Many times the location of the vent is unknown. Basalt can just flow out of the surface of the ground and flood over many square miles. Or the rock can be transported hundreds if not thousands of miles away from any noticeable vent.
and is it common for basalt where there were no volcanos
Nice gold- it paid for the beer or gas,,,, so close to my back yard. Makes me think about the large buried granodiorite boulder in my backyard-right in the path of an old stream channel heading down the hill( now blurred by subdivision).
Depending on where you were in RAmona ,it could be as you say an ancient river bed or just the conglomerate rock that's thrown all over SAn DIego county. It's so widespread( down intoMexico, too) I think it's an indicator for the violence of the impact in the Yucatan 66 million years ago.