Beech Bottom Dyke, is a large ditch running for almost a mile at the northern edge of St Albans, Hertfordshire flanked by banks on both sides. It is up to 30 m (98 ft) wide, and 10 m (33 ft) deep, and it can be followed for three quarters of a mile between the "Ancient Briton Crossroads" on the St Albans to Harpenden road until it is crossed by the Thameslink/Midland mainline railway at Sandridge. Beyond the railway embankment it continues, to finish just short of the St Albans to Sandridge road.