A characterization of the data, including its intended use and limitations.
Abstract: This data layer includes the information portrayed on the Surficial Materials Map of Connecticut (Stone, J.R., Schafer, J.P., London, E.H. and Thompson, W.B., 1992, U.S. Geological Survey special map, 2 sheets, scale 1:125,000). The Surficial Materials Map of Connecticut portrays the glacial and postglacial deposits of Connecticut in terms of their aerial extent and subsurface textural relationships. Glacial Ice-Laid Deposits (thin till, thick till, end moraine deposits) and Postglacial Deposits (alluvium, swamp deposits, marsh deposits, beach deposits, talus, and artificial fill) are differentiated from Glacial Meltwater Deposits. The meltwater deposits are further characterized using four texturally-based map units (g = gravel, sg = sand and gravel, s = sand, and f = fines). In many places a single map unit (e.g. sand) is sufficient to describe the entire meltwater section. Where more complex stratigraphic relationships exist, "stacked" map units are used to characterize the subsurface (e.g. sg/s/f - sand and gravel overlying sand overlying fines). Where postglacial deposits overlie meltwater deposits, this relationship is also described (e.g. alluvium overlying sand). Map unit definitions (Surficial Materials Polygon Code definitions, found in the metadata) provide a short description of the inferred depositional environment for each of the glacial meltwater map units. This map was compiled at 1:24,000 scale, and published at 1:125,000 scale.
Connecticut Surficial Materials is a 1:24,000-scale, polygon and line feature-based layer describing the unconsolidated glacial and postglacial deposits of Connecticut in terms of their grain-size distribution (texture) as compiled at 1:24,000 scale for the Surficial Materials Map of Connecticut. Glacial meltwater deposits (stratified deposits) are particularly emphasized because these sediments are the major groundwater aquifers in the State and are also the major source of construction aggregate. These deposits are described in terms of their subsurface distribution of textures as well as their extent. The texture of meltwater deposits through their total vertical thickness in the subsurface is shown to the extent that it is known or can be inferred. In some places only one textural unit (such as SG - Sand and Gravel) describes the whole vertical extent of the meltwater deposits; in other places 'stacked units' (such as SG/S/F - Sand and Gravel overlying Sand overlying Fines) indicate changes of textural units in the subsurface. Polygon features represent individual textural (surficial material) units with attributes that describe textural unit type and size. Examples of polygon features that are postglacial deposits include floodplain alluvium, swamp deposits, salt-marsh and estuarine deposits, talus, coastal beach and dune deposits, and artificial fill. Examples of glacial ice-laid deposits include till, thin till, thick till and end moraine deposits. Examples of glacial melt-water deposits include gravel, sand and gravel, sand, and very fine sand, silt and clay. Additional polygon features are incorporated to define surface water areas for streams, lakes, ponds, bays, and estuaries greater than 5 acres in size. Line features describe the type of boundary between individual textural units such as a geologic contact line between two different textural units or a linear shoreline feature between a textural unit and an adjacent waterbody. Data is compiled at 1:24,000 scale and is not updated.
Supplemental information: GEOLOGIC DISCUSSION - The following text is excerpted from the text on sheet 1 of the Surficial Materials Map of Connecticut, Stone and others, 1992. It has been modified as necessary for use with the 1:24,000 scale digital data, and is not considered a valid substitute for the information found on the published map. For a more complete understanding of the geologic principles behind the Surficial Materials data it is advisable to consult the published map, which contains cross sections, diagrams and text not available in digital form.
DISCUSSION OF SURFICIAL MATERIALS - The unconsolidated deposits overlying bedrock in Connecticut range from a few feet to several hundred feet in thickness. These earth materials significantly affect human development of the land. Most of the unconsolidated materials are deposits of continental glaciers that covered all of New England at least twice during the Pleistocene ice age. These glacial deposits are divided into two broad categories, glacial till and glacial stratified deposits. Till, the most widespread glacial deposit, was laid down directly by glacier ice and is characterized by a nonsorted matrix of sand, silt, and clay with variable amounts of stones and large boulders. Glacial meltwater deposits are concentrated in both small and large valleys and were laid down by glacial meltwater in streams and lakes in front of the retreating ice margin during deglaciation. These deposits are characterized by layers of well-sorted to poorly sorted gravel, sand, silt, and clay. Postglacial sediments, primarily floodplain alluvium and swamp deposits, make up a lesser proportion of the unconsolidated materials of Connecticut. Alluvium is largely reworked from glacial materials and has similar physical characteristics.
The distribution of surficial (unconsolidated) materials that lie between the land surface (below the pedogenic soil) and the bedrock surface is shown on this map to the extent that it is known or can be inferred. The cross sections and the block diagram shown on the published map (Stone and others, 1992) illustrate the characteristic vertical distribution of glacial till, glacial meltwater deposits, and postglacial deposits encountered in Connecticut. The areal distribution of till and stratified deposits is related to the physiographic regions of the State: the eastern and western highlands and the central lowland. In highland areas, till is the major unconsolidated material, present as a discontinuous mantle of variable thickness over the bedrock surface. Till is thickest in drumlins and on the northwest slopes of hills. Glacial meltwater deposits that average 10-40 feet in thickness overlie the till in small upland valleys and commonly in north-sloping pockets between bedrock hills. In the central lowland, especially in the north half, glacial stratified deposits are the predominant surficial materials. These deposits generally overlie till; however, well logs indicate that in some places till is not present and the stratified deposits lie directly on bedrock. The extensive stratified deposits of the central lowland average 50-100 feet in thickness, and in the northern part they almost completely mask the till-draped bedrock surface. Postglacial materials locally overlie the glacial deposits throughout the State. Alluvium occurs on the floodplains of most streams and rivers. Swamp deposits occur in poorly drained areas. Talus occurs along the bases of steep bedrock cliffs, principally along the traprock ridges within the central lowland. Salt-marsh and estuarine deposits occur mainly along the tidal portions of streams and rivers entering Long Island Sound. Beach deposits occur along the shoreline of Long Island Sound.
The units on this map delineate textural changes in the subsurface as well as areally at the surface. An earlier map at 1:125,000 scale of central Connecticut (Stone and others, 1979) shows only surface textural units; a separate map in the same series (Langer, 1979) shows subsurface deposits of fine-grained materials. Several previous 1:24,000-scale quadrangle maps in Connecticut show three-dimensional textural units and refer to them as 'superposed deposits' (see Stone, 1976 and Radway and Schnabel, 1976, as examples). On this map, the term 'stack unit' (Kempton, 1981) is used in place of superposed deposits.
DISTRIBUTION OF TEXTURES IN GLACIAL MELTWATER DEPOSITS - The distribution of textural units is extrapolated from both point data (well and test-hole logs, gravel pits, and shovel holes) and from interpretation of landforms based on the principles of morphosequence deposition and systematic northward ice retreat (Koteff, 1974; Koteff and Pessl, 1981). These concepts provide a model by which grain-size distribution can be predicted from the morphology of a deposit, given primary data about the textures at specific points. A morphosequence is a package of sediments deposited contemporaneously by meltwater flowing from the glacier margin to a specific base level. Within a morphosequence, grain size decreases and sorting improves from the ice-marginal (proximal) end of a deposit downstream to the distal end. Landforms are transitional within a sequence as well, ranging from ice-contact forms (eskers, kettles, kames) at the head (proximal end) of a deposit to uncollapsed forms (delta-foreset slopes, lake-bottom plains, valley trains) downstream (distal end). Coarser grained sediments are associated with the proximal parts of morphosequences, finer grained sediments are associated with distal parts; given this principle, textural distribution can be mapped using point data that serve as controls.
The relationship between textural variations and morphosequences is illustrated by a cross section on the published map, which shows the distribution of texture units in the northern Quinnipiac River valley. This north-south section transects seven chronologically numbered morphosequences. Dashed lines drawn to the six southern sequences represent the probable generalized surface gradients of the heads of these deposits, prior to collapse (due to melting of buried ice) and subsequent stream entrenchment. From north to south within each of these sequences, the textures grade from coarse- to fine-grained sediments and the topography changes from collapsed to non-collapsed landforms. The longitudinal and vertical relationships illustrated by this section are common in other valleys as well.
Stack units similar to those on the section described above occur throughout the stratified deposits of Connecticut. Many deposits having similar superposition of materials of differing texture were produced by geologic processes that occurred repeatedly in time and space during the deglaciation of Connecticut. For example, the SG/S/F and S/F stack units commonly occur in glacial-lake deltas. The SG/F stack unit commonly results from fluvial meltwater (or postglacial stream) terrace deposition on slightly older lake bottom deposits. The F/SG and S/SG units commonly occur in the distal parts of morphosequences where the sand or fines overlap the collapsed, coarser, proximal parts of other (older) sequences. Many basic texture units (G, SG, S, F) likewise have broadly common origins. Units of gravel or sand and gravel often occur in the proximal parts of deposits, or were commonly laid down in glaciofluvial environments. Units of sand and fine-grained sediment are typically associated with distal parts of sequences and were usually laid down in lacustrine environments.
THICKNESS OF MATERIALS - The thickness of surficial materials in Connecticut varies considerably because of such factors as the high relief of the bedrock surface, changing conditions of deposition during deglaciation, and various effects of postglacial erosion and removal of glacial sediments. For more information on the thickness of deposits and the point data used to determine stacked units, it is beneficial to review the complete and figures on the published map.
DESCRIPTION OF MAP UNITS
GLACIAL ICE-LAID DEPOSITS - Glacial ice-laid deposits (tills) consist of nonsorted, generally nonstratified mixtures of grain-sizes ranging from clay to large boulders. The matrix of most tills is composed dominantly of sand and silt. Boulders within and on the surface of tills range from sparse to abundant. Some tills contain lenses of sorted sand and gravel and, less commonly, masses of laminated fine-grained sediments. The color and lithology of till vary across Connecticut, but generally reflect the composition of the local underlying and northerly adjacent bedrock from which the till was derived. Till blankets the bedrock surface in variable thickness, ranging from 0 to about 200 ft, and commonly underlies stratified meltwater deposits. Tills deposited during two separate glaciations occur in superposition within Connecticut (Pessl and Schafer, 1968). The upper till was deposited during the last (late Wisconsinan) glaciation; it is the most extensive till and is commonly observed in surface exposures, especially in areas where till thickness is less than 15 ft; it is described in the thin till unit description below. The lower till or 'old' till was deposited during an earlier glaciation (probably Illinoian). The lower till has a more patchy distribution; it is principally a subsurface deposit, generally overlain by upper till, and therefore not shown as a separate map unit; the lower till does however constitute the bulk of material in the areas where till thickness is greater than 15 ft; it is described in the thick till unit description below. In all two-till exposures, the base of the upper till truncates the weathered surface of the old till. The lower part of the upper till commonly displays a zone of shearing, dislocation, and brecciation in which clasts of lower till are mixed and incorporated into the upper till. End moraine deposits occur principally in southeastern Connecticut. These deposits were laid down by ablation processes along active ice margins during retreat of the last (late-Wisconsinan) ice sheet.
Glacial Ice-Laid deposits include Thin till (T), Thick till (TT), and End moraine deposits (TS).
GLACIAL MELTWATER DEPOSITS - Glacial meltwater deposits (stratified deposits) consist of layers of well-sorted to poorly sorted gravel, sand, silt, and clay laid down by flowing meltwater in glacial lakes and streams which occupied the valleys and lowlands of Connecticut during retreat of the last ice sheet. Textural variations within the meltwater deposits occur both areally and vertically because meltwater-flow regimes were different in glaciofluvial (stream), glaciodeltaic (where a stream entered a lake), and glaciolacustrine (lake bottom) depositional environments. Grain-size variations also resulted from meltwater deposition in positions either proximal to or distal from the retreating glacier margin, which was the principal sediment source. A common depositional scenario contained a proximal, ice-marginal meltwater stream in which horizontally bedded glaciofluvial gravel and/or sand and gravel were laid down; farther down valley, the stream entered a glacial lake where glaciodeltaic sediments were deposited consisting of horizontally layered sand and gravel delta-topset beds overlying inclined layers of sand in delta-foreset beds. Farther out in the glacial lake, glaciolacustrine very fine sand, silt, and clay settled out on the lake bottom in flat-lying, thinly bedded layers. Mappable textural variations are present in the vertical section of meltwater deposits in many places. This stacking of textural units commonly resulted from locally changing conditions of meltwater deposition. For example, glacial lakes drained upon ice retreat from particular positions. This may have been followed locally by distal glaciofluvial (stream) deposition from ice positions farther up valley. The resulting vertical section shows meltwater terrace sediments consisting of horizontally bedded fluvial sand and gravel which overlie lake-bottom sediments of very fine sand, silt and clay (shown as unit SG/F on the map). In other places glaciodeltaic deposition over an extended period of time in a particular glacial lake caused deltaic deposits (sand and gravel topset beds over sand foreset beds, unit SG/S) to prograde farther out into the lake and to overlie lake-bottom sediments; such deposits are shown as stack unit SG/S/F on the map.
Meltwater deposits are shown on this map as four basic units: gravel, sand and gravel, sand, and fines. Grain-size terminology used to define the textural range within these units is modified from Wentworth, 1922. Stack units are also shown; these are combinations of the four basic units in various orders of superposition. The map units described below show the texture of meltwater deposits through the total vertical section to the extent that it is known or can be reasonably inferred. In some places only one textural unit (such as unit SG) describes the entire vertical thickness of the meltwater deposits. In other places stack units (such as units SG/S/F or S/F) indicate changes of textural units in the subsurface. Common depositional environments for each textural unit are given in parentheses after each unit description.
Glacial meltwater deposits are represented by the following categories. Fine deposits include Fines (F). Coarse deposits include Gravel (G), Sand and Gravel (SG), and Sand (S). Stacked Coarse deposits include Gravel overlying Sand and Gravel (G/SG), Gravel overlying Sand (G/S), Sand and Gravel overlying Sand (SG/S), Sand and Gravel overlying Sand overlying Sand and Gravel (SG/S/SG), Sand overlying Gravel (S/G), and Sand overlying Sand and Gravel (S/SG). Stacked Coarse Deposits Overlying Fine deposits include Gravel overlying Sand overlying Fines (G/S/F), Gravel overlying Fines (G/F), Sand and Gravel overlying Sand overlying Fines (SG/S/F), Sand and Gravel overlying Fines (SG/F), and Sand overlying Fines (S/F).
POSTGLACIAL DEPOSITS include Floodplain alluvium (A), Alluvium overlying undifferentiated coarse deposits (A/SG), Alluvium overlying Sand (A/S), Alluvium overlying Fines (A/F), Alluvium overlying undifferentiated coarse deposits overlying Fine deposits (A/SG/F), Alluvium overlying Sand overlying Fines (A/S/F), Alluvium overlying undifferentiated Fine deposits overlying coarse deposits (A/F/SG), Alluvium overlying Fines overlying Sand (A/F/S), Swamp deposits (SW), Swamp deposits overlying undifferentiated coarse deposits (SW/SG), Swamp deposits overlying Sand (SW/S), Swamp deposits overlying Fines (SW/F), Swamp deposits overlying Sand overlying undifferentiated coarse deposits (SW/S/SG), Swamp deposits overlying sand overlying Fines (SW/S/F), Swamp deposits overlying Fines overlying Sand (SW/F/S), Salt-marsh and tidal-marsh deposits (SM), Salt-marsh and tidal-marsh deposits overlying Sand (SM/S), Salt-marsh and tidal-marsh deposits overlying Fines (SM/F), Salt-marsh and tidal-marsh deposits overlying Sand overlying Fines (SM/S/F), Talus (TA), Beach deposits (B), and Artificial Fill (AF).
REFERENCES
Deane, R.E., 1967, The surficial geology of the Hartford South quadrangle, with map: Connecticut Geological and Natural History Survey Quadrangle Report 20, 43 p.
Haeni, F.P., and Anderson, H.R., 1980, Hydrogeologic data for south-central Connecticut: Connecticut Water Resources Bulletin 32,43 p.
Kempton, J.P., 1981, Three-dimensional geologic mapping for environmental studies in Illinois: Illinois Geological Survey Environmental Geology Note 100, 43 p.
Koteff, Carl, 1974, The morphologic sequence concept and deglaciation of southern New England, in Coates, D.R., ed., Glacial geomorphology: Binghamton, N.Y., State University of New York, Publications in Geomorphology, p. 121-144.
Koteff, Carl, and Pessl, Fred, Jr., 1981, Systematic ice retreat in New England: U.S. Geological Survey Professional Paper 1179, 20 p.
Langer, W.H., 1979, Map showing distribution and thickness of the principal fine-grained deposits, Connecticut Valley urban area, central New England: U.S. Geological Survey Miscellaneous Investigations Series Map I-1074-C, scale 1:125,000.
Mazzaferro, D.L., 1973, Hydrogeologic data for the Quinnipiac River basin, Connecticut: Connecticut Water Resources Bulletin 26, 54 p.
Pessl, Fred, Jr., and Schafer, J.P., 1968, Two-till problem in Naugatuck-Torrington area, western Connecticut, in Orville, P.M., ed., New England Intercollegiate Geological Conference 60th Annual Meeting, New Haven, Conn., Oct. 25-27, 1968, Guidebook for fieldtrips in Connecticut: Connecticut Geological and Natural History Survey
Guidebook 2, Trip B-1, 25 p.
Radway, J.A., and Schnabel, R.W., 1976, Map showing unconsolidated materials, Avon quadrangle, Connecticut: U.S. Geological Survey Miscellaneous Field Studies Map MF-514-C, scale 1:24,000. Ryder, R.B., and Weiss, L.A., 1971, Hydrogeologic data for the Upper Connecticut River basin, Connecticut: Connecticut Water Resources Bulletin 26, 54 p.
Stone, J.R., 1976, Map showing unconsolidated materials, Windsor Locks quadrangle, Connecticut: U.S. Geological Survey Miscellaneous Field Studies Map MF-450-E, scale 1:24,000.
Stone, J.R., London, E.H., and Langer, W.H., 1979, Map showing textures of unconsolidated materials, Connecticut Valley urban area, central New England: U.S. Geological Survey Miscellaneous Investigations Series Map I-1074-B, scale 1:125,000. Wentworth, C.K. 1922, A scale of grade and class terms for clastic sediments: Journal of Geology, v. 30, p. 377-392.