Journal ofCoastal Research 1132-1144 Royal Palm Beach. Florida Fall 1999
The Evolution of the Island of Neuwerk, Elbe Estuary, North Sea Alex Chepstow-Lustyt, J iirgen Ehlers[, and Gerhard Linke] t University of Cambridge :j:Geologisches Landesamt Botany School. Downing Billstr. 84 Street D-20539 Hamburg, Germany Cambridge CB2 3EA, England ABSTRACT _
CHEPSTOW-LUSTY, A., EHLERS, J., and LINKE, G., 1999. The Evolution of the Island of Neuwerk,Elbe Estuary. .tllllllll:. North Sea. Journal of Coastal Research, 15(4), 1132-1144. Royal Palm Beach (Florida). ISSN 0749-0208. ~ On samples recovered from trench profiles on Neuwerk, a little island on the German North Sea coast. palynology. euus~_~ 1f heavy metal and cesium analyses were carried out in order to reveal the historical development of the island. All three analytical approaches yielded good results with regard to the relatively recent developments. The cesium de --+ &-- terminations suggest that sedimentation during most of the last fifty years has been in the order of 2.1 rn rn/yr , but under the influence of increased storm surge heights and frequencies has recently accelerated to about 6.0-7.5 mm/yr. The earlier history of the island is revealed by stratigraphical investigations. Aclear sequence ofvegetationsuccession is recorded, which is followed by a series of plant communities maintai ned by animal grazing.
ADDITIONAL INDEX WORDS: Pollen analysis. heavy metals. cesium . Waddell Sea.
INTRO DUCTION tury, which were destroyed again before 1914. In 1927 sa nd fencing and planting of Ammophila arenaria resulted in dune Neuwerk (Figures 1 and 2) is situated in the inner part of and salt marsh form ation. The island has been eroding on the German Bight, adj acent to th e Elb e Estuary. It lies half th e NW side and growing on th e SE side ever since, resulting way betw een the outer margin of the intertidal zone and th e in a shift of well over 1 km since 19:30, i.e. about 16 m/yr ma inland coast. Tid al range in this area is slightly above 3 (LANG , 1970 ; EHLERS, 1988 ), m. According to the classification of HAYES (l975) this would suggest a mesotidal environme nt. EHLERS (l988) has intro Tri sch en (north of th e Elbe estua ry ) was first mentioned in a legal document in 1610. It developed from a 'very low sand' du ced a different classification ada pted to North Sea condi tions. According to this scheme th e area is located in the 'ma (l689) to a little island with patches of salt marsh (since crotida l' zone. No permanent barrier islands are found be 1854) and dunes (since 1885). In 1907, aft er a sequence of tween th e easternmost East Frisian Island ofWangerooge in setbacks and recoveri es, a vegetate d area of 80 ha had de Niedersachs en (Lower Saxony) and the Eiderstedt Peninsula veloped . The island was dik ed in 1925 by a farm er, but ero in Schleswig-Holste in, i.e. in a n area where tidal range ex sion in th e west outpaced accretion in th e eas t. The dik e was ceeds 2.90 m. Instead, th e are a is occupied by wide, open tid al breach ed in 1942 a nd th e polder was lost. Tri schen has shift flat s with highly mobile sand banks, a few of which have ed eastward about 1 km sinc e 1930 (LAN G, 1975;EHLERS, developed into fragil e islands in the course of th e last cen 1988). tury: Neuw erk differs from those islands in th at it consists en Mellum (betw een the Jade and Weser estuaries) was tirely of sa lt marsh . It neither developed from a sa nd bank known to be a n elevate d sand bank since the Middl e Ages; in relatively recent times, nor was it carved out of coas ta l however, salt marsh es pr otected by low dunes only starte d to wetlands in historical times like th e Hall igen in Schleswig develop shortly before 1879 (REINECK, 1987 ). The island is Holstein , but it has been kn own as a marsh island since th e highly mobil e; between 1962 and 1978 it migrated 250 m to Middle Ages, being first men tion ed in 1286. Its origina l nam e wards th e SE , i.e. approximately 15.6 m/yr. 'Nova 0 ' (Ne w Island) does not neccessarily suggest that it Scharhorn (between th e Weser and Elb e es tuaries, seeward had recently formed, since place nam es have a tradition of of Neuw erk) was kn own to be a high sand bank since th e not bein g cha nged (LANG, 1970 l. Th e island has und ergone 13th century. Geological investigations suggest that it formed some land losses, but it ha s not shifted like its three younger mu ch earlier, when th e sea-level rise reach ed th e north Ger neighbours mentioned above. However , its origin and its re man coas t, some 3,500 years BP (LINKE, 1969 ). Th e nam e lation to th e sa lt marsh development on th e mainl and coast, Scha rhorn first a ppea red in docum ents dating from 1466. It south of Cuxhaven , is not clea r (LINKE, 1979,1 982). developed dunes more th an 5 m high in th e late 19th Cen- A number of boreholes in and a round Neuwerk have re vealed th at th e island is not sit ua ted on top of a silted-over 9704 8 received 28 May 1997; accepted ill revision 28 October 1997 Pleistoc ene core (Figures :3a and h). Th ere is only a thi n cover The Evolution of the Island of Neuw erk 1133
4 0 I-: 7 0
Bllnndshuk
.,. " ~~o N IJ---+---+---+---':f---F--4"' .
NORTH SEA
Helgoland ~ 5 0
~JO Bremen
II ~ll km E3 E3 E3
Figure 1. Locat ion map of Neuwerk a nd th e north German coas t.
of salt marsh sediments overlying Holocene marine sands island was much larger (LANG, 1970). Th e salt marshes were which reach down to a depth of approximately 15-20 m (LINKE , used for cattle and sh eep grazing, and agri culture started on 1970). the polder in 1564 . The relatively sandy soil of the polder allows the production of rye, wheat, oats and beans (VON Historical Evidence LEHE, 1952). Though the Isle of Neuw erk was first mentioned in a doc Neuwerk is a marsh island, surrounded by very sa ndy tid al ument of 1286, it probably form ed mu ch earlier. Th e impres fiats. Old reports mention sanding-up of th e island on a num sive stone tower on th e island (Figure 4), erecte d by th e City ber of occasion s. A docum ent dealing with the storm surge of of Hamburg 1300-1310 as a protective measure against 1717 reports that the sa lt marshes in th e west were largely beach robbery , gave rise to its present name (Neuwerk = covered by sa nd, and that the polder field s were spoiled by New Work s) (DANNMEYER et al., 1952 ; VON L EHE, 1952, sand wash ed in through the breached dike. Heavy dam age 1973). The island was diked betw een 1556 and 1559 , and was also caused by another storm surge in 1825. A map of since th e polder area measured during the first primitive sur 1787 shows a broad belt of sand dunes at th e northern edge vey of Neuw erk in 1574 is almost identical with its present of Neuw erk. The dunes also a ppear on a map of 1854. They size, it can be assumed tha t no major changes have occurred are still featured on the first edition of th e 1:25,000 map of since. Nothing is known, however, about the exte nt of the 1878, extending to a height of 2.8-3.3 m above NN (German und iked sa lt marsh es at th at time, although allegedly the Ordnance Datum). Th ey could still be t ra ced until 1904 , when
Journal of Coasta l Research, Vol. 15, No. 4, 1999 1134 Che psto w-Lusty, Ehlers and Link e
Figure 2. Aerial photograph of Neuwerk (1989 ).
th eir remnants disappeared under coastal protection struc been completely halted by th e constru ction of th e revetments, tures (LANG, 1970). Neuw erk is now protected by a re vet and there are even signs of minor accretion in th e east. ment, completed in 1931 (UHL, 1952 ). The salt marsh es are additiona lly protected by a low summer dike of about th e Vegetation sa me age . The exis te nce of the island is not threatened as it is situated on high tidal flats a cons iderable distance from The veget ation of the undiked sa lt marsh es ofNeuwerk has th e Elbe river and othe r major channels. Coas tal erosion has been studied by TOX EN et at. (1957), MANG (1982) and R UD -
Journal of Coasta l Research, Vol. 15, No.4, 1999 The Evolution of the Island of Neuw erk 1135 s N Neuwerk
storm su rge depo sits dike tidal flats intercalated with (not to scale) salt marshes aeo lian sa nd tidal flats
MTHw .!1.-- NN clay:wilh Scrn~ (a ria MTt w . she H S bl 1 ife po s t1 1Q n . : :: :~
Holocene t idal.sands tl'le sa nd
O m grave]
i5 m Om 500 m ~=~=~ sw NE Neuwerk
polder salt marshes tidal flats di ee " and N'th sntrayerv 1" 0 1 10 soarer largel y orst c -be c bv aeone r- sand olo"gl'-.ng with bn cs;fragments
MTHw
NN
MTLw '7 ~ 0 Holocene tidal sands nne sand ~ 3 m
o 250 '"
Figure 3a and b. Geological cross sections through th e island of Neuwerk. based on data from Linke. Figure 3a is 'Profile a' in Figure 2. Figure 3b 'Profile b'
OAT (1987). It resembles that of the undiked marsh islands ium perenne and Leontodon autumnalis flourish (BURD 1989 , (Halligen) of North Fri sia ie.g , LAUX , 1988). However, in con KORBER-GROHNE 1992). Anim al grazing and the large pres trast to the latter, intensive grazing has altered the natural ence if sea bird colonies will have left traces of eutrophication. vegetation. The 180 ha of salt marshes are grazed from The salt marshes are dominated by seven vegetation com March to November by 165 cattle, 26 horses and a few sheep munities: (numbers of 1986). Plants vulnerable to trampling, e.g. Li monium vulgare, have almost vanished, whereas resistant (I) Lolium-Trifolium communities. Major parts of the salt plants like Plantago or plants furthered by grazing like Lol- marshes are rarely flooded and are thus inhabited by the
Jou rn al of Coastal Research, Vol. 15, No.4, 1999 1138 Chepstow-Lusty, Ehl er s and Linke
Upper Neuwerk
~.,
:v~ ., l~ "",,'<:' '1l~~ (l&~ ~0
30 "------30
./ 40 ------, 40
50 50
...... 60 ~ 60
70 70
80 / \ 1" 90 90
L...L ~ l-J LL.....-.L LL~1 II I II I 1,1 I ,I, I.! . I , I. I .I 1 ,I, I,l 1 o 0 20 0 0 20 0 0 0 0 20 40 60 80 0 1 2 0 5 10 15 20 25 30 35 40 45 0 5 10 15 cm2cm·3 % Figure 8a and b. Pollen diagram of selected taxa from Upper Ne uwerk.
case. Th e details of th e analyses are given in the pollen dia RESULTS gram s (Figu res 8 and 9). Over 300 gra ins of terrestrial pollen Palynology and spores were counte d. Aqu atic pollen and spores (i .e. Ty pha and S phagnum ) a re represented relative to the total sum Marine sediments are usually not an ideal medium for th e of terrestrial and aquatic types. preservation of pollen . When AVERDI ECK(1980) sampled Ho Th e sa mples from th e Upper Neuwerk section have also locene deposits on th e island of Sylt, he gave only pollen di been analyse d for As, Cd, Cr, Cu, Fe, Ni, Mn, Pb and Zn. In agrams of th e ba sal peat layers but did not interpret t he pol order to allow comparison s between samples from differently len conte nt of the salt marsh sediments. It has to be antici composed stra ta, th e determinations were restricted to th e pated th at in such material a large prop ortion of th e pollen fines. For an aly sis, the fraction < 2 u.m was sepa rate d, dried has travelled a long way prior to deposition. In a preliminary at 50°C, and the residue broken down in nitric acid/hydro test of sa lt marsh samples from Sylt, however, it was found fluori c acid under pressure. Each sample wa s an alysed twic e. that pollen preservation wa s sufficiently good to attempt pol For cesi um mea surements, samples of 5 litres soil material len analy ses of a salt marsh sedimentary sequence. were taken from th e Upper Neuw erk profile. Th ey were mea In th e undiked easte rn part of th e Neuw erk sa lt marsh es sured in the 'Ins titut fur Physik ali sch e Che mie der Uni ver (Figure 2), two profiles were excavated and sa mpled for pol sitat Hamburg' using Germanium (Li) detectors and gamma len analyses, heavy metals and cesium determination . The spectrometry. With th e long measuring period of 72,000 sec profiles (Figu res 5, 6 and 7) show an alte rnation of silt- and ond s per sa mple, th e lower bounda ry of determination was clay-rich strata with several ern to dm thick sand beds. The at 0.2 Bequ erel (Bqj /k g, seque nce differs from those found elsewhere in the Wadd en
Joumal of Coasta l Research, Vol. 15, No.4, 1999 The Evolution of th e Island of Neuwer k 1139
Upper Neuwerk
,.. Depth /~ em 0 o - -..... /' 10 "- 10 /' -- 20 / - 20
30 "---- 30
40 ./ --"- 40 ./ /' 50 50 /
60 ~ 60
70 70
80 80 " <, 90 90
I I I, I ~ ~ L LLJ.L L.1.-.l. L.L-..L- L-L l. I t , 1 .I , I, I , I,I , I , ! , !, !,!I ,I ,!, I. I 20 40 60 0 20 0 0 0 0 0 0 0 20 0 0 0 4 8 12 16 0 5 10 15 20 25 30 35 40 45 0 5 10 15
Figure 8b.
Sea (e.g. on Sylt , EHLERS et al., 1994, or on Fohr, MEYE R brought in as a result of a majo r erosional phase from COROT, 1992). The thick sa nd laye rs th ere reflect the mu ch surrounding areas, either by storm surges or by aeolia n st ronge r influen ce of storm-related events. transport. Th e layers yielded a low pollen concen tration, but contain a high percentage of Sphagnum. Th e source Upper Neuwerk for this is uncertain, but it probably originates from th e erosion of Holocene peats on th e mainl and. The low sa lt From bottom to top th e profile (Figu re 8) ma y be divided marsh community continues. Relatively high abundances into 4 broad un its: of Juncaginaceae (Triglochin maritima ) are found . The (1) Clayey silts (> 90 em). Th e silts are intertidal sediments sands were probably deposited over a very short peri od containing Che nopodiaceae (e.g. Salicornia ) and Artemi of time. sia probably derived from other salt marshes in the a rea, (3) Silts (35-55 em). Th ese silts are slightly coarser th an as well as an in situ Chenopodiacea e salt marsh com tho se of the previous phase of silt deposition. Th e Che n munity. The samples yielded high pollen concentrations. opodiaceae dominated community is replaced by a sedge The Gramineae component was low. The arboreal pollen (Cyperaceae) dom inated community, probably Scirpus were derived from long distance tran sport from the main maritimus (KORBER-GROHNE, 1992 ) which is in turn re land and can be neglected here. Th e high proportion of placed by a grass (Gramineae) community as the site be Artemisia is probably caused by A. maritima. comes drier. A first bloom of Compo sitae (Liguliflorae) (2) Sands (55- 90 em). Th e sands derive from the sandy tid al occurs, including probably both Leontodon and Taraxa flat s around th e island and are thought to have been cum spp., which corresponds with th e first high organic
Journal of Coastal Research, Vol. 15, No.4, 1999 1140 Ch ep stow-Lusty, Ehlers a nd Linke
LowerNeuwerk
~.. ~ " ~~.. ..rI JIl~ 1I~ J> .J'<:' ~-{J ..<9+ 'f>~J> ~I' ~ C1' (J1' l,~.. 50 -, 50 -, I / \ 60 I 60 / \ \ II . II II I , I ,I, I ,I I, 1, , ! I , !. I ! I ! I I , ! I I, I,I , I o 0.4 0.8 12 1.6 0 5 10 15 20 25 30 35 0 2 4 6 8 cm2 em· J "- Figure 9a a nd b. Poll en dia gram of selecte d tax a from Lower Neuwerk. concentration in the sediment, perhaps allowing this com community is domi nated by grasses which account for a ma munity to develop. The Compositae (Lig.) comm unity jor in situ component of the Gramineae in th e pollen curve, then contributes to the organic horizon until nutrients and th e environment is well drained and dry . are either exhaus ted or conditions become drier and sand Below 45 ern no traces of agriculture were found . We as ier. It is possible that the Compositae (Lig.) community sume that th e change in pollen composition th ere reflects th e may arise before the grass dominated community, the lat onset of cattle grazing. At 45-35 em depth the influ en ce of ter favouring dri er conditions. farming is rather clear. On the basis of th e pollen alone it is (4) Interbedded sands and organic horizons (0-35 ern) , At the hard to judge whether this change in composition of th e pol base of this unit is the first indication of cereals and as len assemblage indicates the onset of human interference on sociate d weeds grown on the island. They were deposited th e island , or not. The Cerealia pollen is difficult to identify. in sandy horizons, possibly developed by aeolian deposi Part of it is tho ught to represent gr ain (rye and barley), but tion. It is under these dri er, better drained conditions part may also come from Elymus, which occurs on th e island. that Armeria is observed. As expected, concentrations of The pollen of the latter can normally be distinguished, but pollen and organic material are lower in th e more sandy pr eservation in this cas e was too poor. Similarly, only part of horizons. th e Plantago pollen was prob ably derived from th e weed The percentage of grasses increases up the section, reflect Plantago major; the rest from th e very similar pollen of Plan ing th e usage of the sa lt marshes for gr azing cattle. The sed tago maritima, a plant characteristic of th e North Sea salt iments at th e top of the profile are organic rich and uncorn marshes. Leontodon maritima is most certainly favoured and pressed, but still have a sandy component. The modern plant encouraged by regular grazing (Kb RBER-GROHNE, 1992). Journal of Coas tal Research, Vol. 15, No.4, 1999 The Evolution of th e Island of Neu wer k 1141 Lower Neuwerk Depth em o o I / --- / 10 \ 10 <, \ ./ <, 20 I 20 ./ \ I "- 30 I 30 \ I \ I 40 / 40 50 -, 50 -, I / \ 60 I 60 / \ \ ! I. I LLLL..L-LLLLL I! I / I, I.I .I.I.'. I ,I ,I .I .I !I, I .I ,I o 20 40 0 20 40 0 0 0 0 0 0 0 0 0 0 20 40 0 5 10 15 20 25 30 35 0 2 4 6 e .,0' % Figure 9b. Lower N euwerk Heavy Metals The ana lysis from the second site, Lower Neuwerk, has Significant changes in th e environmental conditions are re produced a different picture (Figure 9). Whereas at Upper flected in th e geochemistry of the sedime nts . Under favour Neuw erk clear patterns of vegetational succession and chang abl e conditions, such changes can be used for dating th e sed ing plan t communities are recorded by th e pollen , at Lower ime nts in question. In a number of cases heavy metal con Neuwerk th e composition is rather uniform throughout the centrations have been utilised to date tid al and sa lt marsh profile. Th ere is only one distinct Compositae (Lig.) peak at deposits (e.g. ALLEN, 1990, 1991 ; DOMINIK et al., 1978; the base of the profile, and th e organic horizons have a much SCHWEDHELM , 1984; IRION and MULLER, 1990). Th e heavy lower content th an the upp ermost section of Upper Neuw erk. metals in th e sediments of th e German Bigh t have tw o sourc Cerealia pollen is present throughout the profile, so from the es: a geogenic one (background) and an indust rial one. onset of deposition of this sedimentary seque nce farm ing was Th e diagrams (Figu re 10) show increased conten ts of lead already well esta blished on the isla nd. (Ph), nickel (Ni), cadmium (Cd) and copper (Cu) in th e up per Alt hough the sites are less th an 200 metres apart, th e pro part of the profile. Th e Cd, Ni, Cu and Pb peak at 10 ern depth files are so different th at no correlation of individua l storm does not neccessarily mean that pollu tion has decreased in layers was possible. Depositional events in these environ recent years. MEYERCORDT (1992 ) suggests, that the de ments, including deposition of orga nic detritu s, are predicted crease in the upp er part of the sa lt marsh profiles reflects to be spatially discontinuous. On the whole, the Lower Neu leaching and tran slocation down profile. Leaching is clearly wer k sequence is interpreted as an equivalent of the upp er reflected by the curves for iron (Fe) and man gan ese (M n ), In 45 ern at Up per Neuwerk , or at least part of it. contrast to MEYERCORDT'S test areas on Foh r and Nor d- J ournal of Coastal Research, Vol. 15, No.4, 1999 1142 Che pstow-Lusty, Eh lers a nd Linke As Pb Cd Cr 20 40 60 80 a 400 800 1200 1600 a 0.2 0.4 0. 7 0.8 a 20 40 60 80 Oem J I Oem J Oem J I Oem J 1Qcm 10 cm l Q cm 10cm 20cm 20 em 20 em 20cm 30 em 30 em 30 em 30cm 4Q cm 40 cm 40 em 40 em 50 em 50 em SO em SOem 6Dem 6D em 60 em 6D em ?Oem lDcm lO cm 70 em SDem eoem 80 em aDem 90 em 90 em 90 em 90 em 100 em - Figure 10. Hea vy metal analysis of the Upper Neuw erk secti on. strandischmoor (for location see Figure 1), ab solute back sand content of the uppermost sa mples is considerably high er ground an d pea k values on Neuwerk were much higher. The than that of th e underlying layers (80.5 and 75.5% vs. 40.0 reason is that M EYE RCOR DT used th e < 20 urn fraction for and 49.0%), indi cating more rapid sedimentation. his analysis, whe reas we used the < 2 p.m fraction measured on th e island of Sylt (E HLERS et al., 1994). Before that date DISCUSSION AND CONCLUSIONS sedimentation seems to have been considerably slower . If the onset of bomb testing at 1952 is considered to be represented This investigation has shown that pollen analysis, heavy in the profile at a depth of about 11 em, the area in question met al and cesium determination can contribute to th e un had experienced vertical accretion of only about 2.1 mm/yr der standing of the development of coastal salt marshes. between 1952 and 1986. This seems plausible , because the Since th e island ofNeuwerk is situated on high sand y tidal Cu Fe Mn Ni Zn 20 40 60 80 a 30.000 60 .000 a 100 200 300 a 40 80 120 160 a 100 200 Oem J I Oem J Oem J Oem J Oem J 10 em 10 cm l Ocm 10 em 10 cm 20 em 20 em 20cm 20 cm .-- 20 em 30 em 30cm 30 cm 30 cm 30cm 40 em 40 cm 40 em 40 cm 4D cm SO em SO em SOem 50 em SOem 60 em 60 em 60 em 6D em 6D em l Ocm l Dcm l Ocm l Dcm l Dcm accm Bfl cm 80 em Bucm Bn cm 90 em 90 em 90 em 90 em 90 em 100 em - 100 em - 100 em - 100 em --- 100 em - Figure 10. Continued. Journal of Coastal Resea rch, Vol. 15, No.4, 1999 The Evolution of the Island of Neuwerk 1143 Cs Pb Ni Cd o 400 800 1200 1600 40 80 120 160 0.2 04 0.7 0.8 o 10 20 30 40 50 Bqlkg Oem ~ I , Oem -' , I I I II Oem Oem .J I r I I , not determined not determined not dete rmined 5em 5 em 5 em 5 em 10 cm 10 ern 10cm 10cm 15 em 15 cm 15 em 15 em - 20cm 20 em 20 em 20 em - 25 em 25 em 25 em 25 em - Figur e I I. Cesium content of the Upper Neuwerk section, compa red to Ph, Ni and Cd. flats, the salt marshes were repeatedly affected by a strong needed to provide a compre hensive interpretation of the evo input of sand, both by storm surges and by aeolian redeposi lution of the island. tion. Until land reclamation in the northeast started in 1929 frequently sa nd was blown in from the high sa ndy tida l flats ACKNOWLEDGEMENT east of th e islan d, the so-called 'Kleine r Vogelsand'. Salt marshes are characterise d by rhythmic sedimentation Thi s study was funded by the EPOC sche me of th e Euro largely controlled by extraordinary events. Wher eas storm pea n Comm unity . Petra Schmidt has kindly done th e cesium surges and major aeolia n inputs are recorded in well-d evel deter mination for us. We would like to thank Pro f. Behre oped layers, yea rs of calm conditions are probably represent (Wilhelrnshaven) for critically discussing the resul ts of the ed by a hiatus. The variability in sedimentation rates is high pollen analysis wit h us. In Cambri dge, valua ble ass istance lighted by the uppermost te n centimetres of the profile. The was provided by Steve Boreh am , Phil Gibbard and J ill Dye. most recent sediment layer reflects the increased storm surge We are grateful for th e use of facilities at the Department of frequency of the last decades. Plant Sciences, and the pollen plotting program Psimpoll of Grazing has a clea r influence on th e plant communities of Keith Benn et t. This work was well co-ordina te d by Prof. Da the sa lt marshes, the presence or absence of which can be vid Smith at Coventry University, and Charlotte Jenkins checked. Pollen analysis suggests th at at Upper Neuw erk a gave excellent help wit h the field work, an d was funded by sedimentary sequence is preserved which might reach back the Montgomery Trust. beyond the time whe n farming started on th e island. Th is interpretation is supported by borehole data which show that liTERATURE CITED th e sa lt marsh sediments of Neuwerk only form a th in blan ALLEN, J .R.L ., 1990. Salt-marsh growth and stratification: A nu ket covering intertidal sands. However, the sedimentary re merical mod el with special reference to the Severn Estuary, south cord is highly discontinu ous. Further investigations will be west Britain. Marine Geology, 95, 77-96. ALLEN, J .R.L., 1991. Sa lt-marsh accretion and sea level movement in th e inner Severn Estuary, southwes t Britain: the archa eological an d historical contribution. Journal of th e Geological Society Lon Table 1. Measured contents of 1·I7C S,D .IC S and J ournal of Coasta l Research, Vol. 15, No. 4, 1999 1144 Chepstow-Lusty, Ehlers and Linke DELAUNE, R.D.; BURESH, R.J., and PATRICK, W.H., JR., 1978. Sed LINKE, G., 1970. Uber die geologischen Verhaltnisse im Gebiet Neu imentation rates determined by t:ncs dating in a rapid accreting werk/Scharhorn. 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