Say you’re walking along the side of a mountain, perhaps below a cliff face – let’s say, for example, the Eiger, in Switzerland. Rocks tend to fall off cliffs and accumulate at the base, resulting in a steep slope of loose rocks. Now, because you’re below the cliff, you may think you can’t fall. But you take a step, and some rocks slip. You take another, and more slip, and soon with every stride you are sliding faster and faster down, down, down, unable to stop, and your voyage culminates in a scream that falls away… a lesson learned too hard.
Oo. How unpleasant. Well, that’s scree for you: if the word seems like scream falling away at the end, it’s just fitting. It has that /skr/ onset, too, often used for loose or rough things (scrap, scrabble, scramble, scribble, scratch, scrub). And these loose, rough rocks do tend to slide, which is so well set forth with the /i:/ (“EEEeeeee…”).
Scree isn’t found only at the foot of a cliff – it can also be left behind by glaciers, for instance – but it is very often associated with the word talus, which means “detritus at the foot of a cliff or similar”. It’s loose rocks on a slope. It’s inviting to think of the letters in scree as resembling so much scree, but the truth is that the rocks in scree are often rather angular, having gotten where and how they are by breaking.
Scree is a word that comes to us from Old Norse – specifically skriða “landslide” (of which we can feel sure there are plenty in Norway’s fjords). Most likely it came to English in the plural, and the dental fricative slipped out before the final alveolar fricative (like saying clothes as “cloze”), to make screes, and that backformed to the singular – the /z/ slipped away too. Skriða for its part came from a Germanic word meaning “slide” or “glide” that came through to modern German as schreiten, “stride”.
There are a couple of other common collocations with scree, by the way: scree plot and scree test. This is not the land that scree is on or the act of nudging scree to check its stability; rather, these terms refer to something one can do in statistical analysis: for each eigenvalue, plot its contribution to the overall variance, from greatest to least. The result will look like a steep slope with a point where it becomes a shallow slope, sort of like the foot of a cliff with a scree slope. That’s a scree plot. The scree test is the act of looking at the scree plot and deciding where the slope shallows out (the “elbow”) and declaring the eigenvalues on the steep side to be important and the rest to be relatively unimportant. So, for example, in the factors causing you to go for a slide down the slope, the instability of the slope and your own obliviousness may make a significant contribution, and your clothes and the temperature may not make that much of a difference, perhaps.
And what’s an eigenvalue? Well, aside from saying that eigen is German for “characteristic” (and “own”, as in “my own”) and that it may have an etymological relation to Eiger, I think I would do best to direct the interested to further self-directed explorations. A warning, though: it’s a bit of a steep learning curve.