bluff face

Qgtv — is the Till we see capping the bluff face and which also mantles most of the upland area

Qgdme — is the Everson glaciomarine drift that I will mention below

The three purple lines at the point are strand lines that mark former shorelines

So, the last big ice sheet advance was the Vashon stade where the Puget Lobe pushed down all the way to around Olympia about 15.000 years ago. When the Puget Lobe receded, the land had been depressed under the weight of the ice, depressed below sea level so the land was covered by marine waters as the ice sheet receded. When the ice sheet melted, the weight was lifted and the land surface rebounded. As the land surface rose above sea level, strand lines (beaches) formed as shown by the three purple lines on the geologic map. The other thing that was happening was the ice sheet and icebergs were floating around on a shallow sea. As the ice sheet and ice bergs melted, they were dropping silt and stones (“drop stones”) that had been picked up along the way as the ice sheet advanced. That’s how the Everson glaciomarine drift (Qdme) was formed, a build up of silt on the seafloor that shed from the melting ice.

Let’s look at some of these drop stones we found along the beach that the ice sheet transported in some cases way more than one hundred miles from their original location (provenance).

I love this boulder as it is an excellent example of the principle of cross-cutting relationships, a way of relative dating in geology. Of the three main things in this rock, the rock and the two veins, which is the oldest and which is the most recent? What is the sequence of events shown in this rock? This could be a Friday sequence in a single photo.

A granite boulder peppered with xenoliths. Granite is an intrusive igneous rock formed from magma (molten rock). As magma intrudes into the surrounding rock, it picks up pieces of the host rock (“country rock”). When the magma cools and crystallizes into granite, the xenoliths become part of the rock.

Xenoliths in granite — closer view

Next up is a chert-pebble conglomerate.

Conglomerate made up of chert pebbles

Closer view of the conglomerate and chert pebbles

That’s it for beach boulder bingo. Let’s walk back along the Bluff Trail.

Along the Bluff Trail with straight-trunked Douglas fir and sinuous madrone trees.

The dogs get thirsty after a long walk.

Buddy

Daisy

That’s all I have for today. What’s going on in your natural worlds?

Thanks for reading the Daily Bucket.

Phenology is how we take earth’s pulse.

We discuss what we see in each Bucket.

We value all observations, as we ponder life’s cycles.

Now it’s your turn.

Please comment about your own natural area, and include photos if possible. We love photos!

To have the Daily Bucket in your Activity Stream, visit Backyard Science’s profile page and click on Follow, and join to write a Bucket of your own observations.

[END]
---
[1] Url: https://www.dailykos.com/stories/2023/5/24/2166515/-The-Daily-Bucket-Ice-Age-Revealed-at-Barnum-Point

Published and (C) by Daily Kos
Content appears here under this condition or license: Site content may be used for any purpose without permission unless otherwise specified.

via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/dailykos/

You are viewing proxied material from magical.fish. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.