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Report of an excursion to Hertingfordbury and Hertford
EX CU RSION TO HE RTI NGF OIW I.IUR Y A N D H E RTFOR D.
115
P LATE 2 2. Ext ens ion of sectio n in Pla te 2I, enl a rge d ab out four times. The da rk pat ches represe nt large " gravel-bo ulders," embedd ed as frozen masses in the boulder-clay a nd contorted g ra vels. NOTE. -This ph otog raph may be compa red with t hose reproduced from former secti ons at Ni ne-mile Ride in t he O ld Win dsor F orest coun try . " Excur sion to W oki ngham an d We llin g ton C ollege," Proc, Geo], Assoc., vol. xi, p. clxi ; a nd" Geo log ica l H istory of the Tham es Va lley " (Fig . 3), Scimce Gossip, 1891,
REPORT OF AN E XCURSION TO HERTINGFORD· BURY AND HERTFORD. S EPTE~IBER 2 0 T H , 1913.
By GEORGE BARROW, F.G.S., Director of the E xcursion. Official Par ty left K ing's Cross at 1. 15 p.m . and left the tra in at H ertingfordbury. They first visited a pit which has already been described in our Pr oceed ings." A few points may be added to that acco unt. The Bould er Clay which rests on th e bedded sands and gravels is of a dark grey colour in spite of the great amount of chalk in it. T he colour is d ue to 'the inco rporat ion of a great q uanti ty of the Mesozoic clays ; Lias, Oxford , Amp thill, a nd K immeridge clays, etc. In most cases the loss of the grey colour is due to dissolution of the chalk and oxidation of th e iron-pyri tes in these clays, and the loss of colour is confined mainly to the top and base of the Boulder Clay. There are one or two patches in which the rat her red der colour may be du e to the incorp orat ion of so me clay originally not grey, or itself decomposed before it was caught up. The point is proving important, as near Finchley, for exampl e, deep sections show this grey colour is often abse nt, the clay involved bei ng mostly London and R eadin g clays, and th e proportion of chalk is so large that, seen from a distan ce of a few yards, this Bould er Clay is often almo st white. It is proving imp ortant to not e the colour of the clay, where sufficiently thick to have a nondecalcified core. The members of the Association are invited to record as many cases as possible, as it shows the track of the ice more accurately than anything else. A large quantity of the clay has been thrown on one side and is decomposing ; from it severa l of the party obtained fossils, mainly of Jurassic age, and in excellent preservat ion. TH E
'" P roc, Geol, Assoc., vo l, xxi, Part 3, 19(9 ) pp. 167·t j J.
II(:l
EXCURSION TO HERTlNGE'ORDBURY AND HERTFORD.
The party then walked on toward Hertford, keeping along the field road close to and on the south side of the railway, the object being to examine some clay, from which bricks were being made some time ago. The clay is an interesting example of the accumulation of wash. The wash-clay has been brought down from the higher sloping ground, formed of the Boulder Clay, and has settled on the flat ground composed of the sand and gravel. It is of quite recent origin and has accumulated in sufficient thickness to be worked for bricks. This" wash" is one of the "terrors" of the geological mapper, as it completely obscures the underlying beds, that are really ill situ. Only a spade is able to penetrate it, a hammer being quite useless in such cases. Most of this clay has now been dug off and the works are nearer the foot of the slope. Here a pit has been opened in what appears to be chalk; but examination showed that the chalk is not solid and in situ, for there are pebbles from the Reading Beds, often arranged rudely in layers, at a considerable depth down in it: This pit is worth further careful examination; it is not clear whether the material in it represents a chalk surface ploughed up by ice, or is the talus of chalk, etc., on the north side of the old valley; the faintly bedded arrangement of the pebbles rather suggests the latter, but no satisfactory conclusion was arrived at at the time of this visit. Walking on to Hertford the party passed completely through the town eastward to the large pit at the south side of the road to Ware, a description of which will be found in the paper already referred to. There the Boulder Clay is both overlaid and underlain by sands and gravels. The clay is of the same grey colour, but there is a point of difference between this and the Hertingford bury pit. At the latter the fossils in the clay are its most striking feature, foreign pebbles of igneous origin are at least rare; at the Hertford pit they are comparatively common and of fairly large dimensions. One large block of granite, almost certainly Scandinavian, is placed at the door of the offices, near the entrance to the pit. Fossils, though present, are not abundant. During the examination of the last pit the members of the Association were entertained by an aeroplane race, but this was not in the programme. The party then walked back to Hertford, where tea was provided at the Salisbury Arms, and afterwards returning to London by the train. Mr. A. H. Williams acted as Secretary of this excursion.
115
P LATE 2 2. Ext ens ion of sectio n in Pla te 2I, enl a rge d ab out four times. The da rk pat ches represe nt large " gravel-bo ulders," embedd ed as frozen masses in the boulder-clay a nd contorted g ra vels. NOTE. -This ph otog raph may be compa red with t hose reproduced from former secti ons at Ni ne-mile Ride in t he O ld Win dsor F orest coun try . " Excur sion to W oki ngham an d We llin g ton C ollege," Proc, Geo], Assoc., vol. xi, p. clxi ; a nd" Geo log ica l H istory of the Tham es Va lley " (Fig . 3), Scimce Gossip, 1891,
REPORT OF AN E XCURSION TO HERTINGFORD· BURY AND HERTFORD. S EPTE~IBER 2 0 T H , 1913.
By GEORGE BARROW, F.G.S., Director of the E xcursion. Official Par ty left K ing's Cross at 1. 15 p.m . and left the tra in at H ertingfordbury. They first visited a pit which has already been described in our Pr oceed ings." A few points may be added to that acco unt. The Bould er Clay which rests on th e bedded sands and gravels is of a dark grey colour in spite of the great amount of chalk in it. T he colour is d ue to 'the inco rporat ion of a great q uanti ty of the Mesozoic clays ; Lias, Oxford , Amp thill, a nd K immeridge clays, etc. In most cases the loss of the grey colour is due to dissolution of the chalk and oxidation of th e iron-pyri tes in these clays, and the loss of colour is confined mainly to the top and base of the Boulder Clay. There are one or two patches in which the rat her red der colour may be du e to the incorp orat ion of so me clay originally not grey, or itself decomposed before it was caught up. The point is proving important, as near Finchley, for exampl e, deep sections show this grey colour is often abse nt, the clay involved bei ng mostly London and R eadin g clays, and th e proportion of chalk is so large that, seen from a distan ce of a few yards, this Bould er Clay is often almo st white. It is proving imp ortant to not e the colour of the clay, where sufficiently thick to have a nondecalcified core. The members of the Association are invited to record as many cases as possible, as it shows the track of the ice more accurately than anything else. A large quantity of the clay has been thrown on one side and is decomposing ; from it severa l of the party obtained fossils, mainly of Jurassic age, and in excellent preservat ion. TH E
'" P roc, Geol, Assoc., vo l, xxi, Part 3, 19(9 ) pp. 167·t j J.
II(:l
EXCURSION TO HERTlNGE'ORDBURY AND HERTFORD.
The party then walked on toward Hertford, keeping along the field road close to and on the south side of the railway, the object being to examine some clay, from which bricks were being made some time ago. The clay is an interesting example of the accumulation of wash. The wash-clay has been brought down from the higher sloping ground, formed of the Boulder Clay, and has settled on the flat ground composed of the sand and gravel. It is of quite recent origin and has accumulated in sufficient thickness to be worked for bricks. This" wash" is one of the "terrors" of the geological mapper, as it completely obscures the underlying beds, that are really ill situ. Only a spade is able to penetrate it, a hammer being quite useless in such cases. Most of this clay has now been dug off and the works are nearer the foot of the slope. Here a pit has been opened in what appears to be chalk; but examination showed that the chalk is not solid and in situ, for there are pebbles from the Reading Beds, often arranged rudely in layers, at a considerable depth down in it: This pit is worth further careful examination; it is not clear whether the material in it represents a chalk surface ploughed up by ice, or is the talus of chalk, etc., on the north side of the old valley; the faintly bedded arrangement of the pebbles rather suggests the latter, but no satisfactory conclusion was arrived at at the time of this visit. Walking on to Hertford the party passed completely through the town eastward to the large pit at the south side of the road to Ware, a description of which will be found in the paper already referred to. There the Boulder Clay is both overlaid and underlain by sands and gravels. The clay is of the same grey colour, but there is a point of difference between this and the Hertingford bury pit. At the latter the fossils in the clay are its most striking feature, foreign pebbles of igneous origin are at least rare; at the Hertford pit they are comparatively common and of fairly large dimensions. One large block of granite, almost certainly Scandinavian, is placed at the door of the offices, near the entrance to the pit. Fossils, though present, are not abundant. During the examination of the last pit the members of the Association were entertained by an aeroplane race, but this was not in the programme. The party then walked back to Hertford, where tea was provided at the Salisbury Arms, and afterwards returning to London by the train. Mr. A. H. Williams acted as Secretary of this excursion.