I went back to Canyon Lake today, and stopped for a third time on the hillside on the loop road. It was mid morning and the pasqueflowers were mostly closed up in buds. The bud shot (center) is the same cluster as in the photo of open flowers.
This white flower is the plains phlox, of the phlox family. More commonly seen in the Black Hills area is the leafy phlox, which is mostly pink or lilac, but can be white also.