Surficial Geology Baker Island Quadrangle, Maine
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Surficial Geology SURFICIAL GEOLOGY OF MAINE Surficial geologic mapping of the Baker Island quadrangle was conducted by Duane D. Braun during the 2014 field season. SOURCES OF MAP INFORMATION ___A surficial geology map shows all the loose materials such as till (commonly called hardpan), sand and gravel, or clay, which overlie solid ledge (bedrock). Bedrock outcrops and areas of abundant bedrock outcrops are shown on the map, but varieties of the bedrock are not distinguished (refer to bedrock geology map). Most of the surficial materials are deposits formed by glacial and deglacial processes during the last stage of continental glaciation, which began about 25,000 years ago. The remainder of the surficial deposits are the products of postglacial geologic processes, such as river floodplains, or are attributed to human activity, such as fill or other land-modifying features. ___The map shows the areal distribution of the different types of glacial features, deposits, and landforms as described in the map explanation. Features such as striations and moraines can be used to reconstruct the movement and position of the glacier and its margin, especially as the ice sheet melted. Other ancient features include shorelines and deposits of glacial lakes or the glacial sea, now long gone from the state. This glacial geologic history of the quadrangle is useful to the larger understanding of past earth climate, and how our region of the world underwent recent geologically significant climatic and environmental changes. We may then be able to use this knowledge in anticipation of future similar changes for long-term planning efforts, such as coastal development or waste disposal. ___Surficial geology maps are often best used in conjunction with related maps such as surficial materials maps or significant sand and gravel aquifer maps for anyone wanting to know what lies beneath the land surface. For example, these maps may aid in the search for water supplies, or economically important deposits such as sand and gravel for aggregate or clay for bricks or pottery. Environmental issues such as the location of a suitable landfill site or the possible spread of contaminants are directly related to surficial geology. Construction projects such as locating new roads, excavating foundations, or siting new homes may be better planned with a good knowledge of the surficial geology of the site. USES OF SURFICIAL GEOLOGY MAPS Braun, Duane D., 2015, Surficial materials of the northern portion of the Baker Island quadrangle, Maine: Maine Geological Survey, Open-File Map 15-22, map, scale 1:24,000. Thompson, Woodrow B., 2015, Surficial geology handbook for southern Maine: Maine Geological Survey, Bulletin 44, 97 p. Thompson, W. B., and Borns, H. W., Jr., 1985, Surficial geologic map of Maine: Maine Geological Survey, scale 1:500,000. SOURCES OF RELATED INFORMATION Figure 1: West shore of Baker Island showing intertidal granite ledges with light gray boulder pile “seawall” beach at the high tide line on the right. In the background on the left is a low green strip, Little Cranberry Island. On the skyline are the glacially sculpted mountains on Mount Desert Island. Figure 2: Shaded relief Lidar image of Baker Island with illumination from the northwest. The interior of the island is dominated by bedrock ledges while the periphery of the island is ringed by cobble-boulder ridges built by storm action. On the east and south sides of the island (exposed to the open ocean) the ridge is made of angular blocks of granite. On the west and north sides of the island (more protected from wave attack) the ridge is made of rounded boulders and some angular blocks. On the northwest part of the island there are beach deposits from sea levels higher than present, where the ground surface is smoother with faint lines representing beach strandlines. Image courtesy of the College of Atlantic GIS laboratory. Baker Island Quadrangle, Maine Open-File No. 16-13 2016 Funding for the preparation of this map was provided by the Maine Geological Survey. Digital cartography by Susan S. Tolman Christian H. Halsted State Geologist Robert G. Marvinney Cartographic design by Christian H. Halsted Surficial geologic mapping by Duane D. Braun Address: 93 State House Station, Augusta, Maine 04333 Telephone: 207-287-2801 E-mail: [email protected] Home page: http://www.maine.gov/dacf/mgs/ Maine Geological Survey 1 0 1 0.5 Mile 1 0 1 0.5 Kilometer 1000 0 1000 2000 3000 4000 5000 6000 7000 Feet Maine Schoodic Head Southwest Harbor Baker Island Bass Harbor Frenchboro Seal Harbor CONTOUR INTERVAL 10 FEET ___Continental glaciers like the ice sheet now covering Antarctica probably extended across Maine several times during the Pleistocene Epoch, between about 2.5 million and 11,700 years ago. The slow- moving ice superficially changed the landscape as it scraped over mountains and valleys, eroding and transporting boulders and other rock debris for miles. The sediments that cover much of Maine are largely the product of glaciation. Glacial ice deposited some of these materials, while others washed into the sea or accumulated in meltwater streams and lakes as the ice receded. Earlier stream patterns were disrupted, creating hundreds of ponds and lakes across the state. The map at left shows the pattern of glacial sediments in this quadrangle. ___The most recent "Ice Age" in Maine began about 30,000 years ago, when an ice sheet spread southward over New England (Stone and Borns, 1986). During its peak, the ice was several thousand feet thick and covered the highest mountains in the state. The weight of this huge glacier actually caused the land surface to sink hundreds of feet. Rock debris frozen into the base of the glacier abraded the bedrock surface over which the ice flowed. The grooves and fine scratches (striations) resulting from this scraping process are often seen on freshly exposed bedrock, and they are important indicators of the direction of ice movement. Erosion and sediment deposition by the ice sheet combined to give a streamlined shape to many hills, with their long dimension parallel to the direction of ice flow. Some of these hills (drumlins) are composed of dense glacial sediment (till) plastered under great pressure beneath the ice. ___A warming climate forced the ice sheet to start receding as early as 21,000 calendar years ago, soon after it reached its southernmost position on Long Island (Ridge, 2004). The edge of the glacier withdrew from the continental shelf east of Long Island and reached the present position of the Maine coast by about 16,000 years ago (Borns and others, 2004). Even though the weight of the ice was removed from the land surface, the Earth's crust did not immediately spring back to its normal level. As a result, the sea flooded much of southern Maine as the glacier retreated to the northwest. Ocean waters extended far up the Kennebec and Penobscot valleys, reaching present elevations of up to 420 feet in the central part of the state. ___Great quantities of sediment washed out of the melting ice and into the sea, which was in contact with the receding glacier margin. Sand and gravel accumulated as deltas and submarine fans where streams discharged along the ice front, while the finer silt and clay dispersed across the ocean floor. The shells of clams, mussels, and other invertebrates are found in the glacial-marine clay that blankets lowland areas of southern Maine. Ages of these fossils tell us that ocean waters covered parts of Maine until about 13,000 years ago. The land rebounded as the weight of the ice sheet was removed, forcing the sea to retreat. ___Meltwater streams deposited sand and gravel in tunnels within the ice. These deposits remained as ridges (eskers) when the surrounding ice disappeared. Maine's esker systems can be traced for up to 100 miles, and are among the longest in the country. ___Other sand and gravel deposits formed as mounds (kames) and terraces adjacent to melting ice, or as outwash in valleys in front of the glacier. Many of these water-laid deposits are well layered, in contrast to the chaotic mixture of boulders and sediment of all sizes (till) that was released from dirty ice without subsequent reworking. Ridges consisting of till or washed sediments (moraines) were constructed along the ice margin in places where the glacier was still actively flowing and conveying rock debris to its terminus. Moraine ridges are abundant in the zone of former marine submergence, where they are useful indicators of the pattern of ice retreat. ___The last remnants of glacial ice probably were gone from Maine by 12,000 years ago. Large sand dunes accumulated in late-glacial time as winds picked up outwash sand and blew it onto the east sides of river valleys, such as the Androscoggin and Saco valleys. The modern stream network became established soon after deglaciation, and organic deposits began to form in peat bogs, marshes, and swamps. Tundra vegetation bordering the ice sheet was replaced by changing forest communities as the climate warmed (Davis and Jacobson, 1985). Geologic processes are by no means dormant today, however, since rivers and wave action modify the land, and worldwide sea level is gradually rising against Maine's coast. References Borns, H. W., Jr., Doner, L. A., Dorion, C. C., Jacobson, G. L., Jr., Kaplan, M. R., Kreutz, K. J., Lowell, T. V., Thompson, W. B., and Weddle, T. K., 2004, The deglaciation of Maine, U.S.A., in Ehlers, J., and Gibbard, P. L., eds., Quaternary Glaciations – Extent and Chronology, Part II: North America: Amsterdam, Elsevier, p. 89-109. Davis, R. B., and Jacobson, G. L., Jr., 1985, Late-glacial and early Holocene landscapes in northern New England and adjacent areas of Canada: Quaternary Research, v. 23, p. 341-368. Ridge, J. C., 2004, The Quaternary glaciation of western New England with correlations to surrounding areas, in Ehlers, J., and Gibbard, P. L., eds., Quaternary Glaciations – Extent and Chronology, Part II: North America: Amsterdam, Elsevier, p. 169-199. Stone, B. D., and Borns, H. W., Jr., 1986, Pleistocene glacial and interglacial stratigraphy of New England, Long Island, and adjacent Georges Bank and Gulf of Maine, in Sibrava, V., Bowen, D. Q., and Richmond, G. M. (editors), Quaternary glaciations in the northern hemisphere: Quaternary Science Reviews, v. 5, p. 39-52. 1:24,000 SCALE Note: The first letter of each map unit indicates the general age of the unit: H = Holocene (postglacial deposit; formed during the last 11,700 years). Q = Quaternary (deposit of uncertain age usually late-glacial and/or postglacial). P = Pleistocene (deposit formed during glacial to late-glacial time, prior to 11,700 yr B.P. [years before present]). Base map features from Maine Office of GIS - 1:24,000 USGS contour lines, E911 roads, 1:24,000 National Hydrography Dataset, USGS GNIS placenames and 1:24,000 political boundaries. Map projection Universal Transverse Mercator, North American Datum, 1927. The use of industry, firm, or local government names on this map is for location purposes only and does not impute responsibility for any present or potential effects on the natural resources. Approximate Mean Declination, 2015 16.5 o W (not to scale) True North Magnetic North Lowell, T.V, 1980, Late Wisconsin ice extent in Maine: evidence from Mount Desert Island and the Saint John River area: unpublished M.S. thesis, University of Maine, Orono, 180 p. Norton, S. A. and others, 2010, Early post-glacial and Holocene history of the Sargent Mountain pond watershed, as seen from the bottom of Sargent Mountain pond, Acadia National Park, Maine: in Gerbi, C., Yates, M., Kelley, A., and Lux, D., eds., Guidebook for field trips in coastal and interior Maine: 102nd NEIGC meeting, p. 45-60. REFERENCES Pp Presumpscot Formation - Fine-grained marine mud (silt and clay with sandy lenses) commonly containing gravel dropstones and, more rarely, marine shell fossils. The mud was deposited in deeper, quieter water during the postglacial marine submergence of the coast. Contact - Indicates approximate boundary between adjacent map units. Expectable line location error is 3-6 m (10-20 ft) to locally as much as 10-15 m (30-50 ft) where the materials are obscured by dense surface vegetation and lack diagnostic landform. Hwsm Salt marsh - Salt marsh - Grass, reed, and sedge wetland, inundated at high tide, that is underlain by fine grained sediment having a variable thickness of 0.3 - 2 m (1-6 ft). Hms Marine shoreline deposit - Beach ridges composed of cobble to boulder-size material 2-3 m (6-10 ft) thick. Pms Marine shoreline deposit - Stratified pebble to boulder gravel and sand that has layering dipping downslope. This deposit was mapped where there is a rounded gravel mantle with distinct strandline features. Deposited during the postglacial marine submergence of the coast. Marine beach ridge or strandline - Subtle ridge or bench feature with an abrupt steepening of slope in the downslope direction in an area of Pleistocene marine shoreline deposits. A strandline marks a temporary pause in sea-level lowering or an especially stormy period as the sea receded. Glacial striation locality - Arrow shows ice-flow direction inferred from striations on bedrock. Dot marks point of observation. Number is azimuth (in degrees) of flow direction. Crescentic mark - Arrow shows direction of ice-flow. Dot indicates point of observation. Number is azimuth (in degrees) of flow direction. Bedrock - Areas shown as solid gray are where 25% or more of the land surface is knobs of bare or vegetation-covered bedrock ledge. Thin (.3-1 m [1-3 ft]) glacial till or marine mud materials overlie the bedrock between knobs. rk Photo locality ! ( 1 The Northern Portion of the 125 This map supersedes Open-File Map 15-31.
Transcript of Surficial Geology Baker Island Quadrangle, Maine
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Surficial Geology
SURFICIAL GEOLOGY OF MAINE
Surficial geologic mapping of th e Baker Island quadrangle w as conducted byDuane D. Braun during th e 2014 field season.
SOURCES OF MAP INFORMATION
___A surficial geology map sh ow s all th e loose materials such as till (commonly called h ardpan), sandand gravel, or clay, w h ich overlie solid ledge (bedrock). Bedrock outcrops and areas of abundantbedrock outcrops are sh ow n on th e map, but varieties of th e bedrock are not distinguish ed (refer tobedrock geology map). Most of th e surficial materials are deposits formed by glacial and deglacialprocesses during th e last stage of continental glaciation, w h ich began about 25,000 years ago. Th eremainder of th e surficial deposits are th e products of postglacial geologic processes, such as riverfloodplains, or are attributed to h uman activity, such as fill or oth er land-modifying features.___Th e map sh ow s th e areal distribution of th e different types of glacial features, deposits, andlandforms as described in th e map explanation. Features such as striations and moraines can be used toreconstruct th e movement and position of th e glacier and its margin, especially as th e ice sh eet melted.Oth er ancient features include sh orelines and deposits of glacial lakes or th e glacial sea, now long gonefrom th e state. Th is glacial geologic h istory of th e quadrangle is useful to th e larger understanding ofpast earth climate, and h ow our region of th e w orld underw ent recent geologically significant climaticand environmental ch anges. W e may th en be able to use th is know ledge in anticipation of future similarch anges for long-term planning efforts, such as coastal development or w aste disposal.___Surficial geology maps are often best used in conjunction w ith related maps such as surficialmaterials maps or significant sand and gravel aquifer maps for anyone w anting to know w h at liesbeneath th e land surface. For example, th ese maps may aid in th e search for w ater supplies, oreconomically important deposits such as sand and gravel for aggregate or clay for bricks or pottery.Environmental issues such as th e location of a suitable landfill site or th e possible spread ofcontaminants are directly related to surficial geology. Construction projects such as locating new roads,excavating foundations, or siting new h omes may be better planned w ith a good know ledge of th esurficial geology of th e site.
USES OF SURFICIAL GEOLOGY MAPS
Braun, Duane D., 2015, Surficial materials of th e north ern portion of th e Baker Island quadrangle,Maine: Maine Geological Survey, Open-File Map 15-22, map, scale 1:24,000.Th ompson, W oodrow B., 2015, Surficial geology h andbook for south ern Maine: Maine GeologicalSurvey, Bulletin 44, 97 p.
Th ompson, W . B., and Borns, H. W ., Jr., 1985, Surficial geologic map of Maine: Maine GeologicalSurvey, scale 1:500,000.
SOURCES OF RELATED INFORMATION
Figure 1: W est sh ore of Baker Island sh ow ing intertidal granite ledges w ith ligh t gray boulder pile “seaw all” beach atth e h igh tide line on th e righ t. In th e background on th e left is a low green strip, Little Cranberry Island. On th eskyline are th e glacially sculpted mountains on Mount Desert Island.
Figure 2: Sh aded relief Lidar image of Baker Island w ith illumination from th e north w est. Th e interior of th e islandis dominated by bedrock ledges w h ile th e periph ery of th e island is ringed by cobble-boulder ridges built by stormaction. On th e east and south sides of th e island (exposed to th e open ocean) th e ridge is made of angular blocks ofgranite. On th e w est and north sides of th e island (more protected from w ave attack) th e ridge is made of roundedboulders and some angular blocks. On th e north w est part of th e island th ere are beach deposits from sea levels h igh erth an present, w h ere th e ground surface is smooth er w ith faint lines representing beach strandlines. Image courtesy ofth e College of Atlantic GIS laboratory.
Baker Island Quadrangle, Maine
Open-File No. 16-132016
Funding for the preparation of this map was provided by the Maine Geological Survey.
Digital cartography bySusan S. Tolman
Christian H. Halsted State GeologistRobert G. Marvinney Cartographic design by
Christian H. Halsted
Surficial geologic mapping byDuane D. Braun
Address: 93 State House Station, Augusta, Maine 04333Telephone: 207-287-2801 E-mail: [email protected] page: http://www.maine.gov/dacf/mgs/
Maine Geological Survey
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___Continental glaciers like th e ice sh eet now covering Antarcticaprobably extended across Maine several times during th e P leistoceneEpoch , betw een about 2.5 million and 11,700 years ago. Th e slow -moving ice superficially ch anged th e landscape as it scraped overmountains and valleys, eroding and transporting boulders and oth errock debris for miles. Th e sediments th at cover much of Maine arelargely th e product of glaciation. Glacial ice deposited some of th esematerials, w h ile oth ers w ash ed into th e sea or accumulated inmeltw ater streams and lakes as th e ice receded. Earlier stream patternsw ere disrupted, creating h undreds of ponds and lakes across th e state.Th e map at left sh ow s th e pattern of glacial sediments in th isquadrangle.___Th e most recent "Ice Age" in Maine began about 30,000 years ago,w h en an ice sh eet spread south w ard over New England (Stone andBorns, 1986). During its peak, th e ice w as several th ousand feet th ickand covered th e h igh est mountains in th e state. Th e w eigh t of th is h ugeglacier actually caused th e land surface to sink h undreds of feet. R ockdebris frozen into th e base of th e glacier abraded th e bedrock surfaceover w h ich th e ice flow ed. Th e grooves and fine scratch es (striations)resulting from th is scraping process are often seen on fresh ly exposedbedrock, and th ey are important indicators of th e direction of icemovement. Erosion and sediment deposition by th e ice sh eet combinedto give a streamlined sh ape to many h ills, w ith th eir long dimensionparallel to th e direction of ice flow . Some of th ese h ills (drumlins) arecomposed of dense glacial sediment (till) plastered under great pressurebeneath th e ice.___A w arming climate forced th e ice sh eet to start receding as early as21,000 calendar years ago, soon after it reach ed its south ernmostposition on Long Island (R idge, 2004). Th e edge of th e glacierw ith drew from th e continental sh elf east of Long Island and reach edth e present position of th e Maine coast by about 16,000 years ago(Borns and oth ers, 2004). Even th ough th e w eigh t of th e ice w asremoved from th e land surface, th e Earth 's crust did not immediatelyspring back to its normal level. As a result, th e sea flooded much ofsouth ern Maine as th e glacier retreated to th e north w est. Ocean w atersextended far up th e K ennebec and P enobscot valleys, reach ing presentelevations of up to 420 feet in th e central part of th e state.___Great quantities of sediment w ash ed out of th e melting ice and intoth e sea, w h ich w as in contact w ith th e receding glacier margin. Sandand gravel accumulated as deltas and submarine fans w h ere streamsdisch arged along th e ice front, w h ile th e finer silt and clay dispersedacross th e ocean floor. Th e sh ells of clams, mussels, and oth erinvertebrates are found in th e glacial-marine clay th at blankets low landareas of south ern Maine. Ages of th ese fossils tell us th at ocean w aterscovered parts of Maine until about 13,000 years ago. Th e landrebounded as th e w eigh t of th e ice sh eet w as removed, forcing th e seato retreat.
___Meltw ater streams deposited sand and gravel in tunnels w ith in th eice. Th ese deposits remained as ridges (eskers) w h en th e surroundingice disappeared. Maine's esker systems can be traced for up to 100miles, and are among th e longest in th e country.___Oth er sand and gravel deposits formed as mounds (kames) andterraces adjacent to melting ice, or as outw ash in valleys in front of th eglacier. Many of th ese w ater-laid deposits are w ell layered, in contrastto th e ch aotic mixture of boulders and sediment of all sizes (till) th atw as released from dirty ice w ith out subsequent rew orking. R idgesconsisting of till or w ash ed sediments (moraines) w ere constructedalong th e ice margin in places w h ere th e glacier w as still activelyflow ing and conveying rock debris to its terminus. Moraine ridges areabundant in th e zone of former marine submergence, w h ere th ey areuseful indicators of th e pattern of ice retreat.___Th e last remnants of glacial ice probably w ere gone from Maine by12,000 years ago. Large sand dunes accumulated in late-glacial time asw inds picked up outw ash sand and blew it onto th e east sides of rivervalleys, such as th e Androscoggin and Saco valleys. Th e modernstream netw ork became establish ed soon after deglaciation, andorganic deposits began to form in peat bogs, marsh es, and sw amps.Tundra vegetation bordering th e ice sh eet w as replaced by ch angingforest communities as th e climate w armed (Davis and Jacobson, 1985).Geologic processes are by no means dormant today, h ow ever, sincerivers and w ave action modify th e land, and w orldw ide sea level isgradually rising against Maine's coast.
References
Borns, H. W ., Jr., Doner, L. A., Dorion, C. C., Jacobson, G. L., Jr.,K aplan, M. R ., K reutz, K . J., Low ell, T. V ., Th ompson, W . B., andW eddle, T. K ., 2004, Th e deglaciation of Maine, U .S.A.,in Eh lers, J.,and Gibbard, P . L., eds., Q uaternary Glaciations – Extent andCh ronology, P art II: North America: Amsterdam, Elsevier, p. 89-109.Davis, R . B., and Jacobson, G. L., Jr., 1985, Late-glacial and earlyHolocene landscapes in north ern New England and adjacent areas ofCanada: Q uaternary R esearch , v. 23, p. 341-368.
R idge, J. C., 2004, Th e Q uaternary glaciation of w estern New Englandw ith correlations to surrounding areas,in Eh lers, J., and Gibbard, P . L.,eds., Q uaternary Glaciations – Extent and Ch ronology, P art II: NorthAmerica: Amsterdam, Elsevier, p. 169-199.
Stone, B. D., and Borns, H. W ., Jr., 1986, P leistocene glacial andinterglacial stratigraph y of New England, Long Island, and adjacentGeorges Bank and Gulf of Maine, in Sibrava, V ., Bow en, D. Q ., andR ich mond, G. M. (editors), Q uaternary glaciations in th e north ernh emisph ere: Q uaternary Science R eview s, v. 5, p. 39-52.
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Note: Th e first letter of each map unit indicates th e general age of th e unit:H = Holocene (postglacial deposit; formed during th e last 11,700 years).Q = Q uaternary (deposit of uncertain age usually late-glacial and/or postglacial).P = P leistocene (deposit formed during glacial to late-glacial time, prior to 11,700 yr B.P . [years beforepresent]).
Base map features from Maine Office of GIS - 1:24,000 U SGS contourlines, E911 roads, 1:24,000 National Hydrograph y Dataset, U SGSGNIS placenames and 1:24,000 political boundaries. Map projectionU niversal Transverse Mercator, North American Datum, 1927.Th e use of industry, firm, or local government names on th is map is forlocation purposes only and does not impute responsibility for anypresent or potential effects on th e natural resources.
Approximate MeanDeclination, 2015
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True North
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Low ell, T.V , 1980, Late W isconsin ice extent in Maine: evidence from Mount Desert Island and th e SaintJoh n R iver area: unpublish ed M.S. th esis, U niversity of Maine, Orono, 180 p.
Norton, S. A. and oth ers, 2010, Early post-glacial and Holocene h istory of th e Sargent Mountain pondw atersh ed, as seen from th e bottom of Sargent Mountain pond, Acadia National P ark, Maine: in Gerbi,C., Y ates, M., K elley, A., and Lux, D., eds., Guidebook for field trips in coastal and interior Maine:102nd NEIGC meeting, p. 45-60.
REFERENCES
Pp Presumpscot Formation - Fine-grained marine mud (silt and clay w ith sandy lenses)commonly containing gravel dropstones and, more rarely, marine sh ell fossils. Th e mud w asdeposited in deeper, quieter w ater during th e postglacial marine submergence of th e coast.
Contact - Indicates approximate boundary betw een adjacent map units. Expectable linelocation error is 3-6 m (10-20 ft) to locally as much as 10-15 m (30-50 ft) w h ere th e materialsare obscured by dense surface vegetation and lack diagnostic landform.
Hwsm Salt marsh - Salt marsh - Grass, reed, and sedge w etland, inundated at h igh tide, th at isunderlain by fine grained sediment h aving a variable th ickness of 0.3 - 2 m (1-6 ft).
Hms Marine shoreline deposit - Beach ridges composed of cobble to boulder-size material 2-3 m(6-10 ft) th ick.
Pms Marine shoreline deposit - Stratified pebble to boulder gravel and sand th at h as layeringdipping dow nslope. Th is deposit w as mapped w h ere th ere is a rounded gravel mantle w ithdistinct strandline features. Deposited during th e postglacial marine submergence of th e coast.
Marine beach ridge or strandline - Subtle ridge or bench feature w ith an abrupt steepening ofslope in th e dow nslope direction in an area of P leistocene marine sh oreline deposits. Astrandline marks a temporary pause in sea-level low ering or an especially stormy period as th esea receded.Glacial striation locality - Arrow sh ow s ice-flow direction inferred from striations onbedrock. Dot marks point of observation. Number is azimuth (in degrees) of flow direction.
Crescentic mark - Arrow sh ow s direction of ice-flow . Dot indicates point of observation.Number is azimuth (in degrees) of flow direction.
Bedrock - Areas sh ow n as solid gray are w h ere 25% or more of th e land surface is knobs ofbare or vegetation-covered bedrock ledge. Th in (.3-1 m [1-3 ft]) glacial till or marine mudmaterials overlie th e bedrock betw een knobs.
rk
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This map supersedesOpen-File Map 15-31.
