High Quality Content by WIKIPEDIA articles! Rock Creek is an 18.9-mile-long (30.4 km) tributary of the Monocacy River in south-central Pennsylvania and serves as the border between Cumberland and Mount Joy townships. Rock Creek was used by the Underground Railroad (at McAllister`s Mill, "slaves would slosh through the water to throw off the tracking dogs that were pursuing them") and flows near several Gettysburg Battlefield sites, including Culp`s Hill, the Benner Hill artillery location, and Barlow Knoll.