strange rock formations

I live near diamond valley lake in Winchester ca. while digging in the late 90's they discovered a lot of fossils, a big find of mammoths, saber tooth, dire wolves etc. they say one of the largest mammoths ever found in north America. larger concentration of fossils then the la brea tar pits. I live about a mile from there. I take my dog out walking near my house and there are outcrops of boulders all over. I walked up into one and the rocks where very strange. they are regular quartz, black and white speckled. huge boulders, but they all have these other smaller rocks stuck in them. they are like lava rock, some are smooth others look more like typical lava rock. some are embedded deep some stick out. it makes the boulders look like choc chip cookies or something. i'm used to seeing the black quartz, I grew up in the mountains but I've never seen anything like this. why are they like that? what kind of rock could it be and what would cause that to happen?