The stream passage of 'New Oxlow', as this eastern area is known, became incised during this stage, and headward incision of a series of steps began. The inlet into Faucet Rake at the lower end of West Antechamber is likely to have operated for the first time during this stage. Subsequent events in Oxlow are obscure, for the lowest levels of development have not yet been observed. Vadose modification of the extreme lower end of the stream passage has occurred since Stage 3, as also has some modification of the West Antechamber inlet. The water entering here now sinks into the boulder floor.
At some point between the shale margin and Faucet Rake, the tuff which represents the Lower Millers Dale Lava has its feather edge. It does not seem to be present near to the margin, with the possible exception of the area around the top of the Winnats Pass, where it may be present in Winnats Head Cave as a series of thin clay wayboards. Possibly the steep depositional dips caused the material to spill over the reef front and to become dissipated on the slope. In Oxlow Caverns the tuff, here referred to as the Nettle Tuff because of its influence on the development of Nettle Pot, is seen at the top of the Second Pitch at an elevation of 1330 ft (4o5.4 m) A .O .D . The horizon is largely washed out, but may be up to 30 cms thick in Oxlow. It is characteristic of the area in which the tuff is present that joint or vein-controlled cavities increase dramatically in size below it. This is so in Oxlow; the cavities are immense below the tuff. Development of such cavities as West Chamber are a combination of enlargement of primary vein cavities by slow moving water during the early stages of phreatic flow, and, subsequent to lowering of water levels, modification by percolating water rendered more aggressive in its passage over the tuff by oxidation of pyrite, in which such tuffs abound. The present main drainage route, thought to follow Faucet Rake, lies deep below the known cave at Oxlow. The nature of this route is unknown.