The Chumash of Southern California used and traded asphalt extensively. Caulking for canoes was one of the most important uses for the substance. The Chumash also used asphalt to plug up holes in abalone shells so they could be used as dishes, and as a glue to fasten spear points and arrow points to shafts. In the mid-1960's highway engineers discovered that when asphalt was blended with about 18% rubber, another Native American invention, the new substance provided increased resistance to melting in hot weather and cracking in cold weather.