Dual functioning seals for use in forming a lubricant seal in a rotary rock bit comprise an annular elastomeric seal body having a dynamic seal surface for sealing against a dynamic surface and a static seal surface for sealing against a static surface. The seal body has an asymmetric axial cross-sectional geometry the dynamic seal surface having a greater radius of curvature than that of the static seal surface. The dynamic and static seal surfaces are formed from different elastomeric materials, and the dynamic seal surface is formed from an elastomeric material that is harder and more wear resistant than the elastomeric material used to form the static seal surface. A rotary rock bit seal constructed in this manner is well adapted to accommodate the different operating conditions and sealing requirements that exist both at the dynamic and static surfaces of the seal to ensure optimal seal performance and, thereby enhance the service life of the rock bit.