Park near the old school and enter from Spycraig Road. Follow the main path for maybe 500m. On the left you'll spot an overhanging flat wall of decent height at the left side of the quarry entrance. This is Ragnarok, at the entrance to the Northernmost of the big quarries.
Continue past this and keep right as the path splits and you'll quickly go past the biggest of the quarries on your left. The obvious entrance to this is very boggy but following the path skirting the left edge of the quarry reveals a ramp dropping down to the back wall where you will find the line of Mjolnir.