This appears to be a little-visited part of Mt Rubidoux, a collection of boulders strewn low on the western face of the hill, not accessed by the road to the cross, as are most of the popular parts of Rubidoux.
You are more likely to see evidence of homeless people living in the rock alcoves than other climbers, but there's still fun stuff up here.
Despite the area being dirty, there are some very difficult friction climbs, some very fun, easy boulder problems, and a couple of really run-out high slabs that should probably be top-roped, though I couldn't quite figure out how. A fun area to explore.
Park on in the lot on the northeast side of Mt. Rubidoux, and walk along the Santa Ana River bike path for a while (couple hundred yards?) until you see a weed-choked gully leading up to what appears to be a promising pile of big boulders. If I remember correctly, you can see the Wall of Glass (a blank slab of a boulder) from the trail. Walk up through the gully or on the little ridge to the left of it to the boulders.
I've placed it on the map where I think it is, and will go out and verify this soon.