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

Toward the end of tlie Paleozoic era there were several great mountain-building disturbances, characterized by compression of the earth's crust in an east-west direction and the intrusion of vast quantities of igneous rock. To the mashing and intrusion is due the change of the old shales and sandstones to the schist and gneisses of the Xoi'walk area and the highland portion of the Glastonbury area. The change of tlie Cambrian and Ordovician limestone to a coarse marble (Stockbridge dolomite) was brought about by the same process. The igneous rocks, in large part, were also crushed and converted to gneisses. During Triassic time the mountains were deeply eroded and much of the debris was deposited in a troughlike valley in central Connecti- cut, making the sedimentary rocks of tlie Suffield area and the low- land portion of the Glastonbury area. These rocks are for the most j)art red sandstones, shales, and conglomerates, but they include some dark bituminous shales and green and gray limy shales. In some places in the red rocks there are footprints of reptiles, both large and small, and a few of their bones have also been found. The footprints and bones have been identified as belonging to Triassic reptiles. Re- Tuains of fishes are found in places in the bituminous shales and fur- ther prove the age of these beds to be Triassic. The deposition of the Triassic sediments was interrupted three times by the gentle eruption of basaltic lava, which spread out across the wide valley floor and which now^ forms the trap ridges between the Farmington and Connecticut valleys, in part in the Suffield area. Into the buried sediments were also intruded other masses of basaltic lava that formed sills and dikes, such as the sill of Manitick Moun- tain, in the western part of Suffield. Subsequently (in Jurassic tiriieO the flat-lying sedimentary rocks and the intercalated trap sheets were broken into blocks by a series of faults that in general 154444^—20 2 18 GEOUiSTD WATER IjST NOEWALK AI^D OTHEE. AEEAS^ CONIsr. cut diagonally across the lowland in a northeasterly' direction. Each block was rotated so that its southeast margin v,'as depressed and its northwest margin elevated. There is no sedimentary record of the interval between the Triassic period and the glacial epoch, but the erosion that took place then has left its mark. During- the Cretaceous period the great block moun- tains formed b_y the faulting T\"ere almost completely worn away. It is believed hj Davis ^ and others that during part of Cretaceous time the sea advanced over Connecticut as far as Hartford, and that the submerged area was covered with marine sediments. No such marine beds have been found, however, and the only e^'idence of such an in- cursion of the sea is indirect. Most of the streams in this region flow southward, but parts of the larger ones have sontheasterly courses. This condition could be explained, by assnmdng that when the postulated Cretaceons beds were raised they were tilted a little to the southeast and the streams across them took southeastward conrses. The more vigorous streams according to this hypothesis were able to cut their southeasterty channels into the diseordsnt rock surface ]3elaw the Cretaeeons deposits, whereas. the smaller streams were turned back to the old channels which existed before the Creta- ceous sedimentation ocemrre^ and which, it is assumed, ra;n south- ward. It Avas noted by Pereival ^ that the highlands may be regarded as " extensive plateaus " which " present, when viewed from an elevated point of their surface, the appearance of a general level, with, rolling or undulating outline, over which the view often extends to a very great distainee, interrupted onfy hj isolated s^maanits- of riclge&, usually of small extent." Rice ^ has described the phenomeBon as follows: If we should imagine a sheet of pasteboard resting upon the summits of the highest eievations of Litchfield Comnty and slopiaig soiitheastward.: in an in- elined plaiae^, that irioaginary slieet of pasteboard, would rest on n.earlj'- all the STimmits oi' both the eastern and western highlands. Barrell * has shown, however, that th.e hilltops approximate not an inclined plane but a stairlike succession of nearly horizontal planes, each a few hundred feet lower than the next one to the north. Traces of these terraces are fotmd in m^any parts of both the eastern and western highlands of Connecticut, but ai'e not discernible in the lowlands. Figure 1& (p. 119) is a compasite projection of north- south profiles of hilltops and ridge crests of a part of the Norwalk 1 Davis, W. M.,, The Triassic fo-rmation of Connecticut : T^. S. G«ol. Survey Eigliteenth Ann. Kept., pt. 2. p. 105, 1898. - Pei'cival, J. C4., Eepo-rt o® the geology o£ Connecticut, p. 477, 1842. "Rice, W. X., and Gregory, H. E., Manual of the geology of Connecticut: Connecticut GeoT. and Nat. Hist. Survey Bull. 6, p. 20, 1906. ■^ Barrell, Josepla,. Piedmont terraces of the- northern Appalachi-ins- and their origin : Geol. Soc. America Bull., vol. 24, pp. 688-601, 191.3. tiKOLOi.lC IIJSTOKV. 19 ;irea ;iml shows three more or less \vell-(k>\eh>|)(Ml phiij*js determined by the concordant elevations. The rocks of this plateau arc the roots of mountains tliut stood tJu-ri' in late Paleozoic ajid early jMcsozoic time and that were eventually Moru away. Erosion prochiccd a more or less smooth inclined i)lajie or s-eries of level planes which when uplii'tcd con- stituted the plateau surface. Since the uplift erosion has deeply trenched the plateau u)itil only a small part of its original surface is preserved. During- the Pleistocene or glacial epoch the continental ice sheet tliat overrode most of the northern United States covered the whole of New England. It was of great thickness, and as it moved slowly southward it remodeled the topography !>y sci'aping away the de- cayed rock accumulated at the surface, by breaking oft and grinding down projecting lodges of rock, and by redepositing the debris. The major featuies of the topography v.erc left unchanged, but tlie de- tails were greatly altered. In general the relief was decreased. The soil mantle was replaced by glacial drift of two tyjjes — till and stratified drift. The till, v.hich A\as deposited directly by the ice, is of moderate thickness, and its surface is similar to the surface of tlie underlying bedrock but somewliat smoother. The stratified drift, wliich Avas deposited by the streams that flowed out from the glacial ice. tilled tlio larger valleys to a considerable extent, making broad ])lains. During the recent epoch there has been no considerable change in ii)e topography. Small amounts of alluvium have been deposited in valleys, some swamps have been filled, and some lakes have been changed to swamps by being fdled witli sediment. Tliere has been slight erosion over the whole region, but the changes are in genera! imperceptible save for the terracing of stratified-drift deposits in the larger valleys. WATER-BEARING FORMATIONS. The water-bearing formations of Connecticut may be divided into two classes — bedrock and glacial drift. The bedrocks are the under- lying consolidated, firm rocks, such as schist, granite, trap, and sand- stone, and they are exposed at the surface only as small, scattereci outcrops. The glacial drift comprises the unconsolidated, loose ma- terials such as sand, clay, and till that occur at the surface in most of the State and overlie the bedrocks. These materials are by far the more important source of ground water and are of two chief varieties — till, also known as "hardpan" or "boulder clay," and stratified drift, also known as '* modified drift " or " glacial outwash.'' On the geologic maps (Pis. III. V, and VII) are shown the areas occupied by the two principal tyi)es of ghn ial drift as well as the 20 GEOUIs^D WATER IN NORWALK AND OTHER AREAS, CONN. outcrops of bedrock. The Triassic sandstone (including" also shale and conglomerate), the trap rocks, the limestone, and the crystalline rocks are differentiated by color on the maps, but no attempt was made to separate the varieties of the crystalline rocks. The rock out- crops are indicated as small patches, which have roughly the shape of the actual outcrops but most of which are disproportionately large because of the small scale of the map. Inasmuch as in the field work it was necessary to follow the roads, many outcrops in the spaces between roads may have been unmapped. GLACIAL DRIFT. TILL. Till, which is an ice-laid deposit, forms a mantle over tlie bedrock of much of Connecticut. Its thickness is in general from 10 to 4:0 feet but in places reaches 80 feet. The average thickness of the till penetrated by 56 drilled wells in the areas under discussion is 24 feet. The till is composed of a matrix of the pulverized and granulated fragments of the rocks over which the ice sheet passed and of larger pieces of the same rocks embedded in the matrix. The principal minerals are quartz, clay, feldspar, and mica, but small amounts of their decomposition products and of other minerals are also found. There has been but little chemical disintegration and decomposition of the till, and it has in general a blue-gray color. Near the surface, however, where the iron-bearing constituents of the matrix have been weathered, j;he color is yellow or brown. ^Vliere the material is in large part derived from the red Triassic rocks the till has a reel- brown to red color. The boulders of the till are characterized by their peculiar subangular shapes with polished and striated facets. Many of the boulders have facets that are in part concave where spalls have been flaked off as the boulders were pressed together in the ice. The boulders are very abundant and are scattered over the fields and in cut banks. As a rule, a number of different varieties of rocks are represented in any one place.