Preston: Geology South of Grantham. 253 
about 40 degrees. The width at the top varied from 20. yards 
to 40 yards, and the bottom of each was reached with one 
exception. The sketch is exaggerated in vertical scale so as to 
show the feature more distinctly. . The first idea was that these 
Openings in the rock were ancient pre-Glacial valleys which had 
been filled in by drift material, but the inclination of the sides 
was far too steep for ordinary valleys, and the sides made 
practically a sharp angle at the bottom. Besides this, the sides 
of the valley, and generally the top of the rock, consisted of 
a layer of limestone breccia from 2 feet to 6 feet thick—angular 
pieces of limestone cemented firmly together by calcite, which 
we should scarcely expect to find on the banks of a valley having 
such a high angle. It was also noted that the trend of the 
fissures was about eee with the course of the Witham. It 
may be interesting now from these facts to try and draw a 
conclusion as to the origin of these large openings. It will be 
noticed from Fig. 3 that the limestone rests on Northampton 
Fig 3 
wy E€ 
"ds GUE ¢ SRG 4 
AF br SOT LI 
san fad 
Be a oe OS c fog ter or 
a yn ae T' 
or roe io 
= = 
Zz apes 
A Luke Vortin Aho fimAne RESALE o 
Frcunece Creacelcyg - : 
Bottled Livre Ofunws Prowtewe xt Parrot 
Sands, underlying which is an impervious bed of Upper Lias 
Clay. We have then a large mass of limestone dipping towards 
the S.E., being separated from an impervious clay by sandy 
beds only. Now, when a mass of pervious rock rests on an 
impervious bed, there is always a tendency to slip in the 
direction of the dip when the rocks are saturated with water, 
this tendency i increasing with the inclination of dip. The dip in 
this case is very small, hence the probability of a slip is not 
August 1858. 
ce Sg th Bt st ah a ec, cL fee GP 
