Strathearn is frontier country in more ways than one. The Highland Boundary Fault, which passes to the south of the town of Comrie, marks the boundary between the hard crystalline rocks of the Scottish Highlands and the softer sediments of the lowland Central Belt. Around 79 AD the Romans built a major camp, said to be called Victoria, at Dalginross, Comrie, possibly to control the entrance to (or exit from) the highlands along Strathearn. Today Comrie is known as a picturesque tourist and retirement village, having recorded the largest proportion of over-65s in Scotland in the 1991 census.
Further up Strathearn, at Dundurn east of St Fillans, stand the remains of a Pictish royal fort. The Dundurn hill fort, while on Pictish territory, was close to the frontier with the land of the Britons to the south and would have controlled access along Strathearn by the Scots of Dalriada to the west. St Fillans, at the east end of Loch Earn, is named after the 7th century Scottish / Irish Saint Fillan of Rath Erenn, a different Saint Fillan to that of Strathfillan and Glen Dochart. Originally a hamlet of thatched cottages and smallholdings named Port Lochearn, St Fillans acquired its present name in 1817 and was developed with grand holiday houses in Victorian times.