Excursion to Mid-Lincolnshire, Easter, 1905
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JI4 EXCURSION TO MID-LINCOLNSHIRE, EAST E R, 19°5 . FROM APRIL 20TH TO 26TH, 1905. Directors: Prof. P. F. K ENDALL, F.G.S., H ENRY PRESTON, F.G.S., R ev. W. L. CARTER, M.A., F .G.S., and Rev. E. NELSON, M.A. E xcursion Secretary: W. P . D. STEBBI NG, F.G. S. (Rfport o.f thf Grantham and Lincol« uisi! 0' H ENRY P RE STON, F.G.S .) TH E principal object of th e Easter E xcursion, 1905, was to investigate th e Geology of Mid-Lin coln sh ire, and inclu ded th e di stri cts around Grantham, An caster, Lin coln, Louth, Market R asen, and Claxby, On Thursday, aoth, som e ear ly visitors reached Grantham at 4.22 p.m. , and accompa nied Mr. Preston sou th of th e town to see expos ures of Upper Lias Clay a nd Plateau Gravel. The U pper Lia s in this dis trict consists of a dense blue-grey clay with bands of sep tarian nodules. These clays comprise the zones of : L eda OVU 11l, with A 11l. bifrons, A m. communis, Am. serpentinus, The paper shales, and have a total thi ckness of about 120 ft. The clay was first seen in Rudd's Bri ckyard, and a very interesting ban d of nodular ironst one was examined, which occ urs at the top of th e fossiliferous zone of the Communis beds, a nd contains numerou s dwarfed specimens of Ammonites of the various zonal types . The bed is filled with oo litic grai ns a veraging about n- in. diam eter, whic h often show a beautiful band ed structure whe n mounted for th e micros cope. The chief feat ure, however, is th e number of species of gasteropod s which this concretionary bed contains, as will be seen from th e following in Mr. Preston 's collec tion: Pieurotomaria perseus, Pieurotomaria sp., Onustus Spi1losus, Turbo Theodori, Cerithium costellatum, Cerithium armatum, Cerithiu»: sp., Fusus (?), Alaria sp., Amberleya capitana, Turriteiia sp. Whe n pass ing ove r the fields to the Great North Road a good gen eral view was obtained of the great escarpment of Li nco lnshire Oolite Limeston e kn own as the Lincolnshi re Cliff, which ha s pra ctically a north and south strike through th e who le co unty. EXCURSION TO MID-LINCOLN SHIRE, EASTER, 1905. 115 The gravel pit on the east side of the North Road just S.W. of Little Ponton was next visited. The gravel consists of rounded and smoothed pebbles of Inferior Oolite, Liassic Limestones, Marlstone, Carboniferous Limestone, and Sandstone, with a few flints, quartzites, igneous rocks, and hard Palreozoic rocks. The following list of derived fossils were obtained by Mr. Preston from this pit: Thecosmilia annuiaris (Kem), Coral Rag? Belemnites otoenii, var. Parosianus, Kel, Rock. GrypJuea dilatata, " Serpula clava (Bean ), Cornbrash. Rhynchonella uarians, " Ostrea obscura (Sow), " Modiola imbricata (Sow), " Pholadomya phillipsi, " Ostrea marshi, " Trigonia moretoni (Morris), Oolite. " costata (Park), " " pallas (Sow), " Pholadomyajidicula, InL Oolite. Ostrea flabe/oides (Lam.),,, " " gregaria, "" Lima pectl1liformis, "" " nedis (Sow), " " Rhyne/zonella tetrahedra, Middle Lias. Gryphcea incurua, Lower Lias. Am. capricornus, "" Productus semi-reticula/us, Carbo Limestone. Crinoid stems, "" Rock, with Spirifer, Orthis, and small univalves, Devonian. (?) Wood, (?) There is little evidence in the pit as to the exact position of this Plateau gravel relative to the Boulder Clay, but it extends westwards for more than a mile, and when the Waterworks Company were laying a main in the field opposite the pit they cut through this gravel bed, and the evidence there obtained shows that it is Post-Glacial in date. Boulder Clay was found filling certain large fissure cracks as well as being inter spersed in the overlying gravel, thus indicating that some, at least, of the clay reached its position before the gravel was deposited. Returning by way of the Waterworks a swampy piece of land was pointed out through which the river Witham runs, and from some trial borings which had been made it was found that the mean thickness of the swamp ground was r a ft. of silty clay and alluvium, and underneath, and extending from side to side of T J6 EXCURSION TO MID-LINCOLNSHIRE, EASTER, J905. the valley was found the dense blue clay of the Upper Lias. The pumping station at Saltersford happens to be on the site of a small Roman settlement, and the rough stony ford by which the Romans crossed the river at this point was found when excavating. The depth of this ford is from 9 to loft. below the river level, and mingling with the stones were numerous fragments of Roman pottery and other remains. From this it is assumed that fully 10 ft. of the alluvium of the swamp has accumulated since the Romans were here. With reference to the origin of this swamp it is significant to note that half a mile lower down the river there is an old water mill, and there being no natural fall in the river suitable for the mill, the water has had to be dammed up to get a head on the wheel. Damming the river up for this purpose would at first fill the valley, and form a long narrow lake which, by continual ~~~ d'uA..(AWI. .tk.-~ t; H uy - f,..;..e-f4 .Hcec 1iuh {;?~~ . ~ . £II[f> l" .•'.:...," ... t' :., f~O'l/tI? nuu..ldu.. ~~,. ~ fnaA£ FIG. I. growth and decay of marsh plants and deposited silt, has caused a deposit of alluvium, through which the river now quietly wends its way. In the Papermill Lane Brickyard (Upper Lias) some very fine fluted pieces of sandstone were obtained from a pit in re-arranged Northampton Sands. These flutings are caused by carbonated water from the overlying limestone percolating amongst the sand grains, and depositing calcite as a cementing material along the runnels. In the evening Mr. Preston gave a brief description of the geology of the neighbourhood, in the course of which he again referred to the very interesting section which was opened in the autumn of 1897 by the Waterworks Company, when they cut through the limestone hill in Little Ponton. This section, a full description of which appeared in the Naturalist for August, ]898, showed several large fissure cracks running parallel to the river, some of which extended below the bottom 01 the pipe trench (22 ft. deep), and had a width at the top upwards EX CUR SION TO MID -LINCOLNSHIRE, EASTER, 1905 117 of 40 yards (see Fig. 1), and a measured section taken near the deepest point gave the following : fl. in. Soil I 6 Gravel 5 0 Clay . 1 6 Sand a nd Marl 1 3 Boulder Clay . 5 0 Limestone Breccia • 5 6 Ha rd Limestone Rock 1 9 2 1 6 The study of these fissure cracks gives an interesting key to the origin of the Lincolnshire Cliff. In the geological maps of Lincolnshire (sheets 70, 83, 86), the westerly escarp ment of the Lincolnshire Limestone is seen to strike almost due south from the north ern extremity of the county until it reaches Grantham, where, on the south side of the town, it takes a S.W. direction. Th e mass of limestone which lies S.W. of the town is a great outlier, being cut off by the valley of the river Witham, and by a fault at the extreme southern edge. It is also noticed that the various streams which rise in this outlier have an eastward flow in conseq uence of the easterly dip of the strata, and have cut their beds to the Lias Clay. Thus the great outlier is cut up into smaller masses all resting on a clay which is inclined towards the east. T he origin of the fissure cracks at Little Ponton was explained as being due to the letting down of the rock along the western bank of the Witham soon after the river had cut thr ough the Limestone. Thi s had allowed the springs to wash out the sand s between the Limestone and the clay, and gradual settlement had caused fracture in lines which are practicall y parallel with the river. These cracks have been enlarged by percolating water in the ordinary manner of swallow holes, and filled in partly by overlying material, and partly by debris of decomposed rock. Thus the tributary streams which have cut up the great outlier of Limestone into smaller masses ; the underlying clay which dips eastwards towards the river j and the lateral cracks which release these masses; all combine to cause slip or creep on the lubricated and impervious Upp er Lias Clay, and denudation carries away the thin edge of the Limestone without any perceptible land -slip. By degrees, then, the Limestone west of the river will be removed, and the escarpment on the east bank of the river will remain as an extension southwards of the Lincolnshire cliff. On Friday, the a rst, the party now numbering thirty, an early start was made northwards of the town. At the Middle Gonerby Brickpits a very fine section was seen of the Middle Lias Clays immediately und erlying the Marlstone Ro ck Bed. The floor of the pit marks the junction of the zone of Am. Capricornus P ROC. GEOL. Assoc.. \ 'OL. X IX, PART 3, 1905 .] 10 lIS EXCURSION TO !\lID-LlNCOLNSHIRE, EASTER, 1905. (Lower Lias), and the thickness of the clay face was about 60 ft., giving practically the whole thickness of these beds.