I also have a theory or observation (or whatever) about steep runs.
For the most part, here in Jackson Hole anyway, a geological phenomenon known as "angle of repose" comes into play. The angle of repose is the slope angle at which different types of dirt, rock, scree, boulders, etc can pile up and still remain stationary. If material is added to the top of that slope, it becomes unstable and some of it sluffs down the slope until the slope is stationary again.
For most of the scree, boulders, and mixed rock here in the Tetons, that angle of repose is probably somewhere around 38-43 degrees. That's why so many of our ski runs like Rendezvous Bowl, the Hobacks, Lower Faces, Cirque, Laramie Bowl, Tower 3, Alta Chutes, etc are all somewhere just a little north or south of 40 degrees.
For things to get much steeper than about 40-42 degrees here, it almost has to be a rock face that's lying at whatever angle. As the rock face starts crumbling or fracturing, it turns into cascading boulders or scree, which ends up at the angle of repose.
As an interesting little aside, there is a type of soil that has, as I understand it, one of the highest angles of repose of anything on earth. It's called loess soil, and it can be stable at almost 90 degree slopes. It's very fine-grained soil that has been wind-deposited over thousands of years. The best examples I know of in the US are in my home state of Iowa: http://en.wikipedia.org/wiki/Loess_Hills
Edited by Bob Peters - 5/5/14 at 7:48pm