A question about southern california?

ok in the inland empire the are threee cites fontana rancha cucamonga and upland and i guess we can add claremont all of these are at thebaseof the san gaerial mountain rancho and upland being at the base of the second largest mountain in so cal soare these cities considered part of the valley or the foothills i dont know exactly wa foothills are but i would imagine that that is ares right befor a mountain??