Post-glacial Sea-level Change Research Articles

Stratigraphic evolution of the inner continental shelf in response to late Quaternary relative sea-level change, northwestern Gulf of Maine

Accumulations of deltaic and littoral sediments on the inner continental shelf of Maine, Gulf of Maine, preserve a record of postglacial sea-level changes and shoreline migrations. The depositional response of coastal environments to a cycle of regression, lowstand, and transgression was examined with seismic-reflection profiles, vibracores, and radiocarbon dates collected from sediments at the mouths of the Kennebec and Penobscot Rivers. Sequence-stratigraphic analysis of these data reveals two distinctly different successions of late Quaternary deposits that represent end members in an evolutionary model for this glaciated coast. Seaward of the Kennebec River, coarse-grained shorelines with foreset beds occur at depths of 20–60 m and outline the lobate margin of the Kennebec River paleodelta, a complex, rock-framed accumulation of glaciomarine and deltaic sediments capped by estuarine and marine deposits. Sand derived from this system today supports large barrier spits and extensive salt marshes. In contrast, the mouth of the Penobscot River is characterized by thick deposits of glaciomarine mud overlain by marine mud of Holocene age, including gas-charged zones that have locally evolved into fields of pockmarks. The distinct lack of sand and gravel seaward of the Penobscot River and its abundance seaward of the Kennebec River probably reflect differences in sediment sources and the physiography of the two watersheds. The contrasting stratigraphic framework of these systems demonstrates the importance of understanding local and regional differences in sediment supply, sea-level change, bedrock structure, and exposure to waves and tides in order to model river-mouth deposition on glaciated coasts. Evolution of shelf deposits was largely controlled by relative sea level, which locally fell from a highstand (+60 to +70 m at 14 ka) contemporary with deglaciation to a lowstand (−55 m at 10.8 ka). The sea-level lowering was accompanied by fluvial incision of older deposits, producing a regressive, basal unconformity. Major rivers deposited abundant sediment over this surface. Sea level then rose at varying rates, extensively reworking formerly emergent parts of the shelf and producing a shoreface ravinement surface in areas exposed to waves. A tidal ravinement surface has developed in sheltered embayments where erosion is due mainly to tidal currents. Incised valleys in both settings preserve transgressive estuarine deposits that contain lagoonal bivalves and salt-marsh foraminifera at depths of 15–30 m. These deposits accumulated ca. 9.2–7.3 ka, locally a period of relatively slow sea-level rise.

Postglacial sea-level change on a rotating Earth: first results from a gravitationally self-consistent sea-level equation

We present and solve a gravitationally self-consistent sea-level equation which governs postglacial sea-level variations on a spherically symmetric, self-gravitating, viscoelastic and rotating Earth. We find that the inclusion of a glacio-isostatically induced rotational excitation can significantly affect previous predictions of both present-day sea-level rates and postglacial sea-level histories which were based on a theory that assumed a non-rotating Earth model. To illustrate, we consider present-day sea-level rates (and tide-gauge corrections) along the US east coast, and relative sea-level curves in the far field of the late Pleistocene ice sheets.

Late Quaternary sea level history in the Paulatuk to Bathurst Inlet area, Northwest Territories

The Paulatuk – Bathurst Inlet region experienced rapid deglaciation in response to marine incursion across isostatically depressed terrain during high relative sea level stands. Marine limits, frequently defined by ice-contact deltas, range from 10 m asl in the west to 228 m asl in the east and were formed from approximately 12.5 to 9 ka BP, respectively. Seven relative sea level curves demonstrate that the mainland coast has shown initial rapid emergence, then progressively less emergence, and finally submergence from Paulatuk to Bernard Harbour during the late Holocene. Regions to the east (Richardson Bay to Bathurst Inlet) continue to experience emergence. Sea level curves have shown persistently higher rebound from west to east. The pattern and magnitude of observed postglacial sea level changes are in accord with theoretical predictions. In areas outside the glacial limit, as well as close to but within the glacial limit, relative sea level initially falls during the early and middle Holocene, and then begins to rise due to the migration of the forebulge in late Holocene times. Submergence becomes progressively younger from west to east. Closer to the centre of ice loading, emergence has progressed since deglaciation.

Present‐day post‐glacial sea level change far from the Late Pleistocene ice sheets: Implications for recent analyses of tide gauge records

We examine the sensitivity of predictions of the present‐day rate of sea level change due to glacial isostatic adjustment (GIA) in two regions located within the ‘far‐field’ of the Late Pleistocene ice sheets (Europe/north Africa and the western Pacific/Caribbean) to variations in the mantle viscosity profile. The regions considered here encompass the location of 11 (of 21) sites used in a recent determination, based on GIA‐corrected tide gauge records, of the global rate of secular sea level change. Our analysis suggests that the mean GIA correction at the 11 far‐field sites may vary by as much as 0.5 mm/yr, depending on the adopted Earth model, and we derive a bound on the (residual) secular sea level rise of 1.1 to 1.6 mm/yr. This bound is consistent with our analysis of the tide gauge record from the U.S. east coast, and it suggests that previous estimates of the residual sea level rise represent an upper bound.

A world perspective on acid sulphate soils

Generalizations about the morphology and genesis of acid sulphate soils at a regional scale are derived from a review of extensive though patchy soil survey data. A general sequence of sedimentation, accumulation of sulphides and burial of sulphitic material by peat or alluvium of low sulphide content, and a parallel sequence of soil profile development are illustrated by examples of unripe sulphitic soils under a natural tidal regime, artificially drained raw acid sulphate soils and ripe acid sulphate soils at various stages of development. The world pattern has been driven mainly by postglacial sea level change but each regional pattern is determined by its unique sedimentary and geomorphological history. Distinctive patterns occur in deltas and estuaries, and in humid and seasonally dry climates. Very extensive areas of ripe acid sulphate soils in areas that are not known to have experienced falls in relative sea level are ascribed to human activity: forest clearance and soil erosion in catchments causing lower dry season river flows, and piecemeal clearance of peat over the floodplains permitting the dry season water table to fall. The implications for further agricultural development in other comparable areas include the absolute necessity to know the extent of potential acid sulphate soils and to maintain their water table above the sulphide datum. Most effective systems of management have been developed by farmers themselves but the sustainability of these systems is often doubtful due to their detrimental effects on the aquatic ecosystem.

Geoidal-eustatic changes induced by the deglaciation of fennoscandia

Relative sea level changes in Fennoscandia have long been known to be a result of processes connected to the last glaciation. The post-glacial sea level changes have commonly been thought of as a result of global variations of the ocean water volume and isostatic movements (connected to the deglaciation). However, changes of the ice loads bring about gravimetric changes, and resulting eustatic changes will vary significantly over the globe. The importance of the geoid changes related to the deglaciation in Fennoscandia is illustrated by theoretical simulations, which show that the resulting sea level movements differ significantly over the globe. During a fast deglaciation there is not a worldwide rise in sea level due to increased volume of ocean water, but in the region near the former glacier there may actually be a fall in sea level. By simulations of the deglaciation history it is also shown that the associated sea level history varies even within a small area, and may differ by up to 20–30 m from the Barents Sea to central Scandinavia. This is really a significant difference, compared to the overall pattern of sea level change of 100 m for the last 15 ka. Thus no eustatic curve is valid globally.

Mathematical geophysics: A survey of recent developments in seismology and geodynamics: Vlaar, N J; Nolet, G; Wortel, M J R; Cloetingh, S A P L (editors) Dordrecht: Reidel, 1988, 407 P

I. Seismology and Three Dimensional Structure of the Earth.- Waves in a 3-D Earth.- 1 Low frequency seismology and three-dimensional structure - observational aspects..- 2 Free-oscillation coupling theory..- 3 Surface waves in weakly heterogeneous media..- 4 On the connection between ray theory and scattering theory for surface waves..- 5 A hybrid solution for wave propagation problems in inhomogeneous media..- Large-scale inversion.- 6 Elastic wavefield inversion and the adjoint operator for the elastic wave equation..- 7 Subspace methods for large-scale nonlinear inversion..- 8 Imaging algorithms, accuracy and resolution in delay time tomography..- II. Convection and Lithospheric Processes.- Geomagnetism.- 9 The determination of fluid flow at the core surface from geomagnetic observations..- Mantle convection.- 10 Long wavelength features of mantle convection..- 11 Mean-field methods in mantle convection..- 12 Infinite Prandtl number spherical-shell convection..- 13 Lateral heterogeneity and the geoid: the importance of the urface kinematic constraints..- Postglacial rebound.- 14 Lithospheric thickness, Antarctic deglaciation history, and ocean basin discretization effects in a global model of postglacial sea level change..- 15 Non-uniqueness of lithospheric thickness estimates based on glacial rebound data along the east coast of North America..- Thermomechanical processes in the lithosphere.- 16 On the mechanics of plate boundary formation..- 17 Supercontinent breakup: effect on eustatic sea level and the oceanic heat flux..

Lithospheric thickness, Antarctic deglaciation history, and ocean basin discretization effects in a global model of postglacial sea level change: A summary of some sources of nonuniqueness

The global model of postglacial relative sea level variations that has been developed over the past decade is employed to investigate the constraints that it may be invoked to place on the timing of the deglaciation of West Antarctica. The analyses presented here confirm the suggestion of P. Wu and W. R. Peltier (1983, Geophysical Journal of the Royal Astronomical Society 74, 377–450) that the model of this event employed in J. A. Clark and C. S. Lingle (1979 Quaternary Research 11, 279–298) may be simply modified to rectify the misfits between theory and observation that are otherwise obtained at Southern Hemisphere sites. A large number of Southern Hemisphere relative sea level data are shown to require that the retreat of Antarctic ice substantially lagged the retreat of Northern Hemisphere ice if the deglaciation of Antarctica was abrupt. The time of onset of Antarctic deglaciation is thereby shown to coincide with the time of most rapid Northern Hemisphere deglaciation. Sensitivity tests are performed which demonstrate that this result is relatively insensitive to the discretization employed to represent the ocean basins; the only exception to this general rule obtains at some coastal sites at which a trade-off is revealed between the delay of West Antarctic melting and the thickness of the lithosphere required to reconcile the observed local variations of relative sea level. At such sites, which are all in the far field of the ice sheets, some attention must be paid to the accuracy of the local finite element representation of the oceans and to the details of the Antarctic deglaciation history.

Postglacial stratigraphy and sea-level change in southwestern New Brunswick

Cores from small basins in southwestern New Brunswick, above present sea level but below the former marine limit, were examined for microfaunal changes indicating a transition from marine to freshwater conditions. Carbon 14 dates of those transition points add to and strengthen a previous sea-level curve from the area. The oldest date places sea level at about 75 m above present at 16 000 years BP. Dates from the lower lakes (+ 10 m) appear to be anomalously old, and it is suggested that after 12 000 years BP erosion became a factor in marine areas. This erosion may be related to the overall breakup of ice in the North Atlantic and may explain why few dates younger than 12 000 years BP are found in raised marine deposits of New Brunswick and Maine.

Geomorphic Development of the Alluvial Plain along the Lower Nagdong River, Korea

The Nagdong River Basin occuping the area of 24, 000km2 with 525km long trunk stream is located in the southeastern part of the Korean Peninsula. The alluvial plain along the lower Nagdong is divided into two areas, i. e. the down-reaches basin and the river-mouth delta.This study intends to clarify geomorphic characteristics and geomorphic development of this plain in relation to post-glacial sea-level change. The investigation is made up of micro-landform classification, stratigraphic analysis, and restoration of depositional environment.The main findings are as follows:1) There are buried valleys below the 50-70m thick alluvium in th delta, and below the 15-10m thick one at Woegwan located about 210km upstream from the river mouth.2) The alluvium of the delta is classified into the following six different layers in ascending order; Basal Gravel (BG), Lower Silt-Clay (LC), Middle Sand (MS), Middle Silt-Clay (MC), Upper Sand (US) and Upper Clay (UC). The LC and MC are considered to be the bottom-set bed deposited in the transgression phase. The MS and US are considered to be the foreset or topset bed formed in the regression phase.3) The above mentioned sequence of alluvial deposits shows that there are two large sealevel fluctuations in the post-glacial time in this plain as demonstrated in the plains of Central Japan (Ikeda, 1964).4) In the longitudinal profile, the LC and MC show a wedge-shape structure which is sharp upstream. These layers were traced up to 50km (LC) and 150km (MC) from the river mouth.5) In the cross-section of down-reaches basin, silty clay lenses are recognized on the both sides of main stream and are identified as back marsh deposits which formed in submerged environment during the transgression.6) Geomorphic development of this alluvial plain are summarized as follows:a) The down-reaches of Nagdong River was drowned in the postglacial transgression. This drowned valley was consequently filled continuously with stream deposits supplied from the neighbouring mountain sides and with flood deposits provided from the main Nagdong River sides. The area of water surface was gradually decreased. The back marshes such as Upo, Samolpo etc., are the latest remnants of water surface.b) The delta plain formed on the river mouth is made up of three different geomorphic units, that is, the main delta, the lower delta and the marginal marsh. In the bay, the main delta was formed by bifurication of main stream resulting in the formation of interdistributary islands. By contrast in the bay mouth which is affected by open sea, the lower delta consisting of beach ridge, swale, and barrier islands has been developed. The development of main delta provided the enclosure of water surface and resulted in the formation of marginal marsh.

Geomorphological Development of Shonai Coastal Plain, Northeastern Japan

Shonai Plain, located in the western coast of Northeastern Japan, is fed by the Mogami river and several small rivers, and margined by a large coastal sand dune. This paper intends to clarify sediment yield change during the last 12, 000 years, confirming the relationship between the movement of shoreline and that of alluvial fan's front. In order to confirm this relationship, the palaeogeography was restored based on the analysis of 758 borehole data, radiometric datings and field observations.In this paper, the following method was adopted to restore the past geomorphological surface according to every sea level. Commonly, the level of alluvial fan's seaward front should take a slight elevation at every past alluvial surface (a m) was obtained as Fig. 9. The gradient of the present surface was adopted as that of line ((2)) which conbined shoreline and the seaward front of alluvial fans. By the above method, the elevation and the location of alluvial fan's seaward front according to each sea-level was estimated. The palaeogeomorphology was restored by 9 periods (Figs. 10-a-i). Shown in Fig. 8, Post-Glacial sea-level change was restored in order to estimate the age of each geomorphological map.The first period (Fig. 10-a) indicates the geomorphology when the sea-level reached at about -50m (12, 000-11, 000yr B. P.). The lagoonal environment was located at the river mouth of the Mogami river. After that, the sea area (lagoon) was invading inlandward until the level reached at -15m (Fig. 10-a). After the time when the sea-level rose above -15m, the shoreline rapidly moved seaward (Figs. 10-f-h).On the other hand, these palaeogeomorphological maps indicate an irregularity between the expansion of alluvial fan and the shoreline progradation. That is to say, during the period when the shoreline migrated inlandward, the alluvial fan formed by the Nikko river was expanded and the fans formed by the Akagawa and the Sasagawa rivers were not (Figs. 10-c-e). When the shoreline began to migrate seaward, the alluvial fan formed by the Sasagawa river was expanded and the fans formed by the Nikko and the Akagawa rivers were not (Figs. 10-e-f). After that, though the shoreline continued to migrate seaward, the alluvial fans front moved inlandward (Figs. 10-g-h).The author assumed that the change of supplied load from each upstream area is rather effective on the expansion of alluvial fan than the shoreline progradation.

Postglacial Pollen Diagrams from South Georgia (Sub-Antarctic) and West Falkland Island (South Atlantic)

Six pollen diagrams are presented, five from South Georgia, and one from near Port Howard, West Falkland Island. These diagrams are the first published for either island. In South Georgia and the Falkland Islands, conditions seem to have ameliorated by c. 10 000 years BP, and peat then began to accumulate. Many elements of the present vascular flora of South Georgia and West Falkland Island were flourishing before 9500 years BP, having in South Georgia possibly survived the last glacial (approximately 20000-10000 years BP) in low altitude refugia. Post glacial conditions have not prevented South Georgia plants from flowering, and the Neoglacial (c. 5000-4500 years BP) is thus not clearly indicated. As an aid to interpreting South Georgia pollen diagrams, surface litter analyses are related to percentage cover of vegetation. The status of Montia fontana, Sphagnum fimbriatum and Poa annua are established as being native, in the case of the former two species, and as being a probable introduction since c. AD 1800 in the case of the latter. Pollen blown from South America may attain 3.0% of the native pollen in South Georgia peat and up to 2.8% in West Falkland Island deposits. There are no indications from West Falkland Island that trees grew in the Falkland Islands during the postglacial (i.e. the last c. 10 000 years). The distribution of peat deposits in South Georgia hints at postglacial sea level changes, which may have altered the quantity of postglacial coastal tussock (Poa flabellata) pollen deposited in the peat.

Late—and Postglacial Glacier Fluctuations and Sea Level Changes in Arctic Canada

Late—and Postglacial Glacier Fluctuations and Sea Level Changes in Arctic Canada

Radiocarbon age of Robinson's Head moraine, west Newfoundland, and its significance for postglacial sea level changes

A radiocarbon date of 12 600 ± 140 years BP (GSC-2295) is reported on marine shells from sands within the Robinson's Head kame moraine at Stephen ville, Newfoundland. The moraine was deposited simultaneously with a delta-kame distal to it during a brief interval of increased ice marginal activity. This is marked at Stephenville by a temporary halt in landward marginal recession, but in some other localities around St. George's Bay by a lobate readvance into the late-glacial sea. The date refers to the time of moraine and delta-kame construction and confirms an earlier estimation. Other radiocarbon dates from the area, relating more or less closely to past sea level positions at Robinson's, south of Stephenville, are used, with an hypothetical isostatic uplift curve and a published eustatic sea level curve, to produce a first approximation to postglacial changes of local sea level in the St. George's Bay region.

Late. And Postglacial Glacier Fluctuations and Sea Level Changes in Arctic Canada

Late. And Postglacial Glacier Fluctuations and Sea Level Changes in Arctic Canada

SYMPOSIUM: POSTGLACIAL SEALEVEL CHANGE AND GEOMORPHIC DEVELOPMENT

The purpose of this symposium was to draw more preasely the curve of postglacial sealevel change and to examine its relationships with the coastal landform development. The speakers and commentators were as follows; (1) M. Nakayama et al. (on Hokkaido), (2) K. Nitobe (on Aomori Prefecture), (3) K. Omoto et al. (on Sendai Bay area), (4) Y. Toyoshima (on San-in), (5) Y. Ota et al. (on Kikaishima), (6) T. Nakata et al. (on Nansei Islands), (7) I. Akojima (on Jomon sealevel), (8) T. Takahashi (on rock-coast and high sealevel), (9) M. Oya (on some examples from Asian continent), (10) Y. Yoshida (on Antarctica), (11) H. Moriwaki (Review of previous sealevel change curves), (12) T. Wako (Comment on sealevel identification), (13) S. Fujii (Comment on Hokuriku and Atlantic coast), (14) M. Nogami (Comment on glacial mass balance and sealevel change rate). The problems discussed in this symposium were well summarized by H. Iseki as follows (1) The sealevel changed with fluctuations around Japanese islands and the proposed curves of sealevel change are proved to have some similarities. (2) The identification of former sealevel became more accurate. (3) Possibility of adoption of other dating than radiocarbon method. (4) Need of glaciological examination of sealevel change.

The Late-Glacial History of the Firth of Clyde: A Reply

The 'flaws' in the paper by Gemmell (I973) commented on by Gray are examined and assessed. A close reappraisal of the results obtained in Arran using Andrews's (1970) nomogram is carried out, particular attention being paid to the assumptions of a reference level dated at 5500 years B.P. and of a continuous uplift since c. 13 500 years B.P. The probable errors resulting from these assumptions are suggested to be of a maximum size of 600-700 years, the original nomogram tending to underpredict the age of late-glacial shorelines. It seems likely that the errors in the proposed lateglacial chronology are rather lower than Gray implies, low enough at least for future workers to use the chronology as a framework for further study. GRAY'S comments on the author's recent paper (Gemmell, I973) question several aspects of that publication, particularly the use of a nomogram produced by Andrews (1970) to derive a tentative dating sequence in the Island of Arran. Gray's paper gives the author the opportunity to clarify some points and to rectify some minor omissions in his paper. It is proposed to consider various aspects of the original paper in the same order as they are raised by Gray. ANLEY, G. (I95I) 'The ra ge of variation of the British climate', GeogrlJ. II7, 43-68 ARTHINUSSEN, M. (1960) 'Coast and fiord area of Finmark (with remarks on other districts)' in HOLTEDAHL, O. (ed.) e logy of Norway, 416-29 (Oslo and London) ORNER, N. A. (1969) 'The Late Quat rnary history of the Kat egatt Sea and the Swedish west coast: deglaciation, shorelevel displacement, chronology, isostasy an eustasy', Sver. geol. Unders. Afh. C., 640 HOTTON, F. W., BLUNDEL , D. J. and WILLIAMS, R. E. G. (1970) 'Birmingham University radiocarbon dates IV', Radioarbon 12, 385-99 HOTTON, F. . and WILLIAMS, R. E. G. (1973) 'Birmingham University radiocarbon dates VII', Radiocarbon 15, 451-68 I SONS, J. B. (I962) 'A rein erpretation of the literature on late-glacial shorelines in Scotland with particular reference o the Fo th rea', Trans. Edinb. geol. Soc. I9, 83-99 I SONS, J. B. (1965) 'Quaternary' in CRAIG, G. Y. (ed.) Geology of Scotland, 467-503 (Edinburgh) I SONS, J. B. (i967 ) 'Glacial stages and radiocarbon dates', Scott. J. Geol. 3, 375-81 I SONS, J. B. (i967b) The evolution of Scotland's scenery (Edinburgh) I SONS, J. B. (I967c) 'Comments on the paper by F. M. SYNGE and N. STEPHENS in Transactions no. 39', Trans. Inst. r. Geogr. 42, 163-8 I SONS, J. B. (I969) 'Drift stratigra hy nd buried morphological features in the Grangemouth-Falkirk-Airth area, central cotland', Trans. Inst. Br. Geogr. 48, 19-50 I SONS, J. B. (1972) 'Dislocation and non-uniform uplif of aised shore ines i the western part of the Forth valley', Trans. Inst. Br. Geogr. 55, 145-59 I SONS, J. B. (1974) 'A L te-gla ial ice-cap in he central G ampians, Scotland', Trans. Inst. Br. Geogr. 62, 95-114 I SONS, J. . and BROOKS, C. L. (I971) 'D ti g of early post-glacial land and sea level changes in the western Forth valley', ature Phys. Sci. 234, 124-7 TEPHE S, N. (I968) 'Late-gl cial and post-glacial s orelines in Ireland and south-west Scotland' in MORRISON, R. B. and RIGHT JR., H. E. (eds) Means of co relati of Quaternary successions, 437-56 YNGE, F. M. and STEPHENS, N. (I967) 'A reply to J. B. Sissons's comments', Trans. Inst. Br. Geogr. 42, I69-73 he late-glacial histo y of the Firth of lyde: a reply 132 J.M.GRAY This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 05:53:51 UTC All use subject to http://about.jstor.org/terms Late-glacial history Of the tirtn oJ (lyae I33

The Quaternary in Scotland: a review

Synopsis Only two interglacial pollen sites are known in Scotland, both in Shetland. There is at present no satisfactory evidence that the decay of the last ice-sheet was interrupted by a significant readvance. Decay was probably complete by 12 500. The Loch Lomond Readvance, which probably began before 10 800, is associated with excellent end moraines (often, significantly, multiple) at more than 100 locations. Former firn lines can be calculated and palaeoclimatic inferences made. The mapped limit of the readvance accords with the distribution of 58 lateglacial pollen sites. Periglacial processes currently operate on the Scottish mountains but were extremely active during the Loch Lomond Readvance, when permafrost (with frost wedges) existed down to sea-level. A summary of the sequence of lateglacial and postglacial sea-level changes (especially in the southeast) includes re-dating of the low-level rock platform and cliff of western Scotland and leads to an unconventional mode of deglaciation for the Glasgow area.

A Late Pleistocene Marine Shell Deposit at Shippegan, New Brunswick

Twenty species of marine invertebrate shells from a gravelly sand deposit close to the present shore at Shippegan, New Brunswick, were identified and shown to be typical of a shallow-water association formed soon after glacial retreat. One specimen yielded a radiocarbon date of 12 600 ± 400 yr. This evidence strengthens previous conclusions on the date of Pleistocene deglaciation and on postglacial sea-level changes in this area.

Geomorphological Development and Post-Pleistocene Sea Level Changes, Northumberland Strait, Maritime Provinces

Radiocarbon dates, continuous seismic reflection lines, echograms, bottom sediments, and surface topography are used to establish the geomorphological development and post-Pleistocene sea level changes in Northumberland Strait. The oldest recognizable topography is a pre-Pleistocene drainage system eroded into the bedrock. During the post-Pleistocene marine transgression four terraces were formed. Their ages and spatial distribution reflect the combined effect of postglacial sea level changes and crustal movement. After glacial withdrawal the strait was occupied by two estuaries separated by an isthmus. In the west, the sea initially covered much of the present land which had not yet recovered from glacial depression. Between about 13 000 and 7000 years B.P. glacial rebound caused temporary regression of the shoreline until rise in sea level again became the dominant force. In the east, rise in sea level always kept pace with any rebound, and has gradually enlarged and deepened the submerged area. Northumberland Strait became a continuous body of water about 5000 years B.P. when the isthmus to Prince Edward Island was breached. Comparison with sea level curves from other areas of the world indicate that Northumberland Strait has been subsiding during the last 7000 years. The rate of subsidence appears to have been greater in the western part of the strait than in the eastern. It is suggested that this part of the Maritime Provinces lies between areas of positive and negative crustal movement.