A little bit about Glacier Bay

To warn you, this is an abbreviated account of everything I’ve learned about glaciers in the last week, so…

So, these glaciers have actually been receding over the last 200 years. They were a product of the little ice age, which peaked in about 1750. That’s when the glaciers in Glacier Bay reached their maximum extents. When Captain Vancouver “discovered” Glacier Bay in 1795, it was only a 5 mile indentation in the coastline. The entire valley was full of ice. But by the time John Muir got here in 1879, the glacier had receded 40 miles into the bay.

Here’s a diagram from the visitor’s brochure.

Maybe everyone else knows how glaciers work, but it’s been news to me, so, here goes: When snow falls in the mountains and doesn’t melt, it accumulates in depressions and builds up over years. Eventually, the mass of snow is so heavy it compresses itself into ice, which begins to flow downhill under its own weight. It picks up rocks as it flows. These rocks grind at the bedrock under them, carving deeper and deeper valleys into the mountains. The ice melts at lower elevations. So, the rate at which a glacier flows depends on how warm it is at the bottom and how much snow falls at the top. And the relationship between those two determines whether it’s advancing or receding.

There are different types of glaciers, classified by where they end. Valley glaciers are contained, piedmont glaciers flow out of a valley and spread out, hanging glaciers drop out of a valley, and tidewater glaciers—the ones I’ve been posting photos of, mostly—drop into the sea.

I’m pretty sure that this is a piedmont glacier:

And that this is a hanging glacier:

But I could be wrong.

Tidewater glaciers calve icebergs off their faces, like the video from Sawyer Glacier. They also push sediment in front of them, which build up as a protective shoal that can allow them to advance pretty far into the water. Johns Hopkins glacier, one of the last one I posted pictures of, flows at a rate of 10-15 feet a day!

Most of the tidewater glaciers here are receding, but a couple of them are advancing. Our charts are from a couple of decades ago, so sometimes they show us IN the glacier when we’re still a mile away.

Yikes. At least it’s never been the other way around.

The glaciers leave barren rocks behind them, which eventually get repopulated. In the mid-20^th century, a guy named William Cooper did a foundational study of plant succession here, actually. Pretty cool, right?

That’s ice on the left in both pictures.