Pikes/Pines: Glacial Till Hill

Pikes/Pines is our new series exploring the natural world of Capitol Hill. Let us know how you like it in comments.

Glacial Till. This is mostly what the ground beneath our streets looks like. (Image: Brendan McGarry)

Glacial Till. This is mostly what the ground beneath our streets looks like. (Image: Brendan McGarry)

The study of geology probably isn’t on the top of your list of exciting activities. After all, what appears more inert and slow than the rocks and soil beneath of feet? However, geology relates literally to the foundation of lives and has interesting stories to tell — even especially on Capitol Hill.

As Pikes/Pines will weave stories of Hill natural history, there’s no better place to begin than with the ground we live on. Despite our manipulated urban setting, our soil’s makeup is the baseline for all that happens here. Part of that story involves the tremendous churning of ice.

At least six times over the last 2 million years, our region was scoured by various extensions of sprawling ice sheets. The Cordilleran ice sheet, which covered much of modern day Western Canada as well as Northern Washington, Idaho, and Montana, pounded the Puget Sound basin, thrusting down from the Fraser River basin. At maximums, the ice in our area would have been around 3,000 feet thick. This repeated advancing and receding, shoved and washed about soil, rocks, and just about anything else in the way, creating successive deposits over previous sedimentary and volcanic deposits. Then, around 10,000 years ago the ice receded completely. Left behind in our Puget Lowlands were layered piles of glacial till, sedimentary jumbles of rocks, sand, and clay. This jumble is what Capitol Hill sits on.

Ignore the labels, this map from Washington State DNR shows us what it looks like beneath the surface. The light tan areas are all deposits related to glaciers.

Ignore the labels, this map from Washington State DNR shows us what it looks like beneath the surface. The light tan areas are all deposits related to glaciers.

A map of Capitol Hill’s subsurface geology reveals that unlike many areas of central Seattle perched on fill dirt or squashed hills, we remain on mostly original glacial till.

Relatively soon after settlers first arrived in 1851, they set about major environmental renovations. By the 1880s Capitol Hill was completely denuded of the original forest. However, it remained mostly geologically intact, aside from minor pushing about of soil for construction. Our high point, Volunteer Park, also remains an unchanged 451 feet.

Glacial till has its challenges to urban life. Ever spent time digging in the ground around here? Unless you’ve amended the soil, you’ll likely discover a gravelly mess interlaced with large rocks, horrible clay deposits, and sometimes large chunks of wood. Native plants manage very well with this, but imported plants often need copious babying and changes in soil to flourish.

The boring to create tunnels for the U-Link light rail has also had to navigate our soil challenges. The rocks mixed in till aren’t all small, the tunnel building put Brenda the drill in contact with many boulders. Till also isn’t exactly the most stable, which means the engineers had to be sure they didn’t sink buildings in their wake (even an inch is a serious concern for property owners).

All said, I deeply appreciate one aspect of our unstable soils: We haven’t managed to cover every single inch with cement.

Over the years, engineers have overcome instability in many places, but green spaces have persisted in areas either too troublesome to build on or because landowners decided it wasn’t worth the effort. These “undesirable” locations have given way to places like St. Mark’s greenbelt. If our neighborhood was completely flat, we’d still have parks, but likely little or no largely unmanaged green spaces, which are reservoirs for nature.

St. Mark's Greenbelt; one of the places unstable slopes permit green spaces.

St. Mark’s Greenbelt; one of the places unstable slopes permit green spaces. (Image: Brendan McGarry)

Unless we have an earthquake or one of our adjacent volcanoes acts up, most of us don’t think about geology every day. I think that’s alright. However, it’s also worth imaging a time when thousand of feet of ice covered Puget Sound basin and how that has shaped our neighborhood, and ultimately its nature.

13 thoughts on “Pikes/Pines: Glacial Till Hill

  1. A fun way to imagine the ice sheet’s thickness is to look at skyscrapers and the radio towers on our hilltops. Maximum thickness would have been about three times the height of Columbia tower. The ice thickness at the South Pole today is a little under ten Columbia Towers. Let’s not melt that.

  2. Delightful addition. Perhaps at some point you could talk about soils re gardening in more detail? And what were/are native plants to this small area? I look forward to more.

  3. Hmm. I’ve never head of Till being “unstable”. It’s a very strong soil – often referred to as “concrete-like” in engineering circles. [It's not nearly as strong as concrete, however.]

    • It certainly can be, thanks for making reference to that. It’s very similar to concrete if you have the right material makeup. But on a steep slopes, intermingled with clay deposits, and organic materials, not as much. It’s also true when you dig around in it (say building a tunnel), layers can give way or move. Overall it’s much better than say swamplands or sand, yes.

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