_ proper to clear water, that is to 
THE YORKSHIRE NATURALISTS’ UNION AT AYSGARTH. 231 
some thin bands of shale, and with, at least, one band of sandstone. 
This latter is seen north of Hawes, at West Burton below the Fors, 
and again at Redmire Fors. It is the lowest stratum exposed any- 
where in the dale. The shaly bands referred to as occurring in the 
overlying limestone give rise, by their unequal rate of weathering, 
compared with that of the limestone, to some of the more interesting 
waterfalls of the dale—Gale Fors, all three of the waterfalls at 
Aysgarth,* Abbey Fors at Askrigg, and West Burton Fors. 
Above the Yoredale Rocks at one time occurred the whole of the 
Millstone Grit and the Coal Measures, amounting in the aggregate to 
many thousands of feet. At the present day nearly all this vast pile 
as been carried away by denudation (acting at various periods in 
the past); so that now only fragments of the lowest members of the 
Millstone Grit survive, and occur as outliers capping the highest 
fells of the district. A fine display of these Upper Carboniferous 
rocks is exposed on the summit of Pennel (or ‘Pen Hill’). 
The chief lithological changes traceable amongst the Yoredale 
Rocks were studied in some detail on the ground. Stated in general 
terms they may be said to be closely connected with the condition 
under which the rocks themselves were formed. All through the 
Yoredale period (most especially during the formation of the lime- 
stones and cherts, less so through that of the coals and shales, and 
_ least through that of the sandstones), deep-water conditions prevailed 
over the area where Wensleydale now stands, while to the north-west 
of that part rivers, flowing south-eastward, were advancing their — 
deltas towards the open sea. Intermittent subsidences during the 
growth of the deltas periodically carried their newly-formed slopes 
of mud and sand north-westward as the sea-bottom sank, and in 
that way periodically superinduced deep-water conditions over areas 
that would otherwise have shallowed through the seaward advance 
of the deltas 
Thus eds the north-west of any given point in 1 the dale delta 
materials, i.e., sandstones, shales, and coals, tend to come on in 
_ greater force; while in the opposite direction, or towards the south- 
_ east, where deep-sea conditions predominated, such terrigenous = 
_ deposits tend to thin away, brew the ——— deposits oe 
Strata, tend concurrently to maintain ‘their thickness or even to 
increase. As a necessary consequence the sandstones lessen in 
importance as they are followed howatds = south-east, passing first 3 
|e shoud be noted that the eke as a a a a 
: ‘Aysgarth ee ee, form of ie ome acute G.G. 
