MOKTIMER: OEIGIX OF CHALK DALES. 
33 
the v\-estern edge of the Wolds, north-eastwards to Wharram-le- 
street, and cuts at an angle of about 40'^ the axis of elevation run- 
ning E. and W. 
In further support of my theory of the formation of the chalk 
valleys, I will endeavour to point out and explain a few of the 
leading features observed along this line of section. 
Commencing at the western extremity, it will be observed that 
the base of the chalk is about 500 feet above the ordnance datum 
line, and the beds dip for a short distance about 5° degrees to the 
S.W. This dip becomes somewhat less and less for about one mile 
N.E., when the beds begin to dip slightly in the opposite direction. 
At this highest point the base of the chalk is 600 feet above 
O.D. line, and is in immediate contact with the Kimmeridge clay. 
Thence to that extensive rent called Thixeudale (which is about 
25 yards wide at the bottom where crossed by the line of section), 
the beds curve slightlj'-, forming a trough or synclinal. The head 
or upper end of this great dale (Thixendale) begins close to 
Aldro on the very summit of the N.W. edge of the Wolds, and 
about half-a-mile from the outer margin of the chalk. Here at 
first it deepens rapidly, but widens gradually ; afterwards it gradually 
deepens for some miles, and widens the whole of its length eastwards 
to near Driffield, a distance of about 15 miles, its course being 
somewhat sinuous. The bottom of its eastern end is covered for a 
distance of 10 miles to a depth of from 10 to 30 feet with chalk 
gravel, the upper 10 feet of which is well ^^ater-worn, and nearly 
free from earthy matter (ground chalk), mixed with subangular 
flints, and a few foreign boulders of various sizes and many kinds of 
rock ; below, the water-worn appearance diminishes gradually down- 
wards, in all the sections I have seen, and finally becomes unwater- 
worn, and mixed towards the bottom with angular lumps of chalk 
and flint to a depth of 30 feet ; a greater depth I have not seen. 
But I know of no section where a smooth, flat surface is shown at 
the bottom of this dale, or any other, which should be the case if 
these valleys were cut down by the erosive action of rivers. The 
other or north-western end of this dale (Thixendale) consists, at the 
bottom for a distance of 4 miles, of Kimmeridge clay, except for a 
