The predominant rock is Old Red Sandstone, which is a sedimentary rock dating from the Devonian period (circa 350 million before present). Evidence of the glacial processes that shaped and formed the now wooded glen, can be seen in the ice-smoothed rocks that occur in the Reserve e.g. on the far bank of the River Walk.

The overlying soils vary from acid brown earths to alluvial brown earths and peat.