The southern chalk district is bounded by an extremely irregular line, commencing on the north side of Inkpen Beacon, and making a circuit by Westbury, Maiden-Bradley, and Wilton to Shaftesbury in Dorsetshire, its continuity being broken by three deep indentations in the upper part of the valleys of the Avon, Wily, and Sadder, where the subjacent formations have been denuded.
The south-eastern part of the county, inclosed by this boundary, is occupied by the chalk which extends eastward into Hampshire and southward into Dorset-shire, and forms an extensive billy tract fur rowed by the valleys of the Madder, the Wily, the Avon, and the Bourn, and a valley watered by a stream which passes Broad-Chalk, Bishopeton, and Humington, which valleys unite near Salisbury to form the Valley of the Lower Avon. South and east of Salisbury the chalk I. covered with the plastic clay formation belonging to the chalk basin of the Isle of Wight, which is also observed in one or two other places in the district Theprincipal hills in this southern chalk district, as in the north ern, are on the boundary, which is for the most part indicated by a steep escarpment Tho principal eminences are Inkpen Beacon, the highest point in the chalk formation In England, 1011 feet high, near the junction of Wiltshire, Hampshire, and Berkshire; Easton Hill and Paisley Hill, both commanding the Vale of Pewsey, and crowned with ancient earthworks; Little Cheverill and Great Cheverill Hills ; Eddington Hill, Battlesbury Camp, Titherington Hill, Chiselbury Camp, with a circular intrenchment, and White Sheet Hill, between Wilton and Shaftesbury. In the central part of the district are Miz maze, Ashley, and Haruham hills, Tower Hill, Amesbury Down, Newton Hill, and numerous others.
This chalk district, known as Salisbury Plain, forms an elevated platform, uncultivated and uninclosed, except in the valleys. Wide downs, covered by a scanty herbage, spread in every direction. The population is collected in the valleys, where, along the streams which water them, the villages stand very close to each other. The chalk is generally bare of wood, except in three or four spots, such as Sever nake Forest, Gravely Wood, Vernditch Chase, and Cranbouru Chase.
The greensand formation, comprehending the chalk-marl with the greensand, crops out from beneath. the escarpment of the two chalk districts occupying the Vale of Pewsey, which separates them, as well as the indentations in the boundary of the southern chalk district.
Consequently the outer edge of the greensand is rather more regular than that of the chalk. The greensand rises gradually from the foot of the chalk escarpment towards its outer edge, which is in many parts traceable by a well-defined and steep escarpment.
From Eddington the outer edge of the greensand may be traced in an irregular lino by Devizes and Pottern to Market Lavington ; then westward by Westbury to the border of Socuersetshire. It occupies nearly all the county west of the chalk between Warminster and Mere. About Warminster and Stourhead Park, in the south-western part of the county, the greensand hills nearly equal those of the chalk in height. Alfred's Tower, near Stourhead, is on a greensand bill 800 feet high.
From beneath the outer edge of the greensand formation the Weald clay, or Tetsworth clay, which usually separates the greensand from the ironeand, crops out It occupies only a narrow tract, sur rounding on every side the country occupied by the superior forma tions, and may be traced through the county with little interruption. In the Vale of Wardour the clay occupies a very narrow strip skirting the greensand. The ironsand does not appear in this county, except in a few places, and is described as being a pudding-stone composed of rounded quartz united by a siliceous cement with a red calx of iron, containing ore formerly in much request for the furnace and the forge.
In tho absence of the ironsand, the Weald clay is found to rest along the northern and north-western borders of the county on the Kimmeridge clay, which belongs to the uppermost division of the oolitic group. This Kimmeridge clay occupies a tract rarely exceeding two miles in breadth, but extending in length from the Berkshire border to Seend, west of Devizes, beyond which it is covered by the westward extension of the overlying formations. At Swindon, in the Kimmeridge clay district, beds of oolitic freestone, similar to the Portland beds, intervene between the Weald clay and Kimmeridge clay, and are extensively quarried. In the northern part of the county the upper oolites are confined to low ground : in the Vale of Pewsey they acquire some elevation, as in Lady Down near Tiebury.