The Second World War camp was uncovered in a field on the route of Scottish Water’s new Glencorse pipeline, off Frogston Road, and is thought to have been founded in the wake of the Allies’ defeat at Dunkirk in 1940.
CFA Archaeology staff have also discovered musket balls that may be evidence of a camp set up by Oliver Cromwell in the 17th century, and hope to find artefacts dating back even further.
Project manager Bruce Glendinning said: “There are historical references to Roman artefacts being found on the site in 19th century but we are yet to find any of these ourselves.
“The area we’re excavating stretches all the way from Penicuik to Glencorse so if it’s there we’ll find it.
“There are also some written reports of Oliver Cromwell staying with a local landowner nearby, and we have uncovered musket balls which could possibly date from that era.”