22 
BISAT : THE MILLSTONE GRIT SEQUENCE 
group thickens out to the north-east in Colsterdale, mainly owing 
to grit bands coming in. Here trial holes were sunk some years ago 
by the Leeds City Council for their water scheme, and the material 
tipped to spoil has yielded a fairly large fauna. The coal seam may 
be taken as forming the base of the fossiliferous group, but un- 
fortunately there are no exposures showing the whole series, and failing 
this, it is not easy to assign any definite thickness to these beds, but 
lacking better evidence, about 80 ft. of beds may be taken as here 
belonging to the marine band, which thickness includes about 40 ft. 
of grit and ganister which seem to have fossiliferous la vers. 
Above these fossiliferous shales there is a thickness of about 250 
ft. of sandstones, plates and shales, often very micaceous and apparently 
barren of any marine fauna. The sandstone bands are not of very 
great thickness, and very often merely seem to be long lenticular 
wedges. The thickest band seen is a well marked grit running 
down the east side of Birk Gill, where it forms the moor top. On 
reaching Healey Pasture quarry it has become finer grained and is 
not definitely traceable further south. Its thickness is 50 ft. to 60 ft., 
it occurs in the middle of the series, and is perhaps represented by rock 
bands seen at Leighton Reservoir and below Healey, but there appears 
to be no means of correlation. The shales form at Leighton about 
60 per cent of the series, are fairly hard and sandy, and usually full 
of hard fine grained grey sandstone streaks, which may in a few feet 
horizontally change the shale into a sandstone. These sandstones 
contain large concretions of sandstones with a calcite matrix. 
The above series of barren beds possesses on the whole strong 
features, and forms a cap in many places to the relatively softer beds 
below. Beds at this horizon form a capping to the Nidderdale hills 
in the neighbourhood of Great AVhernside, but are best seen in the valley 
of the Burn, where characteristic exposures of alternating sandstone 
and sandy shale may be seen in most of the small gills and in the 
river Burn itself. 
At the top of this series of barren beds there have been small 
exposures on the south side of the valley at Leighton Reservoii*'which 
show what appears to be a definite line of erosion of greater magnitude 
than the small signs of contemporaneous erosion that occasionally 
may be seen in these beds. Not only is the erosion line fairly well 
marked, but also the overlying sediments are of totally different 
character. The sandstones are thin and contain thick clay partings, 
and the shales are extremely argillaceous, a character which the shales 
