1) This is one of several words used by Yorkshire coal-miners for an obstruction in the vein of coal that was being worked, or for a ‘fault’, either of which could cause serious delays.
Alternative terms were dike, gall and horse, all dealt with separately. A ‘mare’ could be a major disruption: in Shibden near Halifax, in 1660, colliers met with a mare or gall … which hindered them from getting the coal: in 1702 a pit at Thornton was said to be not Irrecoverably Lost by reason of Horses or Mares Lyeing and being therein. A later lease links ‘mare’ with other words which evidently had much the same meaning: 1775 for want of driving through any galls, mares, ridges or lips, Northowram.