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Report of excursions to the north Mimms swallow-holes
59 REPORT OF EXCURSIONS TO THE NORTH MIMMS SWALLOW-HOLES. By W. WHITAKER, F.R.S., F.G.S., Director of tlte Excursions, I.-AUGUST 28TH, 1915. The party travelled by the 1. 15 p.m. train from King's Cross to Potter's Bar. From the station they walked westward to the valley of the Mimms Hall Brook (head-water of the Coin e), and then northward, down the valley, to Water End (Mims Street of the older map). The state of things was so much the same as on the visit of April, 19°8, that there is no occasion to repeat the description ; but this time the Water End swallows showed hardly a trace of mud, and therefore were investigated in a perfectly dry state; so much so that it was suggested that another excursion should be made at a time after there had been rain plentiful enough to fill the whole presumed water-course, so that members might see for themselves that water did sometimes run that way. Tea was taken at Water End, after which the party started again, with renewed vigour, to see some swallow-holes that had not been visited before on any excursion of any Society. The walk was eastward, across Potterells Park, and then south-eastward to the southern part of Moffat's Park, where the stream in the wooded bottom of the side-valley was seen to sink into a swallow-hole, with a pool of water, whilst below there was no stream. In the bottom of this well-marked swallow-hole (first described in 1898, Trans. Herts. Nat. Hist. Soc., p. 8) Chalk was seen, showing that there is a small error in the geologic map, which places the site at just about the base of the London Clay. It was agreed, however, that the place was so well hidden that, in days when geologic surveying was carried out much more rapidly than now, and on an old map, there was good excuse for the surveyor (long since departed) missing it. Now the six-inch Ordnance Map records the swallow-holes here. Thence the walk was continued southward to Potter's Bar station, close to which the pumping station of the Barnet Water Company was noticed, the well passing through 122 feet of Tertiary beds to the Chalk. Some members seemed to have a virtuous feeling that they had done two excursions, one before and one after tea, and so they did not object to the fairly late return. 2.-MARCH 11TH, 1916. What may be called the adjourned excursion was held on the above date, the party again travelling by the I. I 5 train from King's Cross.
60
EXCURSIONS TO THE NORTH MIMMS SWALLOW-HOLES.
Nature had kindly arranged for things to be in a very different state on this occasion. The winter had been a wet one, and the thawing of the heavy snow that had fallen but a few days before caused a liberal flow of drainage-water from the clay hills and slopes around the head of the valley. The party therefore found a swiftly flowing stream all along, from Mimms Hall, where it was first reached, to below Water End, whence the return-journey was made. This was in striking contrast to the mostly dry water-course seen in Septem ber, and the various swallow-holes along the stream were now all more or less hidden under water, including the well-marked set at the little chalk-cliff in the bend of the brook a third of a mile north of Mimms Hall; but the swirl of the water into them could be seen in places. The only exception to this was the set of swallow-holes in the meadow on the right (eastern) side of the stream about a quarter of a mile north-west of Hawkshead Farm. These are in a well-marked small funnel-shaped hollow, a good many yards away from the stream, and can act only when the water rises to such an extent that the stream overflows its banks. The set of swallow-holes was dry at the time, but it was clear that only a few days before there had been an overflow, and that the water had swept over the bank of the stream, and across the meadow into the swallow-holes in the depression, one of which showed an open tubular hole into the Chalk. All along the valley there was good evidence that the water had been some three feet higher than it was at the time, and had spread over beyond the stream-channel in places. On reaching Water End the difference between the view in September, 1915, and that now seen was most striking. Then there was a small dry common, through which meandered what looked like a stream-course, though it was dry and often grassgrown, with funnel-shaped hollows along its tortuous course, also dry, grass-grown and looking as if they had nothing to do with water. Now neither stream nor swallow-holes were to be seen; all were covered by a lakelet. The Chalk was saturated with water, and therefore all that came down from the higher lands remained at the surface, and flowed along the once drystream-course. After tea the return-journey was made by way of Hawkshead Lane and the footpath alongside the railway to Potter's Bar. The party was well satisfied, despite the general dampness. All Hertfordshire swallow-holes will be described together in a Geological Survey Memoir on the Water Supply of the county, the work on which is far advanced.
60
EXCURSIONS TO THE NORTH MIMMS SWALLOW-HOLES.
Nature had kindly arranged for things to be in a very different state on this occasion. The winter had been a wet one, and the thawing of the heavy snow that had fallen but a few days before caused a liberal flow of drainage-water from the clay hills and slopes around the head of the valley. The party therefore found a swiftly flowing stream all along, from Mimms Hall, where it was first reached, to below Water End, whence the return-journey was made. This was in striking contrast to the mostly dry water-course seen in Septem ber, and the various swallow-holes along the stream were now all more or less hidden under water, including the well-marked set at the little chalk-cliff in the bend of the brook a third of a mile north of Mimms Hall; but the swirl of the water into them could be seen in places. The only exception to this was the set of swallow-holes in the meadow on the right (eastern) side of the stream about a quarter of a mile north-west of Hawkshead Farm. These are in a well-marked small funnel-shaped hollow, a good many yards away from the stream, and can act only when the water rises to such an extent that the stream overflows its banks. The set of swallow-holes was dry at the time, but it was clear that only a few days before there had been an overflow, and that the water had swept over the bank of the stream, and across the meadow into the swallow-holes in the depression, one of which showed an open tubular hole into the Chalk. All along the valley there was good evidence that the water had been some three feet higher than it was at the time, and had spread over beyond the stream-channel in places. On reaching Water End the difference between the view in September, 1915, and that now seen was most striking. Then there was a small dry common, through which meandered what looked like a stream-course, though it was dry and often grassgrown, with funnel-shaped hollows along its tortuous course, also dry, grass-grown and looking as if they had nothing to do with water. Now neither stream nor swallow-holes were to be seen; all were covered by a lakelet. The Chalk was saturated with water, and therefore all that came down from the higher lands remained at the surface, and flowed along the once drystream-course. After tea the return-journey was made by way of Hawkshead Lane and the footpath alongside the railway to Potter's Bar. The party was well satisfied, despite the general dampness. All Hertfordshire swallow-holes will be described together in a Geological Survey Memoir on the Water Supply of the county, the work on which is far advanced.