BISAT 
THE MILLSTONE GRIT SEQUEN'CE 
2a 
in the whole of the 250 ft. intervening between this horizon and the 
underlying Colsterdale beds never assume, and only remotely approach 
in extreme cases. The channel formed by this erosion line appears 
to run north-east and south-west. 
The shales above the erosion line have yielded Lingula mytiloides, 
and in the sandstone beds which follow a fauna especially rich in 
lamellibranchs is developed. This sandstone bed is the main shellbed 
of the Cayton Gill series, which may be taken as commencing at the 
erosion line. Unfortunately only the lowest and least fossiliferous 
part of this series was exposed at Leighton, but at Roundhill Reservoir, 
Arnagill Valley, High Ash Head Moor, and in the river Burn, near Shaw's 
Bridge, Swinton, more fossiliferous beds are exposed, and boulders 
may be seen strewing the line of outcrop. A dark, platy sandstone 
higher up in the series is seen at Arnagill Valley and High Ash Head 
Moor, and has yielded numerous brachiopods, but no lamellibranchs. 
A similar band on the bank of the Ure, near Hackfall, probably repre- 
sents the same horizon. The boring at Roundhill noted on page 
59 of Fox-Strangways' Geology of the Country north and east of 
Harrogate," passed through the whole of this series, and indicated 
about 60 ft. of shales in the upper part, which unfortunately do not 
appear to be exposed anywhere in the district. There is some little 
difficulty in determining the exact thickness to assign to the Cayton 
Gill Beds as the top is nowhere seen owing to lack of exposures. The 
total thickness is here taken as 150 feet, which allows the shales im- 
mediately below the lower grit band of the Brimham Grit group to be 
included with the grits. 
Beds of this age form the moor top to the east of Lofthouse, 
and stretch northwards round the bend of Xidderdale to Great Haw. 
In the region between Great Haw and the head of Grimes Gill a hard 
white ganisteroid sandstone appears low down in the Cayton Gill 
series, apparently underlying the main shell bed. 
The base of the series at Leighton is mainly formed of clay shales, 
and this appears to be the case wherever exposures are available, 
similar shales being seen at Woo Gill, Nidderdale, and traces in Arna- 
gill Valley and Ger Beck, Tranmire. These shales have only yielded 
Lingula, and that but sparingly. Shales a little higher are more 
fossiliferous. 
The sandstone beds seen at Fountains, near Ripon, are much 
harder and more calcareous than those of this district. 
