26 BRITISH AVONIAN CONODONT FAUNAS 
cyclothems typically developed in north-eastern England. In the type area on the 
Askrigg Block it overlies the Great Scar Limestone and is overlain by the Millstone 
Grit. The upper contact is partly conformable, partly unconformable ; the lower 
contact is an interdigitation. The typical cyclothem of the Yoredale Formation 
comprises a thick marine limestone overlain by shale, which is in turn succeeded 
by sandstone (Moore 1958, Walker 1964). The limestones resemble those of the 
Great Scar Limestone and the southward failure of the shales and sandstones results 
in a diachronous (interdigitating) contact between the Yoredale Formation and the 
Great Scar Limestone, obliterating at least three cyclothems. In Northumberland 
and Durham the interdigitation occurs at lower and lower levels within the Great 
Scar Limestone (here called the Melmerby Scar Limestone). The top of the Yoredale 
Formation extends higher than on the Askrigg Block in consequence of the occur- 
rence of limestones and the non-occurrence of the typical coarse pebbly sandstones 
of the Millstone Grit, in the generally cyclothemic sequence. In terms of the 
goniatite zones, the Yoredale Formation varies in age as follows :— 
Teesdale early Bz to Ry 
Wensleydale (=Yoredale) late Be to Ey, 
Grassington P,, only 
The succession in the type area of the Yoredale Formation in Upper Wensleydale 
is complex, the major cyclothems between two successive thick limestones often 
including minor rhythms, which possess characters of the typical rhythmic unit, 
except for the persistence of the limestone (Phillips 1836, Moore 1958, Walker 1964). 
The succession (Moore 1958 : 94) is as follows :— 
IX Main Limestone 
VIII Underset Limestone 
VII Three Yard Limestone 
VIB 
VIA 
VI Five Yard Limestone 
aga 
V_ Middle Limestone 
Ivc 
IVB 
IV A 
IV Simonstone Limestone 
III Hardraw Scar Limestone 
II Gayle Limestone 
I Hawes Limestone 
Names have not been applied to the thin impersistent limestones, which are 
designated by an index number, indicating the major cyclothem to which they belong, 
and by an index letter, showing their position in the cyclothem. 
