{Middle Lias Clay. gl Lias Clay and Limestone. Except some small springs thrown out by an ironstone rock in the Middle Lias, at Bracebridge, at 45 feet above the sea level, no underground water is yielded by the Lias, and it may be regarded as an impermeable floor on which water absorbed on the porous rocks lying above it are supported, and carried by their dip beneath the sea level, from whence they are prevented from rising by the overlying impermeable Oolitic Clays. Above the Lias Clays occur the inferior Oolitic Basement Beds, consisting of the Northampton sand, and the overlying clays, shales, and sands of the Lower Estuarine series, varying much in character in different parts of the district. From Navenby to Burton four to eighteen feet of massive ironstone beds occur; between Burton and Ingham clay and sands without ironstone replace the ironstone; but northwards, the Northampton sands are represented by the compact hard ironstone called "the Dogger;" the sands above at Ingham are also cemented into "pan," and associated with grey clay. These ironstone beds occur at about 200 feet above the mean sea level below the Cathedral, and dip eastwards, and descend beneath the sea level at a point on the Witham, half-a-mile on the Lincoln side of Washingborough Station, or 200 feet in two miles, which is equal to an inclination of rather more than two degrees in that direction. Strong springs rise in a small combe about 200 yards south of the Wragby Road, and a little more than a mile E.N.E. of the Cathedral, which are due to the faulting of ironstone beds against the Lias. The structure of the ironstone varies from very compact to cellular, the latter being called "fire-stone" by the well-sinkers, who imagine the rock has been exposed to fire. Northwards from the gorge of the Witham at Lincoln, these various beds, capped by the Lincolnshire Oolite, form a fine escarp ment, often locally known as the "Cliff," ranging nearly north and south, while the beds dip gently eastward. Small springs are thrown out at various points along the outcrop, as at the Burton Mill spring, issuing from six feet of ironstone. From the variable character, small outcrop, and the widespread presence of iron, I consider the Inferior Oolite Basement Beds to be of no value for water supply purposes. But the upper part of the Inferior Oolite or Lincolnshire Oolite is an important water-bearing stratum; it has a wide area of absorption, and is exceedingly porous, or, more correctly, it is much traversed by fissures, which afford a ready passage to rain water flowing over them from high levels, and readily discharging the same as springs at low levels. The Lincolnshire Limestone, south of Lincoln, is remarkable for having, from its water-bearing properties given rise to two lines of villages, the one at its base or the foot of the escarpment, where the water is supported by the underlying impermeable Lias clay, the other on the east, where the porous limestone dipping under impermeable clays, leaves no escape for the contained waters, the rock being water-logged up to the top of the overlying clay, the overflowing springs being the water absorbed over the whole porous area. 1. The Lincolnshire Limestones of this area generally consist of:- 2. Whitish or Buff Limestones, with Oolitic grains. 3. Hard Grey Limestones (hydraulic). Seams and partings of calcareous marls crowded with fossils occur at various horizons, and have the effect of dividing up the water contained into several distinct sheets, but none of these shale beds are absolutely continuous, so that the various sheets of water intermingle at certain points. The upper beds are well seen at Blankney and Metherington, between which issue very powerful springs. At Great Spring Head the line of permanent saturation occurs more than a mile west of the overhanging lip of impermeable rocks. The water issues from fissures in a pink limestone with fine sandy grains. These springs are derived from the rain falling over the area lying between Harmston and Coleby, Dunstan Pillar, and the Green Man. Its breaking out at a higher level than most of the other springs appears to be due to the obstruction to the passage of water down the dip of the strata, caused by the N.W. and S.E. fault ranging between Dunston and Nocton. At the Green Man, 154 feet above the mean sea-level, there is a well 25 feet deep with bore-hole, the water at time of visit (July, 1891) did not rise to bottom of well. A slimy pond by the roadside was used 40 years ago, and has a puddled bottom; no spring occurs. At Warren House there is a double bucket well; the water flows strongly to the south-east. The water is lifted by horse-power. A quarter of a mile to the north is a deep dug combe, pointing to a seasonal variation in the level of the plane of saturation. This is also apparent in the park approaching Blankley Hall, where a combe has evidently been recently occupied by a stream, now (July) reduced to a single clear, rather shallow, pond. Shallow wells, with ordinary pumps, supply the village of Blankney. Traversing the same district along the strike of the Lincolnshire Limestone, from Lincoln southwards along Ermine Street, at the cottage 4 miles south of the city, water stands at 35 feet from the surface, and yields a copious (cottage) supply. This is also the case at the cottage at the corner of bye-road just north of the monument ; the water level is just 30 feet from the surface and varies about five feet. The Pillar is 198 feet above the mean sea level, so that the water level is 161 feet above the same. At the first cottage south of the monument a single bucket-pump reaches water, and a very small stream was flowing down the combe between. With the exception of the above well the whole of the facts observed south of Lincoln, in the Lincolnshire Limestone, point to less local irregularities of water level, and a more uniform eastern trend than is observable north of the city, where the limestone is broken up by hard bands, probably of ironstone. The aggregate thickness of the limestone, as measured by the Geological Survey at Washingborough, amounts to 65 feet, in which occur the following impermeable beds :- Or about 13 feet in all, leaving 52 feet of porous material, of which half may be considered as permanently saturated with water, or 26 feet. Taking the width of outcrop between Coleby and Dunston at five miles, a pumping station placed beside the railway south of Dunston Station, draining the rock half-a-mile north and south respectively, might be expected to produce, in dry years, the daily quantity of 700,000 gallons of water. The best site for a boring to obtain the waters in the Lincolnshire Limestone north of the city is Scothern, and after that Dunholme. In either case the boring should commence in the Oxford Clay, so as to have the cover of an impermeable material. Such borings would be of an artesian character. This has been proved on the west side of the village of Dunholme by a boring 106 feet deep, the water rising five feet above the surface, in a pipe 14 inches in diameter, with a discharge of 20,000 gallons in 24 hours. This boring The occasional presence of "swallow-holes" in the Lincolnshire Limestone, into which the streams are precipitated, the cracks and fissures that seem to accompany the outburst of springs, all tend to show that this rock is capable of holding a very large quantity of water. The chief motion of underground water takes place along cracks and fissures in underground defined channels, rather than through the pores of the rock. Two very adverse conditions arise from this: First, that when water passes through fissures, and not through pores of the rock, the natural process of filtration is not exerted. Second, that in works of this class (as will be seen in the chalk) there is an unsatisfactory element of chance as to whether any particular boring may intercept a fissure along which water is travelling or capable of travelling. In the case of the Lincolnshire Oolites, as they are carried by the dip under overlying Oolite and Cretaceous rock under the German Ocean, there is no probability of the beds appearing at the margin under the sea, and consequently the water is passing under the overlying strata, though fully charged with water previously absorbed, and an artesian pressure which varies in amount according to whether the head of water rises after rain or sinks after drought, as is seen in Dunholme Boring, which increases in flow after rain, the rain in question having been absorbed several miles to the west. This ready response to rainfall is a most unsatisfactory element in the Lincolnshire Oolite, and is well evidenced by the great variability in the springs gauged by Mr. Teagne, C.E., at Welton, running inAugust, 1878 June, 1887 June 15, 1891, after heavy rainfall July 29, 1891 105,000 gallons 168,000 Sep. 16, 1891 The whole of the rainfall absorbed by the Lincolnshire Oolite is thrown out as springs, which act in direct response to the rainfall, and any wells or artesian boreholes to obtain a large supply will have to pump at a greater rate than the water can naturally flow out at the springs at the high level, and it is doubtful whether the pumps could do this, unless they should be fortunate enough to be placed in a line of natural fissure. Regarding the springs as interest and the water stored under the clay area as capital, it follows that no water can be permanently pumped beyond the capacity of the springs, and then only at their expense. |