(Weeks 1969) has described a number of sites where slip surfaces have been encountered on slopes where the angle is less than the angle of ultimate stability against land sliding. The purpose of this note is to report two sites in Northamptonshire where slip surfaces have been observed on similarly low-angle slopes, and to suggest a possible mechanism for their formation.
Reference to Fig. 1 shows that the two exposures are both at the 'feather edge' of cambered slopes of the Inferior Oolite (in this instance the Northampton Sand), overlying Upper Lias Clay. In both cases the Northampton Sand forms a structureless rubble, the junction with the Lias clay being about an inch above a slicken-sided slip surface of considerable extent. The slip surfaces are ascribed to solifluction movements.
The first site (A in Fig. 1), at Wellingborough [SP 903 693] was a temporary section in a cutting slope which exposed a slip surface that could be traced for nearly 150 ft parallel to its strike. The second site, (B), a road improvement scheme at Isham [SP 881 745], provided a section aligned obliquely to the direction of greatest slope. This section is shown in Fig. 2. The down-slope extent of the Northampton Sand rubble was marked by a topographical feature where the slope increased from 4 to 6°, and where the slip surface ran out to ground level.
In each case there is some evidence that the slip surface may be somewhat undulatory, though in general