Whitaker, W.

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EXCURSIO~ TO SAXDERSTEAD AXD PURLEY. 217from the north-east, which requires further investigation. TheDirectors regarded all as marginal ice-features, but some presentthought that they could be explained by springs. As far as canbe seen the channel eventually joins the great flat-based valleyalready referred to.On the way to Aidbury the party walked along another typeof channel, a ledge or step cut back into the hillside and runningnear the 700 foot contour north of Down Farm for severalhundred yards. It hardly seems possible for this to have beencut by springs and the explanation offered was that it was gougedout by a marginal stream when the ice lay about ISO feet thickin the valley or hollow between Pitstone Hill and the mainChilterns.EXCURSION TO SANDERSTEAD AND PURLEY.SATURDAY, APRIL 30TH, 192I.REPORT BY THE PRESIDENT, W. WHITAKER, F.R.S., Directorof the Excursion.THE object of the Excursion was to note the difference betweenEocene and post-Pliocene Gravels, and to see a remarkable smallsecncn in the former.Meeting at Sanaerstead Station the party walked a littleway northward, to a pit in River Gravel adjoining the easternside of the railway (main line). This pit was being filled in, but,by the kindness of Mr. J. F. Carter, Borough Engineer of Croydon,its eastern end had been kept open for the Association. Thesection here seen was as follows, below soil :-Brown loam, up to about five feet.Gravel, chiefly of subangular flints, of varying thickness, upto perhaps ten feet, resting very irregularly onChalk with flints, small hummocks of which stood up in thegravel.It is unusual hereabouts to find loam over the gravel, and thejunction of the latter with the Chalk is not often seen.A move was then made south-eastward, to Briton Hill, ne3.rthe foot of which a small pit had been lately opened on the easternside of the road. For notice of this interesting and unexpectedsection we are indebted to Mr. M. A. Ockenden, who correctlydetermined the beds shown and notified the occurrence to theleader of the Excursion early in January.The level of the pit is about 360 feet above Ordnance Datum,and it is in the well-known pebble-beds of the Blackheath Series,the nearest occurrence of which is at Sanderstead Plantation,218 EXCURSION TO SANDERSTEAD AND PURLEY.about half a mile eastward, where their level is about 550 feet.It is true that at Croham Hurst, about two-thirds of a milenorthward, they are at about 400 feet; but that is in the direc-tion of the general dip of the beds, so that they should be lowerthere than at Briton Hill.It is clear then that we have here an outlier of the BlackheathBeds at a much lower level (by 150 or 200 feet) than one wouldhave expected. How has this been brought about? Severalexplanations suggest themselves; faulting, channel-erosion,piping, or a mixture of any of them.There is no evidence of faulting; but the known irregularityof these beds supports the erosional view. On examining thesection it was seen that there were small masses of reddish plasticclay, clearly belonging to the Reading Beds; but an examinationof the chief of these, by Mr. Stamp and others, led to the conclusionthat it was more or less vertical and apparently succeeded bywhat might be Thanet Sand, and this leads to the conclusion thatthere is piping, as would be expected with such porous beds.Many small trial-holes had been made eastward of the pit,some of which showed the pebble-beds, and some Chalk or clay-with-flints.The difference between the two neighbouring gravels is verymarked, this older bed consisting of flint pebbles only.The walk was then extended a little westward, near PurleyBeeches, to a small schoolroom in Lower Sanderstead, which,through the persuasive power of Mr. Ockenden, was put at thedisposal of the party for tea.After tea many of the party went on to the Brighton Road,half a mile south of Purley tram-terminus, where there is a largegravel-pit alongside the road, on its western side. This pit,now partly abandoned, had been greatly extended since thenew Geological Survey was made, and showed an interestingsection at its southern and newer end , where the gravel was seento be overlain by a layer (up to two feet or more thick) of whtish,sandy marl, in which were several land-shells. One of the partyhad previously found some eight species (including a bivalve).The leader suggests that this was a tufaceous deposit, due tosprings that had occurred at the side of the valley-bottom intimes when the Chalk was far more saturated with water than isnow the case.The party therefore had the advantage of seeing three sectionswhich were practically new, that is to say. had not been described.

Excursion to Sanderstead and Purley

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Excursion to Sanderstead and Purley

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