Map sheet number OL31 in the OS Explorer series covers an area of the North Pennines AONB, Barnard Castle, Alton, & Allenheads. Highlights of the area include: Hamsterley Forest, Middleton Common, Milburn Forest, Cow Green & Burnhope Reservoirs, River Tees, Teesdale, River Wear, Weardale & a section of the Pennine Way. With this map you will receive a code for use on your i OS or Android smartphone or tablet. Find our more at Mobile Download FAQ. The OS Explorer map for North Pennines is available in both the standard paper version & weatherproof ' Active' version.