GREAT OOLITE SERIES: CASTLE BYTIIAM. 423 
and purplish ironstone at the base. The top-bed of tho Great 
Oolite Limestone comprises slabs of Ostrca-limestone (six to eight 
inches thick) with O. subrugulosa. Below are compact grey 
earthy and shelly limestones, evenly bedded and divided by bands 
of clay. The more prominent bed is near the base, and it is a hard 
blue-hearted limestone (2 to 3 feet thick), full of fossils ; but many 
of them, both Corals and Mollusca, are replaced by calcite. Some 
Oasteropods occur, together with Lima, Modiola, Perna, Ptero- 
perna, Trigonia, &c. Specimens of Ostrea Sotverbyi, and O. 
subrugulosa, as might be expected, are well preserved. Some of 
the blocks of limestone show fucoidal markings. The rock re- 
sembles the Great Oolite Limestone of Bedford, and consists of dark 
and light grey material. The joint-faces weather white. The 
total thickness of the Great Oolite Limestone is about 10 feet. 
The thick bed at the base was shivered in places, but I was 
informed that many large blocks had lain out all the winter and 
had withstood the frost. It is not likely however to prove a very 
serviceable building-stone. 
Below it there occurs a layer of fissile earthy limestone with 
Ostrea (about 1 foot thick), and it rests on the Upper Estuarine 
Series. This Series comprises brown and green clays (like those 
of the Great Oolite Clay), with a baud of earthy limestone with 
plant-markings. A thin gravel rests on the Upper Estuarine 
Series, in the cutting west of the road that leads to Bytham Park. 
At Potter's Hill, to the west of Castle Bytham, there was a 
cutting through the oolitic freestone (Lincolnshire Limestone), 
which was shown at the east end, and the rock was capped by 
Upper Estuarine Beds and again by Boulder Clay (see also p. 208). 
Further west the Boulder Clay has cut more deeply into the 
strata down to the level of the railway ; but it rises again and a 
fine section of Upper Estuarine Beds was exposed. Like the 
Great Oolite Clay, these beds contain a marly oyster-bed, and 
this appeared as a white band in a variable series of clays, the 
tints of which (in descending order) were brown, dark blue, green, 
grey, white, green, and dark blue. Altogether these Upper 
Estuarine Beds are from 15 to 18 feet thick. At the eastern 
end of the cutting the nodular ironstone-bed was shown above the 
Lincolnshire Limestone. The western end of the cutting was 
not completed at the time of my visit. The Boulder Clay 
contained a large nest of sand, and a block of Great Oolite 
Limestone, measuring 7 feet x 4 feet x 18 inches.* 
Castle Bytham to Sleaford. 
In Lincolnshire there is little difficulty in separating the Upper 
Estuarine Series from the Lincolnshire Limestone on which it 
reposes ; but it is by no means so clearly to be distinguished from 
the Great Oolite Limestone above. The lower portions of that 
formation comprise limestones, marls, and clays with oyster-beds, 
* There was no indication of Great Oolite Limestone in the cutting, nor of the 
fault shown on the Geological Survey Map. 
