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

from rocks in the immediate vicinity. Tlie conglomerates in tlie Connecticut Vrillej!- area are obviously derived from the gneisses, schists, and pegmatites, which are the prevalent rocks of the highlands. * * * The shales, like the sandstones and conglomerates, are prevailingly red, owing their color likewise to the presence of ferric oxide. Some strata of shale, however, con- tain in considerable quantity hydrocarbon compounds derived from the de- composition of organic matter. These bituminous shales are accordingly nearly black. In the Connecticut Valley area there are two thin strata of these bituminous shales, which have been shown, by careful search for out- crops, to have a very wide extent. The red sediments, however, are dominant. There is great variation in the material composing the beds and in their struc- ture, and the changes in the rock are very abrupt. The stratification is un- evi-n and irregular, and the beds are wedge-shaped or lenslike rather tiian uniformlj'- thick over wide extents. Although the beds wei-e originally horizontal and in continuous masses they have been tilted to the east 15° or 20° and have been broken into blocks. The f(.»rct'S which caused the faulting also opened many joints and fissures, along which there has been little or no movement. These joints are in general par- allel to the bedding or nearly at right angles to it, though joints are found with every conceivable inclination. The sandstones and conglomerates have more abundant and more extensive joints than the shales, for they are rigid and relatively brittle rather than plastic and tenacious. The joints ai"e rarely more than 50 feet apart and in general are found at intei'vals of 2 to 8 feet. The joints are more abundant and wider near the surface than they are in depth. OCCURRENCE OF OROUXD WATER. Ground Avater occurs in the Triassic sedimentary rocks in four ways — in pores throughout the rocks, along bedding planes, in joints, and along fault zones. Though originally derived from rain and snow the Avater has, for the most part, reached the Triassic beds by infiltration and percolation from the saturated mantle rock. Wafer in pores. — The sandstone, shale, and conglomerate consist of particles of quartz, feldspar, mica, and other less abundant min- erals and of pebbles of older rocks, all cemented together by fine clay and films of iron oxide. The spaces between the grains are not com- pletely filled with the cementing material but are partly open and may contain water. In the aggregate large quantities of water are held in this way, but on account of the smallness of the openings the water is not readily given off. Bare outcrops, as in quarries, are for the most part dry on the surface, though the interior of the rock may be moist. In the sandstones and conglomerates the water in the pores is slowly given off to joints, from which it may be recovered by means of drilled wells. The shales have pores so very fine that the}^ yield but little water. In some places the shales are so imper- vious as to act as restraining beds that concentrate the water in the pores of the coarser beds. Water in hedding flanes. — There is a tendency for the water in the pores to be concentrated in and transmitted along the lower WATKR-BKARING 1 OKMATIONS. 29 parts of tlic conrher beds Avlierc tliey rest on finer ami relatively impervious beds. It is probable that a few of the wells drilled in Triassic rocks draw their sui)plies from such horizons. \]'ot('r in jo/' /its. — Joints are the most important source of water in the bedrock of Connecticut. They are more abundant and are wider in tlie sandstojie and conglomerate than in the shale. These extensive flat crevices are good watei' bearers because they arc large and offer little capilhiry resistance to the (irculation of water, because they draw on and make available the supply of water stored in pores, and because they are of relatively great extent, Most of the drilled wells and a feAv dug wells in the Triassic areas draAv on the joints. Water in fault zones. — The faults that break the Triassic rocks of Comiecticut into great fault blocks are not single fractures but rather zones containing many parallel planes along which move- ment has taken place. Because of the great number of water-bearing joints in such zones Avells drilled along fault lines are likely to yield very large supj)lies of Avater. TSAP BOCKS. l.'ISTiUlU 'I'lox. Trap rocks underlie parts of Peak Mountain, in Suffield and East (iranby, and of Manitick Mountain, in Suffield. There is also a suiail dike in the eastern part of the town of Westport, in the Nor- walk area. Their extent is so small that they are not an important source of water. LITIIOLCK.Y AND (»CCl KRF.XCK OF GROrXD AVATKR. The trap rocks are den-e. heavy dark-gray to nearly black rocks and aie more or less completely ciystaliine on a small scale. Like the sedimentaiy rocks in which they are inclosed, the traps are cut by numerous joints, some of wliich Avere made by the initial cooling and shrinkiige of the rock and others by the jarring incidental to faulting and tilting. The joints are more abundant near the margins of the masse'-^. Trap rocks have a tAvofoid bearing on the occurrence of ground Avater. The joints may contain Avater and the sheets may act as im- pervious layers to restrain the circulation. Trap rocks haA^e a A^ery low porosity and carry vii-tually no Avater in pores, and they contain rio Avater corresponding to that along bedding planes of sedimentary rocks. Water circulates through the joints and fault zones in traps just as in sandstones, but in general less abundantly. EA'idence of such circulation is given by the yellow and broAvn stains of iron along 30 GROUND WATER IK" NORWALK AXD OTHER AREAS, CONIST. the joints, due to oxidation and hydration of the iron-bearing min- erals. The immediate source of the water in the trap rock is the water in the formations with which it is in contact; this water enters it through the network of interconnecting joints. Because of its hardness and resistance to erosion the trap forms bold hills with cliff's. This is a disadvantageous form so far as water storage is concerned, because of the facility with which water will drain oiit= CSYSTALLINE SOCKS. DISTEIBIJTIOK. Crystalline rocks, so named because their constituent mineral par- ticles consist of crystals rather than fragments, underlie all of the Norwalk area except about 9 square miles in Eidgefield, in which the l^eclrock is limestone, and all of the Glastonbury area except the sandstone area in the northwestern part of the town of Glastonbur}". The extent of these rocks is about the same as that of the eastern and western highlands, because the characteristic typographic features of the highlands depend in large part on the resistance of these rocks to erosion. LITHOLOGY. The areas under consideration contain three t5^pes of cr3^stalline rocks — schists, gneisses of igneous origin, and gneisses of complex origin. Schists.— TjipiGSil schists are metamorphosed sandstones and shales, which in turn are consolidated sands and muds. The mountain- making movements to which this region has been subjected squeezed and folded the sedimentary rocks. At the same time the great changes in temperature and pressure metamorphosed the rocks com- pletely; the quartz sand grains were crushed and strung out, and the cl&jQj material was changed to crystalline mica. The mica flakes were turned roughly parallel to one another and so give the rock a pronounced cleavage, called schistosity. Though other minerals are present the quartz and mica are dominant. In the Norv^ralk area the Berkshire schist is of this tj^pe, and in the Glastonbury area the Bolton schist. Gneisses of igneous origin. — In connection with the dynamic meta- morphism of the region great masses of molten rock were intruded into the sedimientary beds. They have been metamorphosed like the schists but to a lesser degree, and the changes are textural rather than mineralogic. The dark minerals of the igneous rock have been somewhat segregated and parallelly oriented, so that the rock WATKll-liKARlNU FORMATIONS. 31 hiis n fair cleavaire. The Thomaston ofranite gneiss^ ainl the Dan- biirv graiiodiorite gneists ^ of the Xoi'walk aveu and tlie (ilastoii- bury' and ISIaromas granite gneisses of the Ghist(>ni>iiry area aie of this type. f'/ieisses of campie-x' origin. — The intrusions of igneous mateiial were in part massive and gave rise to the gneisses of igneous origin, as desorilwd ai)ove. and they were in part in the form of multitudi- nous thin injections into the schists. Certain parts of the schist have been so extensively injected that their character is materially altered, and they have become gneisses of complex origin. The thin intru- sions for the most part follow the planes of schistose cleavage and somewhat obscure them, but others cut across them. The Waterbury gneiss^ of the Norwalk area and the Hebron gneiss^ of ti;o (ilastonbury area are of this type. OCCLRREXCE A>:f> CIRCULATION OF GROUND WATEi:. Water in lamellar spaces. — In the scliists and to some extent in tl>c gneisses of complex origin, but not in the granite gneisses, there is a little water in the spaces between the mica flakes where they are bent around quartz grains. Most of the opening-s are flat, thin, and not extensive, and they interconnect very imperfectly. In tlie most thoroughly crumj)Ied schists there are small tubular openings along the furrows and ridges. Tlie schistose structure aids in promoting rlie circulation of ground water chiefly because it gives rise to nu- merous joints. Waier in joints and along faults. — The forces that caused metamor- phisrn also made many fractures in the rocks. The fractuit?s are even more numerous in these rocks than in the sandstones, ]iut they bear water in the same way. Inasmuch as it is virtually impossible to trace faults in the crystalline recks they v\'ill be considered here only as compound or enlarged joints in which circulation is espe- cially vigorous.