1) In mines, a fault or intrusion.
The word ‘geology’ has a Greek origin but it had entered English via medieval Latin in the fourteenth century, with a meaning far removed from our modern understanding of the word. Geology as a science, concerned with the earth’s crust and the underlying strata, dates back only to the late 1700s, and much of the vocabulary that we associate with the subject has developed since then: the word ‘fault’ for instance, as used by geologists, is on record only from 1796. The vocabulary of miners is therefore of great interest, for they had been encountering and naming rock strata features for centuries. Their use of ‘gall’ to describe a fault or an intrusion goes back to the 1600s at least. A bill of complaint that was filed in Chancery in 1659 had to do with events in the Halifax area earlier in the century and, when the matter came to trial, one witness testified that they had sunk a new pit and met with a mare or gall … which hindered them from getting the coal, Shibden. ‘Gall’ in this sense is listed in the OED but only from 1805 and only in Scotland. It was possibly the most common of several words given to such features, and later examples indicate that it was usually what geologists call an intrusion, that is an influx of rock or extraneous matter into the stratum of coal: 1691 working thorrow a gall, Farnley
1754 for going through a gall 1s, Beeston
1812 Breaking down at galls in the bord 8s 0d, Soil Hill.
2) A difficult word, used of places where the land is wet and spongy from springs (EDD).
1285 cum toto prato falcabili, ut in capitibus, herbagiis, ranis et watergallis, Wensley. Note: 1538 hys wrongfull making of dyks or galltrappys in the kings hye strette, Normanton.