The sedimentation and geotechnical properties of the Cromer Till between Happisburgh and Cromer, Norfolk
Authors: Kazi Assadullah , Knill John Lawrence 
Published: November 2007

Summary

The cliff-section between Happisburgh and Weybourne, on the north Norfolk coast, provides one of the most representative exposures of glacial sediments in the British Isles and this sequence is now generally accepted as being the product of the Lowestoft Stage. The Cromer Till, as defined in the Geological Survey
Memoir
, forms the lowermost part of the glacial sequence and is composed of two tills separated by a layer of laminated silts and clays; these units may be termed as follows:The Lower Cromer Till is a dense, stiff, fissured grey or dark grey sandy boulder clay which lies on the Cromer Forest Bed, the Leda Myalis Beds and the associated deposits. The Laminated Beds rest on the hummocky, locally faulted, surface of the till. At Happisburgh two divisions of laminated sediments have been recognized, the upper unit being finer-grained and having a more evenly-bedded structure. The Upper Cromer Till is a dense, fissured grey-blue sandy boulder clay containing a significant proportion of chalk pebbles.

The sequence within the Cromer Till represents the three phases of an ice advance followed by retreat associated with the development of a glacial lake followed by a re-advance. The coarse-grained lower unit of the Laminated Beds at Happisburgh is not present at Cromer and this may reflect the early history of the glacial lake while ice was still present in the vicinity of Cromer. The upper unit of the Happisburgh sequence is coarser than that at Cromer and this condition is reflected by variations in liquid limit measured on scrape samples. A wide range of sedimentary structures, which have been formed by traction and fall-out processes associated with turbulent suspension, occur within the Laminated Beds. The associations of structures present are the product of specific bed forms and can be related to flow régimes. Many beds are the result of rapid deposition from turbidity currents.
All the sediments studied have been heavily over-consolidated and are fissured to a greater or lesser degree. Disturbed, block and scrape samples were collected in the field and soil mechanics classification, strength and consolidation tests carried out. Particular attention has been given to different methods of assessment of the effects of pre-consolidation load and the variation in properties of the Laminated Beds with orientation. The influence of the pre-consolidation load has been examined by a field study of fissuring in the Laminated Beds based upon the dissection of excavated blocks, undrained and drained shear-box testing of samples taken from the Lower Cromer Till and consolidation testing of samples from the Laminated Beds to estimate the pre-consolidation load. This study has revealed that the strength of the Lower Cromer Till is significantly greater near Cromer than at Happisburgh. In addition, the pre-consolidation loads at Cromer are equivalent to an ice thickness of some 300 ft above the present level of the top of the Cromer Ridge as compared with a possible ice thickness of about 200 ft at Happisburgh. The fissure measurements tend to confirm the previous studies of pebble and fold-axis orientation in that the direction of ice movement was probably slightly south of east. The geological and geotechnical evidence, therefore, together provide a picture of an ice sheet moving and thinning towards the east and south-east.