The story of the Puget lowland is one of plate tectonics (forearc basin, donchaknow), but it’s also one of continent-spanning glaciers, and those glaciers dragged evidence of plate tectonics over and left it strewn practically all over my doorstep. The drumlin we’re on is lousy with erratics. I’d discovered several recently, and been itching to get after them with the rock hammer before bad weather set in. Luckily, our beautiful weather held out until I got my shot at vacation at the beginning of October. I grabbed the hammer and headed up the drumlin for some quality rock-breaking time whilst the kitteh basked in the sunshine.
These beauties are at the top of a nice paved trail.
When I first spotted them, it was an early morning just before work, and I didn’t have time to linger. They looked a bit granitic, I thought, and there was that big black streak – dike? Xenolith? Between weathering and lack of time, I wasn’t sure.
So I took after them with the hammer. I got a gneiss surprise – orthogneiss, in fact.
Let me show you something neat you can do with the local orthogneiss. Geologist Ron Tabor pointed it out in his book Geology of the North Cascades. Look at orthogneiss one way, and it’s streaky:
Turn it 90°, so that you’re staring in to the streaks, and it seems spotty.
This isn’t foliated, but lineated – which means it got more stretched than squished.
The big, dark crystals of hornblende and biotite just shimmer against the pale feldspar and quartz. You can even see its sparkle on lightly-weathered bits of the rock in the darkness of a forest. It’s gorgeous stuff.
I wasn’t able to get a decent sample of the black streak – it was too hard, and there wasn’t a good angle to break a chunk off from. When I whacked it, great fat red sparks flew, and I became afraid I’d set the forest on fire, which might have seriously inconvenienced the neighbors and possibly turned them off to geology, so I stopped pounding. But I got a bit.
It’s just as shimmery as the orthogneiss, and seems at a glance to be pretty much the same stuff, just darker and finer. Not knowing my metamorphic rocks the way I should, I’m not sure what to make of it. Is it just some orthogneiss where the dark minerals got concentrated? A xenolith? A dike? The latter seems unlikely to me, unless it’s a dike that intruded before the stuff started getting metamorphosed – it’s streaky and flaky and looks like it’s been quite hot and under pressure in its past.
One thing I’m fairly certain of: this set of boulders was probably dragged down from the North Cascades. There’s portions of that area that are practically solid orthogneiss – around Ross Lake, for instance.
Our second erratic is along a non-paved trail, and seems at first glance to be grandiorite, or tonalite, or something else in the non-metamorphosed granite family.
It seems to have been much less hot and bothered than our first erratics. And we’ve got tons of this stuff up in the Cascades, too.
If it gets buried deep enough for long enough, if heat and pressure get to it, then it could become orthogneiss. I can’t give a solid diagnosis as to exactly what it is, as I didn’t take after it with the hammer. I was too anxious to get at these beauties.
These two really caught my attention the first time I encountered them. You see, the one in the foreground is extremely hard. The second is soft and spalling, weathering in sheets.
When I mentioned it, some folks thought it might be basalt weathering to clay. So I whacked on it a bit to find out. I don’t think it’s basalt. I can bore a sandy hole right in it with the pick.
And it looks very much like sandstone. Possibly volcanoclastic. In point of fact, it looks nearly exactly like outcrops of the Blakeley Formation I’ve encountered.
Whereas the other boulder is certainly not that. I’m not sure what it is, but it’s extremely hard to break.
The only things I’m relatively certain of: it’s not plutonic. It’s not sedimentary – although it’s possible it used to be and has been metamorphosed, but I’m leaning toward something extrusively igneous.
The hammer didn’t strike sparks from it. Outside of that, I’ve no idea.
One day, I’ll be better at identifying random rocks in the field. I’m already far more adept than I used to be. And I’m getting over the self-consciousness about whacking on rocks when non-geologists are present. I mean, it’s not exactly ordinary behavior, a solitary woman hammering on a boulder. It earns stares. But at least you lot have made me comfortable saying, “Hello, I’m a geologist,” so there’s that. And perhaps this winter, I’ll get round to appending this to my clothing:
Then, should anyone get curious, we can skip the formalities and get to talking about why geologists break rocks to begin with, and why, in an area where bedrock is several thousand feet down, there are so many rocks available to break. And that story starts with ice from Canada, plucking rocks and leaving them all over the place…