The Hudnalls in the Lower Wye Valley is well worth a visit. Having not been cut for over 100 years it's rich in flora and fauna resembling a temperate woodland or rainforest. For a full description see this link: http://webarchive.nationalarchives.gov.uk/20140711133551/http://www.naturalengland.org.uk/Images/the-hudnalls-welcome-sheet_tcm6-36376.pdf
The Hudnalls makes up a fantastic walk and or hike from St Briavels through to the Devils Pulpit overlooking Tintern. See the map for location and details.