I would question the idea that this is a drainage adit. Pre 1554 i would
expect adits to be fairly short. The scale of coal mining at the time would
probably mean that mining would stilll be confined to shallow seams
therefore only requireing a shallow adit. Economically a short adit makes
more sense. If it is an adit why bother running for such a long distance as
cut and cover when the topography suggests it could run in an open trench
for much of that distance, again a cheaper option. A double adit sounds odd
and i can not think of anything like this on other sites until much later.
Usually if an adit becomes unsuitable for the mines needs then it would
either be re driven at a larger cross section, or more likely a new lower
adit will be driven.
As this is on a golf course can you be sure that you are looking at historic
features. Golf courses are often highly manipulated landscapes?
Meerstone Archaeological Consultancy