In an 1878 map of the Dranesville District of Fairfax County, on Difficult Run, there is a building identified as belonging to Henry Waples and another identified as Difficult G(rist) M(ill). These appear to have been where Waples Mill Road and Fox Mill Road intersect just west of Difficult Run. Here is also some information from the Oakton Patch:
https://patch.com/virginia/oakton/historic-earthworks-on-fox-mill-road