Complex sentence: The valley contains the cities of Clayton, Concord, Martinez, the unincorporated area of Pacheco, Pleasant Hill, which is home to Diablo Valley College, and most of Walnut Creek, however the southern end of Walnut Creek is a part of San Ramon Valley.
Split into simpler sentences: The valley contains the cities of Clayton, Concord, Martinez, the unincorporated area of Pacheco, and Pleasant Hill (home to Diablo Valley College), and most of Walnut Creek. The southern end of Walnut Creek is a part of San Ramon Valley.