Yes, that's the adit. Very well hidden and as Elfyn points out, it goes in about 4m before the first blockage. Surface features indicate that it probably went considerably further, not very far below the surface, to a distance of maybe circa. 100m. from the adit/river. One of the "holes/pits" is very symmetrical with, maybe, the remains of a low wall around it (see other pic).
Other surface remains indicate a building and perhaps, some form of river defence.
Technically, private land. Not actually in the National Park.
Aspects of the management of this area are an absolute disgrace.