They're the washed duck canvas work pants (I think). They're pretty stiff and heavy even after having been washed a bunch. As such I've pretty much stopped wearing them in warmer weather because I end up sweating in them and it rapidly becomes pretty uncomfortable.
For reference, the only Kuhl pants I have are the Hi-Lo Cords, which are much thinner and more flexible as compared with the Carhartt pants. The cut also seems to work better with my leg shape for some reason. I have pretty big quads and the Carhartt's end up being pretty tight around my legs and waist. The Kuhl pants also don't seem to give anything up in the durability department despite being thinner, though I admittedly haven't put them through anything particularly abusive.