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

In many places it is difficult to determine whether the mantle rock is till or stratified drift, and a decision is reached only after weigh- ing several factors. The presence of distinct stratification is in- dubitable evidence that the deposit is stratified drift, but in localities where there are no outcrops or where the outcrops do not show dis- tinct stratification the determination may be uncertain. Till areas are in general characterized by the presence of numerous large boulders, which in many places have been used for building stone fences. These boulders are subangTilar and faceted and may have concave surfaces and glacial scratches, which would distinguish them from the well-rounded boulders found in a few places in very coarse beds of stratified drift. Moreover, areas of stratified drift have characteristic topographic features, such as broad ]3lains and ter- races, with kettle holes, eskers, and kames. Because of its great porosity the upper part of the stratified drift in many places is dry much of the time and therefore favors certain types of vegetation wdiich either get along with little water or are able to send their roots down very deep. Under such conditions there are likely to be many white and yellow pines, cedars, and scrub oaks, with an under- growth of sweet fern and poverty grass. The till has no distinctive floral characteristics. OCCURRENCE AND CIRCULATION OF GROUND AVATER. Some of the water that falls as rain or melts from snow soaks into the ground. A surface layer of sand or gravel or a thick mat of leaf mold or of needles, as in woods, probably affords the most favorable conditions for high absorption. On steep slopes the rain runs off ^VATKK-H1■:AKI^■^< vo]l\iati(3NS. 25 rapidly nnd rolativoly littlo ontors the <xt<)iin<l. Whon the *>;r<)iiii(l is frozen it boc'omos almost iiii])OJ'vions, and ab^orjilion is at a minimum. Heavy rains concentratod iii a short tim(> -will in L'ciuMal rcsnlt in loss absorption than an equal amount of rain over a longer time. The amount of "water that may be absorl)ed is very great. With a rainfall of 48 inches a year, each acre Avould receive in the course of a year over 1,300,000 gallons. If one-fourth of this (luantity Mere to soak into the ground and be concentrated into a single spring, that spring would discharge an average of over 2 qnai-ts a minute throughout the year. Water nu)ves through the groiuid for the most part because of gravity. The water sinks through the pores of the soil until it reaches an impervious bed or the ground- water level, and then it moves laterally. Except in the stratified drift lateral movement over great distances does not occu.r in Connecticut, because the porous soils are cut into small discontinuous areas by the numerous ledges of bedrock. In large valleys occupied by stratified drift there is in general an underflow in the direction of the surface streams. Inasmuch as the porous soil cover over the bedrock is as a rule not ver}' thick, the direction of movement is for the most part the same as the slope of the surface of the ground. The rate at which water moves depends on the amount of Avatei', the steep- ness of the slopes, and the porosity and permeability of the water- bearing materials. Porosity is the ratio of the volume of the crevices between the grains to the total volume of the substance, and does not depend on the size of the pores. Permeability is the capacity of the material to transmit water and depends largeh^ on the size of the individual pores. Large crevices like those of gravels favor rapid circulation. Some fine clays have as high porosity as the gravels, Imt because of the interstitial friction in the minute pores they are virtually impermeable. At some depth the pores of the earth are saturated with water. The rains and melting snows supply water which would saturate the rock deposits throughout but for the lateral escape of the ground water. The upper surface of this saturated zone is known as the water table. In Connecticut the water table is in general near the surface of the ground in and after seasons of high precipitation in areas where the mantle rock is thin or discontinuous and Avhere the surface is relatively level. High, level terraces are an exception to this rule. The water table is likely to be particularly high in small deposits that fill depressions in the surface of the bedrock. Along the edges of streams, lakes, and swamps the water level is at the surface. It is relatively low in times of drought on steep slopes and in ]) laces 26 GROUIs^D WATER IN NOEWAL.K AND OTHER AREAS, CONN. FiGURB 4. — Diagram showing the usual relation of the water table to the land surface on hills and iu valleys. where the mantle rock is thick. The depth to the water table fluctuates with the seasons and may be increased by drainage of wet grounds, by heavy draft on wells, and by transpiration from vege- tation, as well as by changes in the rates of precipitation and evapor- ation. The improvements made by man on farms and the engineering works in cities artificially lower the water table. In Connecticut the greatest fluctuation is on steep hillside slopes from which the water drains rather readily. In such situations there is also rapid though often tempo- rary replenishment of the ground water after rains. There are in Connecticut no extensive water-bearing formations such as the Da- kota sandstone, which is used as a source of water supi^ly in much of the Great Plains. The ground waters in Connecticut are derived from rain or melting snow near by. In manj^ places water lies at the base of the mantle rock, where rapid downward movement is prevented by the relatively im- pervious bedrock. Many wells dug to solid rock and blasted a few feet into it take advantage of this supply. This water bed also feeds Avater into the fissures of the bedrocks. The till and stratified drift contrast greatly in texture and there- fore in their ability to hold up the water table. Because of its greater permeability the stratified drift not only absorbs water more readily than the till but also loses it more readih^ In most regions the water table is nearer the ground surface in valleys than on hills, as shown in figure 4. In much of Connecticut, however, where the vallej^s are filled with stratified drift and the hills are covered with till, the reverse con- dition exists. Because of the much slower rate at which the water percolates through the till the water table is held up nearer the surface on the till-covered hills than in the valleys of stratified drift, as is diagrammatically shown in figure 5, With respect to their capacity for yielding water there is also a very important difference between the two types of glacial drift. On account of its high porosity and permeability, the stratified drift in Figure 5. — Diagram showing the relation of the water table on till-covered hills to the water table in valleys of strati- fled drift in glaciated regions. WATKK-BKAUINU FOKMATIO^S. 27 many places (ontaiub large (iuanlities of water which it will yield I'reel.y to wells and which may be readily replenished when raijis come. The till, on the other hand, contains nmch less available water and gives it out at a much slower rate. (See p. 21.) The stratified drift is the more valuable for obtaining large supplies f loiu wells for municipal and mdustrial u^es, but the till U likewise of great value, as it is widely distributed and in general yields enough water for domestic use to inexpensive dug wells. The till is also the reservoir which feeds most of the small springs that make gravity ^ujjplies for many farms. TRIASSIC SEDIMENTAHY HOCKS. DISTRIBUTIOX. The mantle rock of the Suffield area is underlain by rocks of Triassic age. Most of these are sedimentary, but the ridge of Peak ^fountain, in East Granbj:' and Suffield, is in part underlain bv' trap rocks. Tlie northwest corner of the town of Glastonbury, in the Glastonbury area, comprising about 18 square miles, is underlain by Triassic sedimentary rocks. No rocks of this age occur in the Norwalk area. LITHOLOOy AND STRATIGRAPHY. The lowest of the Triassic beds lie unconformably on the up- turned edges of the crystalline rocks and may be seen in contact with these rocks at a few points along the western border of the area they underlie. The boundary against the crystalline rocks on the east is believed to be a major fault. According to Rice and Gregory,^ the Triassic sediments would naturally be characterized in a broad way as red sandstone. The sandstones, sometimes coarse, sometimes fine, consist mainly of jiTiiins of quart:'., feldspar, and mica resulting from the disintegration of the older rocks v»'hioli form the walls of the trough in which the sandstones were deposited. The prevailing red colors of the sandstone are not due to the constituent grains, but to the cementing material, which contains a large amount of ferric oxide. * * * While the name sandstone would properly express the prevalent aad typical character of the rock, the material is in some strata so coarse as to deserve the name of conglomerate and in others so fine as to deserve the name of shale. In the conglomerates the pebbles may be less than an inch in diameter, but tliey are sometimes much coarser. In some localities occurs a rock which has been called " giant conglomerate," in which some of the boulders are several feet in diameter. The conglomerates occur chiefly near the borders of the Triassic areas, and in these it is especially easy to recog- nize the rocks from the disintegration of which the pebbles have been deri-i.tnl. In general, it may be said that the pebl">les in any particular area are derived ^ Rice, W. N., and Gregory, H. E., Manual of the geology of Connecticut : Connecticut Gcol. and Nat. Hist. Survey Bull. 0, pp. 163-lB."., 1906. 28 geoujs^d watee in noewalk and othee aeeaS; conn.