The clothing was found on the South and North side of the creek. That means that the girls were partially or fully undressed (Libby) before they crossed the creek. That means that they either dropped some clothes as clues, or RA threw them into the creek, before they walked up the ravine.
Richard Allen admitted that a white van drove by and interrupted the sexual assault. He had to be on the South side of the creek to be interrupted by the van.
The white van passed at approximately 2:25, ten to fifteen minutes after they were abducted. Then they crossed the creek to the ravine, where they were murdered an hour later.