This mid-weight, duo layer thermal crew, consists of a fully synthetic inside and Merino blend face. The bi-layer construction creates an air pocket in-between both layers for additional insulation and thermal regulation. The synthetic backside, helps to push sweat and moisture vapors to the surface to keep the wearer dry and comfortable. Merino wool, is natures best at thermal regulation and odor control. Terramar's Merino wool is blended with nylon for an added degree of durability.