Complex sentence: Cyclo-cross began to become popular in the US in the 1970s and in 1975 the first race was held in New England but the first US National Championship was held in Berkeley, CA.
Split into simpler sentences: Cyclo-cross began to become popular in the US in the 1970s, and in 1975 the first race was held in New England. The first US National Championship was held in Berkeley, CA.