I was looking for a geologic map of Rattlesnake ridge, which I didn't find. I did find this one of a study farther north. In the lower corner is a map that does show faults & folds on Ahtanum and Rattlesnake. I think it's interesting that one of those faults, to this layman's eye, looks to be where it is spreading.

The photo I've attached is likely taken from the Parker post office parking lot. I haven't walked down there to be certain but that ugly tower in the foreground is across the highway from Parker.  The top of the ridge was in the fog yesterday and so far still is this morning so I can't actually see anything.

https://pubs.usgs.gov/sim/3212/sim3212_sheet.pdf