Bulkington lies in a vale formed of Oxford Clay. The land here is flat, about 50 metres above sea level, and drained by a network of streams which are fed by springs at the edge of the chalk escarpment which can be seen as a line of hills to the south of the village. The chief of these is Semington Brook (also known as Bulkington Brook to the south of the village), which flows north-westward from the eastern boundary of Bulkington; passing to the south of Bulkington it forms the northern part of the Keevil-Bulkington boundary. Another stream, Summerham Brook, joins Semington Brook west of Bulkington, having formed the northern boundary of the parish. The confluence of the two brooks is at the Baldham Bridge on the A361. Semington Brook continues to meander westwards before joining the River Avon at Whaddon to the north-east of Trowbridge. The water eventually flows through Bradford-on-Avon, Bath, then under the Clifton Suspension Bridge and on to Avonmouth on the Bristol Channel.

Semington Brook and Summerham Brook