Sorel - Caribou (Air) - Women's: These colors are only available on Zappos.com ; Waterproof, full grain leather upper. Removable 9mm ThermoPlus felt InnerBoot provides superb warmth, moisture transfer, comfort, and durability. Acrylic/wool blend snow cuff. 2.5mm bonded felt frost plug in the shell to provide an extra layer of insulation and warmth. The frost plug is a 2.5 mm piece of eva that is in the heel and forefoot. Handcrafted waterproof vulcanized rubber shell. Rubber lug outsole. Sorel Rated: -40F/-40C.