Ground Water in the Norwalk, Suffield, and Glastonbury Areas, Connecticut

mile wide along the south part of the Connecticut River boundary and also the area northwest of a line from South Glastonbury past the foot of Eightmile Hill and Minnechaug Mountain. It is under- lain by relatively soft sedimentary rocks that have been eroded more completely than the more resistant schists and gneisses of the high- land. The highest point in the lowland is 420 feet above sea level, whereas the i3eaks of Birch Mountain are about 920 feet above sea level. The difference of general level between the lowland and high- land was produced by preglacial erosion, as were also their major topographic features, but the minor details are in large part due to glacial and postglacial processes. Along Connecticut River and m the valleys of Salmon and Roaring brooks there are areas under- lain by stratified drift deposited by streams that issued from the glacier during its recession. Parts of these areas retain their original flatness, but the higher parts have been extensively eroded. Most of Glastonbury is drained hj streams tributary to Con- necticut River, the largest of which are Salmon Brook, which has its mouth at Naubuc, and Roaring Brook, which empties at South Glastonbury. About 6 square miles in a strip along the Marl- boro and Hebron town lines is drained by headwaters of Blockledge River and Dickinson Creek, which flow through Marlboro and join Salmon River, which is tributary to the Connecticut at East Haddam. WATER-BEARING FORMATIONS. Five types of bedrock have been recognized in Glastonbury — Tri- assic red sandstone, Glastonbury granite gneiss, Hebron gneiss, Maromas granite gneiss, and Bolton schist.^ Schist and gneiss. — The Bolton schist is for the most part a typical silvery-gray mica schist composed essentially of quartz, feldspar, and mica (chiefly white), with minor amounts of accessory minerals, such as biotite (black mica), garnet, staurolite, and in places magnetite, graphite, pyroxene, and chlorite. Originally this formation was a series of clays, silts, and sands that were consolidated into shales and sandstones, v^^hich in tiy:'n have been subjected to great pressure and heating and have been metamorphosed. In places in Glastonbury the rock is very dark gray, as it contains a good deal of graphite de- rived from organic materials in the original sediments. The Bolton schist underlies a northward-striking belt a quarter of a mile wide that runs through the eastern part of the village of South Glaston- bury, and a belt half a mile wide nekr the east boundary, including Birch Mountain, from which it extends south-southwest. This for- mation is very resistant and everywhere makes ridges that stand up above the neighboring formations. ^ Gregorj', H. E., and Robinson, H. H., Preliminary geolo^cal map of Connecticut : Connecticut Geol. and Nat. Hist. Survey Bull. 7, 1907. IS. S. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 470 PLATE XH MASSlVl!: GRANITE GNEISS, EAST GLASTONBURY, CONN. GLASTONBUllY. 155 The Glastonbury granite gneiss underlies an area of about 26 square miles, including Minnecbaug Mountain, Kongskut Mountain, Eightmile Hill, and Meshomasic Mountain. It is quarried for building stone at a number of places. According to Gregory,^ it may be cUvirted into two parts — ;i bvoud western portion, decidedly gnelssoid aud usually dark colored, with a large quantity of biotite and hornblende ; a narrower eastern portion, more granitic, and in places reaching the raassive- ness of a true granite. * * ^^^ As seen in the abundant expovsures west of the Portland Reservoir, it is a dark, well-foliated, almost schistose gneiss of fine grain, which on the cleavage surface shows alternating patches of black biotite and white feldspar. * * * The presence of biotite aud hornblende, arranged in parallelism with aggregates of feldspar, gives a distinct foliation and banding to the rock. * * * This more schistose variety forms the hills southeast of Glastonbury and occurs in the bed of Roaring Brook in South Glastonbury. * * * The more nia??sive variety of this gnaeiss is seen in tlie small quarries north of East Glastonbury. The rock here is a light- colored fine-grained biotite gneiss or granite, which soruetimesi is quarried in blocks 2 or 3 feet in thickuess, with no sign of a parting. * * * In regard to the origin of the Glastonbury gneiss there is strong indication that it is in large part igneous ; and this applies both to the more massive eastern portion and the more gneissic variety on the west. Plate XII rs a view in one of the small quarries in the massive phase of the Glastonbury granite gneiss and shows the massive character and the jointing of the rock. The Maromas granite gneiss underlies a strip a quarter of a mile wide w^st of the Bolton schist area of South Glastonbury and also a strip half a mile to a mile wide east of the eastern Bolton schist area and along the Marlboro and Hebron town lines. It is for the most part a typical granite gneiss, composed essentially of quartz,, feldspar, and mica with subordinate amounts of other minerals. Much of it has been crushed and mashed, which has given it a gneissic texture. Other phases are more basic and darker colored because of the pres- ence of hornblende. The Hebron gneiss underlies a very small area in Glastonbury along the boundary where it touches the towns of Marlboro and He- bron. According to Gregory,^ it " varies from a granitic gneiss to a highly fissile schist," and '' where typically developed is a fine- grained gneiss, with a relatively small amount of feldspar." As regards their water-bearing capacity these granite gneisses and the schist are essentially alike, but no supplies from them have been obtained in Glastonbury. The dynamic stresses that metamorphosed theni also made many fractures, which form systems of parallel, flat openings. In general there is a set roughly horizontal and one or 1 Eice, W. N., and Gregory, H. E., Manual at the geology of Connecticut : Connecticut Geol. and Nat. Hist. Survey Bull. 6, pp. 116-119, 1906. 2 Idem, p. 141. 156 GEOUND WATER IF FORWALK AND OTHER AREAS, COISTIT. more about at right angles to it. Rain water which has soaked into the overlying unconsolidated soil in part finds its v^ay into the inter- connecting fissures. Elsewhere in the State wells drilled into similar rocks obtain dependable and reasonably abundant supplies of water from the fissures. Drilling into the schist or gneiss at an^^ point in Glastonbury would probably yield equally satisfactory results. Small amounts of water are contained in the minute lamellar openings be- tween the mica flakes of the schist and the schistose phases of the gneisses, but they are negligible. Red sandstone. — The lowland portion of Glastonbury is underlain by red sandstone with which are associated some red shale and con- glomerate. During the Triassic period a great valley, bounded on the east by a great fault, existed in central Connecticut. Into this valley were washed sands, clays, and gravels that were consolidated to form sandston&s, shales, and conglomerates. The beds were origi- nally flat-lying but continued faulting along the eastern border tilted them to the east. At the same time jarring and crushing opened many joints and made new fractures in the rocks. Water is carried in them in the same way as in the fissures in the gneiss and schist. Two drilled wells which presumably obtain their water in this way were visited in Glastonbury. There may be porous zones in the sandstone and conglomerate which carry considerable amounts of water, but none have been recognized. Till. — ^The bedrock of the higher parts of Glastonbury, exj^ept the areas of actual rock outcrop, is mantled with till, the product of direct glacial deposition. It forms a layer ranging in thickness from a few feet to 50 feet, and comprises all the debris plucked and ground beneath the ice sheet. In a matrix of fine rock flour, clay, silt, and sand larger fragments are embedded. Because of the intimate mix- ture of particles of irregular shapes and the packing of the smaller into the interstices between the larger by the great weight of the ice, the till forms a dense tough mass and has moderate porosity and permeability. Water which has fallen as rain is in part absorbed by the till and slowly transmitted by gravity. Below a certain depth, which differs from place to place and from time to time, the minute pores of the till are saturated with water, which may be recovered by means of dug wells. In general it is advisable to dig the well to bedrock, as the zone immediately above the bedrock is the most thor- oughly saturated and therefore yields the best supplies. In some places this is impossible, but wells which extend below the lowest level reached by the top of the saturated zone during droughts are reasonably certain to procure constant supplies of water sufficient for ordinary domestic and farm needs. In iVugust, 1916, measure- ments were made of 42 wells dug in till in Glastonbury. Of these wells 11 were said never to fail and 15 were said to fail, but the re- GLASTONBUKY. 157 liability of the other IC was not ascertained. The data concerning the depths of the 42 wells are summarized in the followinjji: table: ,Si(»n)uir!/ u( irclls diKj in till in (il<isto)ihni-y. Total depth. Depth to water. Depth of water in well. Maximum Feet. 42.4 10.4 21.0 Feet. 32.9 5.3 16.9 Fat. 7.5 Minimum 1.8 Average 4.1