SealSkinz® Chillblocker Sock, this mid-calf high waterproof sock is insulated with a liner made of expedition-weight double velour Polartec® Power Stretch™ fleece. The outside layer is a knit of nylon/lyca® spandex offering a durable, flexible, stretch covering. In between the two fabric layers is our waterproof MVT membrane. These three layers are securely bonded together throughout. This combination of materials provides a very soft, comfy, warm, stretch-to-fit sock suitable for a whole host of cold, wet, nasty outdoor conditions.