A Mystery

During a walk looking at the lost industries on the moors north of Rochdale we came across this site just below Catstones Quarry. There were plenty of signs of burning and several large fused blocks of stone and carbonaceous material. Old maps give no clue as to what this migt represent. We did consider beehive coke ovens but were are no collieries in the immediate vicinity and why transport coal to this remote spot for burning? Any theories as to what was going on will be very welcome.