TRANSACTIONS OF SECTION C. 645 



The Neocomian Ironsands are in full force hard by ; at thia spot they have 

 been denuded before the Gault was deposited, and their ruins have accumulated 

 towards the base of that subformation. 



The Purbeck and Portland rocks of the Swindon outlier have been so often 

 described that they need no special notice.^ The shell-marks and tufaceous lime- 

 stones of the Purbeck lie in a hollow worn out in the Portland rocks, and the 

 two are most sharply marked off from each other. A limestone crowded with 

 Cerithtum Portlandicum marks the upper limit of the Portland. It has been 

 irregularly denuded, and here and there small isolated blocks, which have escaped 

 denudation, project up into the Purbeck. The Portlands have a shingly character 

 in some beds, which must have been formed in shallow water. 



2. Report of the Committee for making new Sections in the Stonesfield 

 Slate. — See Reports, p. 304. 



3. On the Terraced Hill Slopes of North Oxfordshire. 

 By Edwin A. Walpoed, F.G.S. 



The green slopes of many of the minor vales of North Oxfordshire are scored with 

 parallel terraces or terraced banks, frequently of such regularity in depth of step 

 and slope as to present to the mind any other origin for their formation than that 

 of the every-day work of natural forces. They have been described as camps, 

 entrenchments, and amphitheatres, and those of other districts Mr. Gomme has 

 described, and has cited the many theories of their origin. 



Mr. Walfovd first drew attention to the Oxfordshire and Warwickshire terraced 

 fields in 1886,^ and dealt at greater length with the subject in 1890.^ 



He gives as causes of formation — 



1. The downward creep of the surface and sub-surface soil. 



2. The occurrence of the terraces upon one precise geologic line, the micaceous 

 marls of the Middle Lias which come in below the Red Rock bed. The marls are 

 porous and non-cohesive. On the slippery slopes the soil must creep. The rain and 

 rain-wash loosen the light soil below and about the roots of the herbage and urge 

 its movement downward. Terraces from an incipient stage, like an ordinary grass 

 ridge, to minor and major terraced banks of varying regularity of form can be 

 traced. Below these marls are depths of compact blue clay (the zone of Ammonites 

 margaritatus if in near contact with the marls). A little below the point where 

 the marls and clays meet is the line of water outflow. Along the line there is 

 constant removal of marl by chemical and mechanical solution. The effect is the 

 loosening and sliding of the land downwards and outwards. This movement is 

 aided by the weight of the overlying mass of rock, sometimes twenty-five feet in 

 thickness. 



3. Free passage of water through the rock and marl is necessary, for the Upper 

 liias clays have, on the Oxfordshire terraced hills, either been wholly stripped from 

 the hill top or pushed back by atmospheric denudation. Regularly terraced slopes 

 are not found on clay-covered hills ; the appearance of terraces is coincident with 

 the wearing away of the clay ' roof.' 



The amphitheatre form of terraced land is always a valley head. The outflow 

 of the stream — the valley-maker — marks ordinarily the base of the amphitheatre. 

 More frequently the terraces of the valley head are small in step and their curvature 

 is broken. Such an instance is Kenhill, near Shennington. An instance of 

 greater regularity of curvature and greater depth of step is the Beargarden, Ban- 

 bury. 



' Blake, Q.J.G.S., xxxvi. (1880), p. 203. 



^ E. A. Walford, Edge Hill : the Battle and Battlefield, p. 24. Banbury, 1886. 

 ' E. A. Walford, < On some Terraced Hill Slopes in the Midland.s,' Journ. North- 

 amj>ton Nat. Hist. Soc., January, 1890. 



