OXFORDSHIRE or OXON, a south-east midland county of England, bounded north-east by Northamptonshire, north west by Warwickshire, west by Gloucestershire, south-south-west and south east by Berkshire and east by Buckinghamshire. It was originally part of the Mercian kingdom, but its boundaries, except for the Thames on the south (71 m. from Kelmscot near Lechlade, Gloucestershire, to Remenham below Henley-on-Thames, with the exception of two points near Oxford) are artificial. They were slightly changed by act of William IV. and Victoria to extend the area of the county (755.7 sq.m.). The 14 Oxfordshire Hundreds include five of the Chiltern Hundreds, the jurisdiction over which belonged to the manor of Benson, and in 1199 to Robert de Hare court. The county encloses small portions of Berkshire (in Bamp ton Hundred) and of Buckinghamshire (in Ploughley Hundred).
Structure and Topography.—The county lies across the central portion of the Jurassic and Cretaceous outcrops. The strike of the strata is in each case from north-east to south-west, for Tertiary movements have tilted them gently down to the south-east so that the transition from north-west to south-east is from older to newer rocks passing in succession from the Lias over the Lower, Middle and Upper Oolite to the Lower Greensand, Gault, Upper Greensand and Chalk. In the north-west, the Marl stone of the Middle Lias and the Limestones of the Oolitic series are permeable and relatively resistant, and the Marlstones and the Inferior Oolite rocks being specially hard, stand out as a sharp north-west edge, continuing the line of the Cotswolds at a lower level (average 50o ft.), but rising to 700 ft. in Edge Hill in the north-east of the county. In the south-east, the chalk is also permeable and resistant, and the hard chalk-with-flints crowning the Chilterns also presents a sharp edge to the north-west.
Between these two upland regions the intermediate geological series forms a broad vale (2o m.) of alternating clays and cal careous, sandy beds. The Upper Greensand forms a low feature at the foot of the chalk hills ; this is succeeded by the Gault, with width of outcrop varying from 4 M. to 12 m. between Dorchester and Sydenham. The lower Greensand appears from beneath the Gault at Culham and Nuneham Courtney and in outliers north of Cuddesdon ; Portland limestone, Portland sands and Purbeck beds lie between it and the Kimmeridge clays, which outcrop be tween Sandford and Waterperry. Coral Rag is traceable from
Sandford to Wheatley and beyond this comes a broad outcrop of Oxford clay followed by the Cornbrash (a brownish rubbly lime stone). This outcrops at Norton Bridge, Woodstock and Shipton, forms a broad plateau between Middleton Stoney and Bicester, and also occurs as inliers at Islip, Charlton, Merton and Black Horse Hill. The county lies almost wholly in the basin of the upper Thames, in which the significant drainage is that of the Cher well-Thames. The drainage pattern as a whole, consists of nu merous consequent streams from the scarplands (Evenlode, Win drush, etc.), which have been captured and diverted into the Cher well-Thames by powerful subsequent streams (Ock, Upper Thames, Ray, Thame), working along lines of weakness in the strike of the rocks, obsequent streams flowing down the scarp edge often being a further result of the capturing.
The Cherwell occupies a broad sag between Edge Hill and the Northampton Uplands. It flows south-south-east, joins the Thames coming from the west at Oxford, after which the com bined stream continues the south-south-east direction, passing by a deep gorge through the chalk between Wallingford and Reading. That the gap must have originated north-west of its present posi tion is argued from the fact that the level of the river bed at this point is zoo ft. while that of the hills on either side is 700-800 feet. It is an old, but rejuvenated drainage system, which, working upon calcareous rocks, has given rise to a characteristic topog raphy; streams graded to base level but with very steep-sided up per valleys and wide intervening spaces between the head streams that have no surface streams ; broad main valleys from which streams have disappeared, or where drainage is beneath the sur face; misfit streams, due to capture by the river Severn of an earlier drainage to the Thames from the Welsh plateau, or to carv ing out of valleys by flood waters following the retreat of the ice. There is little glacial drift except in the north-east of the county. Gravel deposits, both plateau (North Leigh, Combe, Tiddington, etc.) and flood plain (Bampton, Oxford, Dorchester, etc.), are very important ; tracts of clay-with-flints, brick-earth and gravel, as well as outliers of the London clay (Nettlebed, Caversham, etc.) occur on the dip slope of the chalk.