Complex sentence: In the United States, Thoroughbred flat races are run on surfaces of either dirt, synthetic or turf; other tracks offer Quarter Horse racing and Standardbred horse racing, or combinations of these three types of racing surfaces.
Split into simpler sentences: In the United States, Thoroughbred flat races are run on surfaces of either dirt, synthetic or turf. Other tracks offer Quarter Horse racing and Standardbred horse racing, or combinations of these three types of racing surfaces.