At Brill there are chalky limestones and green glauconitic beds,
below which the pebbly layer is well seen overlying a few feet of brown
and greenish sand, which passes gradually down into the dark grey
Hartwell Clay, which again merges downwards into the Kimeridge
Clay. The clay is used for brickmaking, and in Roman times there
was a pottery at this locality.
The Hartwell Clay which represents the Lower Portland Beds of other localities contains Belemnites souicbi, Ammonites biplex, Area longipunctata, Astarte hartwellensis, Thracia tenera, Perna mytiloides, etc. It has been dug for brickmaking also at Whitchurch.
At Long Crendon there are several exposures of the Portland Beds, and in one pit near the southern windmill four formations were shown in succession :
| ft. | in. | ||
| Gault | Clay | 8 | 0 |
| Lower Greensand | Sand, clay and ironstone | 3 | 6 |
| Purbeck .... | Limestone and clay with Cyprides . | 6 | 0 |
| Portland Beds . . | Limestone with Trigonia gibbosa, etc. | - | - |
Mr. A. Morley Davies estimates the thickness of the limestones of the Portland Beds hereabouts at 32 feet, beneath which is about 2 feet of sand and the pebble bed with lydites, as near Aylesbury. Still lower there is about 30 feet of light-coloured sandy beds with clayey sands, and with a bright green sand at the base. These are the Lower Portland Beds equivalent to the Hartwell Clay of Aylesbury and to the Portland Sands in the south-west of England.[1]
The Upper Portland Beds of Buckinghamshire form dry brashy
soil, which is largely under arable cultivation. Springs are thrown at
the junction with the Hartwell Clay, and good supplies of water are
locally met with. At Dorton below Brill there is a famous chalybeate
spring.
PURBECK BEDS
Several of the more prominent of the outlying hills of Portland Beds in the Vale of Aylesbury are capped by Purbeck strata, as at Oving and Whitchurch, Quainton, Coney Hill, Brill and Long Crendon ; other outlying patches occur at Haddenham and Cuddington, Stone and Hartwell, and at Bishopstone, while their presence has been noted by Mr. Morley Davies between Towersey and Kingsey, and by Fitton at the Warren south of Stewkley, as well as at other localities. Of Purbeck as well as of Portland Beds we have but isolated remnants of formations which may formerly have extended a good deal further north ; but while the record of the Portland Beds is complete, nowhere in this region have we the full thickness of Purbeck Beds. They comprise a variable series of marls, compact and fissile limestones known as ' Pendle,' and calcareous sands, with here and there a cherty layer. The organic remains betoken their freshwater and estuarine
- ↑ See J. F. Blake, Proc. Geol. Assoc. xiii. 74 ; and A. M. Davies, ibid. xvi. 21, 22.
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