Although the GVHS centres upon the post 1875 industrial deep coal-mines, it is just as important to look at the small, often family-run workings. The business for the latter was often conducted at a roll-top desk at home. The maps of the locality are often peppered with "trial shaft" or "disused this, that, or the other". Some of these workings were small Slant or Drift mines, or even personal workings for domestic use only.
Although my family were involved in running such small collieries, neither of the two sites were in the Garw, however one does give a good insight into the problems involved. Finances, flooding, quality of product, the ability to sell a sufficient amount of coal to continue expansion, and so on.
Perhaps some form of survey from OS maps may help with the distribution of workings in the Garw.