The outer layer is hand-crafted using an updated form of Ventile, a densely-knit cotton fabric invented by the British Navy to protect pilots shot down in the North Sea during WWII. That, along with a construction process that employs only two exterior stitches, should keep your feet dry and warm no matter what weather you're riding through.
The inner is made from a double layer of French calf leather (because socks are for hobos) and the outsole uses Vibram rubber over vegetable leather to maintain a solid grip on the pedals. You can find them here.