The Stokenchurch Gap, also known as the Aston Rowant Cutting or Aston Hill cutting (from the nearby village of Aston Rowant), or locally "The Canyon"  is a steep chalk cutting, constructed through the Chiltern Hills in Oxfordshire, England during the early 1970s for the M40 motorway. It is 1,200 metres long and a maximum of 47 metres deep, and is located approximately eight miles (13 km) from High Wycombe and close to the village of Stokenchurch. Despite the names, the cutting is in the parish of Lewknor rather than Aston Rowant or Stokenchurch.
The cutting is mainly through the Upper Chalk, with some Middle Chalk seen at its base. The cutting is a Site of Special Scientific Interest of geological importance. It provides the best Coniacian section in central England.
The curved route of the cutting was designed to mitigate impact on the skyline, and to fit the local topography. The construction of this section of the M40 (junctions 5–8) was at the time extremely controversial, since the cutting through the Chiltern Hills was driven through the middle of the ecologically-important Aston Rowant NNR. Conservation groups were appalled at the unexpected decision of the Inspector at the Public Enquiry to reject their claims that the National Nature Reserve designation implied a fully protected landscape which should in no circumstances be damaged. Their alternative proposals for a tunnelled solution, or a route on-line with the existing A40, were rejected.
To a great degree, the Inspectors decision, and the subsequent endorsement of that decision by the Secretary of State, proved a "wake-up" call to conservation groups across the country, so that when a similar case arose in the 1980s regarding the Winchester-Bypass section of the M3 at Twyford Down a far more robust — though ultimately unsuccessful — response was forthcoming.
Drivers often note significant changes in weather conditions on passing through the cutting, sometimes with clear conditions on one side, and rain and low cloud on the other. It is also well known for making people's ears "pop" as they travel through, due to changes in air pressure and gradient on the motorway.
Red kites are commonly seen flying within the cutting or on the surrounding downland, making their return after their persecution by game keepers in Victorian times following a successful reintroduction programme organised by the RSPB and Natural England, which selected the Aston Rowant NNR as one of four initial sites in the UK for a captive release programme using birds brought in from Spain. The scheme has been so successful that the population has grown to over 200 pairs and is now self-generating and supplies birds for similar release programmes elsewhere in the UK.