36 
MORTIMER: ORIGIN OF CHALK DALES. 
shallow end of another hollow, just as cracks are frequently found 
to do at the foot of a landslip. They are all undoubtedly due to 
subterranean disturbances of the earth's crust. They are in fine, a 
series of small independent cracks in the rock, or incipient dales, and 
had the strain which produced them been a little greater, probably 
these rifts would have developed into one long united fracture 
or valley, resembling and running parallel with Back Dale and 
Thixendale. A section of one of these hollows (fig. 3) obtained by 
making an excavation across the bottom, shows it to be a V shaped 
cavity of unknown depth, filled (excepting some four feet of sub-soil 
at the top) with small un water- worn chalk gravel, mixed with 
various sized pieces of angularly formed chalk, probably fallen 
from the sides of the hill. A careful removal of the debris from its 
southern side to a depth of 13 feet, exposed a jagged and uneven 
rock surface. In other places, not on the line of section, 
similar openings in the rock, and narrow ends of dales, are 
frequently found filled, or partly filled, with similar small un water- 
worn gravel. 
About one-and-a-half miles to the north-east of these hollows, a 
pit in the chalk near High Towthorpe exposes a fault (fig. 4) com- 
pletely charged with this small unwaterworn chalk gravel, one third 
of which is firmly cemented together, in the form of a concrete wall 
which can be traced coming to the surface for a hundred yards, on 
opposite sides of the pit, and running in a nearly straight line E by 
W. This filled-up fracture in the rock, widens downwards by the 
slanting outwards of its S side as shown in the section (fig. 4). The 
oblique position of this side of the fracture could be caused only by 
dislocation and a downthrow. 
Returning to my line of section, the next striking feature of 
interest is the Back Dale mounds. I have already shown that they 
are slips from the adjoining hill side^ but it remains to be told how 
they obtained their present position. It will be observed that the line 
of section exposes the Kimmeridge clay at the bottom of this dale ; 
after a few feet of detritus, clay has been reached in making post holes. 
I believe that the base of the chalk on the south side of this 
dale is several feet higher than on the north side. This would 
