Little published information is available specifically for Cashel. The underlying geology is mainly Dalradian grits and slates and there are several faults through the bedrock. A quartz dolerite dyke is evident in Cashel Glen.

The drift material covering the solid rock consist mainly of glacial till deposited some 28,000 to 13,000 years ago. Typically the till ranges from clayey consolidated deposits
with few pebbles to crudely bedded sands and gravels.

There have been numerous glacial events, and the last re-advance of ice (the Loch Lomond re-advance), which was responsible for reworking these deposits, created Loch Lomond as we sec it today. The end of the Loch Lomond re-advance some 10-11 thousand years ago is evident in the morainic sands and gravel mounds in the lower Cashel and Blair Burn areas.

The upper reaches of the Cashel and Blair Burns show excellent examples of river capture. As glacial erosion deepened the Loch Lomond valley the streams cut down and decapitated the headwaters of the Burn of Mar, which originally flowed to the south east and probably into the Forth.

The major geological feature of the Highland Boundary Fault passes only a short distance to the east of Cashel.