290 
THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
particular district which alone can assist Northampton, 
indeed so much is the Rock-bed reduced in the northern 
parts of the county that I should think an average of three 
feet would be sufficient. It is, perhaps, important to mention 
that the area that would first be affected by filling up the 
Marlstone would be that nearest Northampton, where the’ 
average thickness may be more than six feet. 
As one of the boundaries of the area to be filled is the 
Nen “fault,” it would be better here perhaps to say what is 
known of this “ fault.” 
The Nen “ Fault ” is first mentioned in the “ Memoirs of 
the Geological Survey,” * and is marked on the map by a 
white line. According to the Memoir and map the “fault” 
extends from Newnham to Kislingbury, i.e. about eight miles, 
and is greatest at Weedon, where on the south side of the 
valley the Upper Lias is brought into contact with the Lower 
Lias, entirely cutting out the Middle Lias. I cannot add 
much to the information given above with respect to this 
neighbourhood, but during the recent works in connection 
with the new railway line from Weedon to Daventry, the 
Middle Lias was well exposed near to the station and 
barracks, and from the thickness of it there we can estimate 
the minimum displacement necessary to bring the Upper 
Lias into contact with the Lower Lias; this would be about 
30ft. It is, of course, very probable that the actual 
displacement, particularly where increased by the subsidiary 
N.W. and S.E. “ fault,” is much more, for the actual zone 
of the Lower Lias against which the Upper Lias abuts is not 
known, and the lowest beds of the Upper Lias are not shown, 
although they are presumably present, as they are well 
developed on the northern side of the same valley. 
At Kislingbury the displacement seems to be much less 
than at Weedon, for a little east of Kislingbury a connecting 
link of Middle Lias is shown across the valley, though the 
Rock-bed is absent. 
At Northampton we have evidence of a “ fault” in which 
the displacement is greater even than that at Weedon, for on 
the northern side of the Nen valley the Middle Lias Rock- 
bed is only 75ft. above sea-level, whereas on the southern 
side the lower water-bearing bed is about 130ft. above sea- 
level. Allowing 20ft. (the recorded thickness at the Spinney 
* Description of Quarter Sheet No. 53, S.E., by William Talbot 
Aveling, F.G.S., and Bichard Trench, B.A., F.G.S. Published in 1860. 
