The stratigraphic unit (or layer) below the Twinhoe Beds of the Great Oolite which form the uppermost part of the Combe Down plateau is the Combe Down Oolite which contained the mined stone.
The top 1.5 to 2m of the Combe Down Oolite consists of a buff, thinly to medium bedded, fine grained, slightly oolitic limestone with abundant shell debris. This stone is stronger, more thinly bedded and has a different fracture pattern than the worked freestone. It is probably for these reasons that the old miners referred to it as the 'Bastard Stone' and it was left by them to form the roof beds over much of the mines.
The worked stone of the Combe Down Oolite is up to 9m in thickness and consists of a buff, medium or thickly bedded oolitic limestone. Although some cross bedding is known to exist in Springfield Quarry, the medium to thickly bedded strata in the area of the Firs and Byfield Mines do not show this phenomenon. Beneath the worked stone of the Combe Down Oolite is 2 to 3m of pale buff/grey, thinly bedded, crystalline, shelly, oolitic limestone of the Lower Ragstone (the lowest unit of the Great Oolite). Due to the greater strength and variable character of this limestone it was not extracted by the old miners.
Two prominent fracture sets in the Jurassic rocks in the Bath area could be clearly seen in the roof of the stone mines at Combe Down. When the alignment of these fractures was parallel to the longest part of a mined 'room' a large amount of unsupported stone would be left along the fracture, leading to a higher risk of mine roof collapse.