+---------------------+-----+-----+ |Northern Boundary. |Sand.|Clay.| +---------------------+-----+-----+ | | ft.| ft. | |Hertford | 26 | 3 | |Beaumont Green, | | | | near Hoddesdon | 16 | 10 | |Broxbourne | 28 | 2 | |Gestingthorpe, | | | | near Sudbury | 50?| ? | |Whitton, | | | | near Ipswich | 60?| 5 | | +-----+-----+ | Average | 36 | 5 | +---------------------+-----+-----+ The mean of the three columns in two western sections gives a thickness to this formation of 57 feet, of which only 19 feet are sand and permeable to water, and the remaining 38 feet consist of impermeable clays, affording no supply of water. The area, both at the surface and underground, over which they extend is about 1086 square miles. MEASUREMENT OF SECTIONS WESTWARD OF LONDON. +--------------------------------------------+ | On or near the Southern Boundary | | of the Tertiary District. | +--------------------------------+-----+-----+ | |Sand.|Clay.| | +-----+-----+ | | ft.| ft.| |Streatham | 30 | 25 | |Mitcham | 47 | 34 | |Croydon | 35?| 20?| |Epsom | 31 | 23 | |Fetcham | 35 | 20 | |Guildford | 10?| 40 | |Chinham, near Basingstoke | 20?| 30 | |Itchingswell, near Kingsclere | 22 | 34 | |Highclere | 24 | 27 | |Pebble Hill, near Hungerford | 9 | 39 | | | | | | | | | | | | | | +-----+-----+ | Average | 26 | 29 | +--------------------------------+-----+-----+