Nick on the Rocks

Have you heard of a place called Slate Peak in northern Washington?

This is the road getting you up here.

The highest place that you can drive a car in the state of Washington.

We're 7,400 feet above sea level but the bedrock here at Slate Peak formed below sea level.

The Cascade Mountains are complicated geologically.

It's more than just stratovolcanos and granite magma chambers.

Up here by Slate Peak there's sedimentary and metamorphic rocks.

And there's a crossroads of interesting intersection between those different kinds of bedrock.

Specifically, there's basement rock that's been lifted more than 20 miles to the surface, and there's an ocean story that's far older, that's recorded in many of these peaks up here by Slate Peak.

Let's look carefully along the road at some detail.

There's a basic rule in geology, and it's beautifully on display right here at Slate Peak.

The rule of cross-cutting relations.

If something cuts across a bunch of rock layers, it has to be younger.

This thing is interrupting the horizontal layers.

Makes sense.

Right?

These black layers were once continuous and at some point younger than that this thing squirts up through the middle.

Let's be specific.

This is 50 million year old magma that's been tied to the Golden Horn Batholith exposed at Washington Pass, and so this igneous dike, this intrusion of magma, came up 50 million years ago, and it's cutting across 100 million year old slate of Slate Peak.

The slate is metamorphic rock.

And before it was metamorphic rock, it was shale.

And before it was shale, that's a sedimentary rock, it was mud, and the mud was in a shallow marine setting.

There are some details in these beautiful, tiny, thin beds of slate that tell us that life used to exist in that marine setting.

Let's go take a look.

So are you surprised that here in the Cascades, so many of the bedrock layers are not lava, and they're not granite?

I mean, this black layer here, bedrock full of holes.

I think most people would look at this and go, oh, this must be basalt lava with a bunch of gas bubbles in it.

But that's not the story.

This is the slate of Slate Peak that used to be shale, and the shale used to be mud.

But the mud has holes in it.

Not because of bubbles, but because of life.

Fragile spiral snails living during the age of the dinosaurs 100 million years ago.

And these snails were burrowing their way into the mud.

The water was warm, it was quiet, it was protected.

And these delicate snail fossils have survived after 100 million years of time.

That's the story here at Slate Peak.

But it's not just a shallow marine thing with freshwater rivers coming into that coastline.

There was an ocean here.

And if you look at many of the peaks surrounding Slate Peak, you can see the ocean.

If you can visualize properly.

So the snails, that's shallow marine water, salt water near the coastline of North America.

There's mud, there's sand.

That's all before the slate was created, right?

But there's a deep ocean story here as well.

These peaks behind me are representative of the Methow Ocean, a vast body of deep ocean water between North America and a microcontinent offshore.

And as those two continents, North America and the microcontinent, got closer and closer together with plate tectonics, that Methow Ocean with all sorts of sediment coming into the ocean from both continents sands, muds, turbidites, all sorts of stuff, conglomerates being dumped into that Methow Ocean.

As the ocean got narrower and narrower, it started filling with sediment to the point where the two continents collided and we sutured that ocean.

We can't find the ocean anymore.

It doesn't exist anymore.

But the rocks, ten miles of bedrock behind me, vertically, is the evidence of this vanished ocean that dominated long ago.

Deep ocean rocks here at Slate Peak in the North Cascades of Washington.