Hi, Susan :) Some wool resist crushing because they're very crimpy and somewhat coarse--say in the high 20s or more microns. Many down-breed fleeces are like this, though not all. I've seen a couple Shetland fleeces of this type, and many other breeds can have this characteristic frequently or less often--sometimes Perendale does, and I've had a Corriedale fleece that was like this.
Fleeces with this uncrushability due to crimp and relative (to fine wools) coarseness are often called 'crisp-handling'. In use, they reisist wrinkling, are pretty resistant to abrasion, won't felt or pill a lot, but will shrink quite a bit due to the crimp relaxing, and be very cushy cloth. They're said to be good for socks, though I haven't tried it and don't know if they would still need mohair for strength or not. I haven't tried spinning one very fine with quite a bit of twist to see what that would do to control the shrinking. I like minimal twist, and it was hard just trying to get the 20 degrees my daughter wanted into this blanket yarn--I think I probably averaged 17-18, it was the best I could do, since she wanted it thick, 20 wpi :) Holly
