STRATIGRAPHY AND DATING OF HOLOCENE GULLY SEDIMENTS IN OS,WESTERN NORWAY
I.]IVIND SONSI'EGAARD& JAN MANGERUD
gully S@nstegaar E. Sjtrtslcgctard & J. Mongenrtl, Geologisk ittstittttt' uvd' B' Alllgt' 1i ' N-5011 Berpen Utiversitet, Norwdy. In those parts of Norway coveredby marine silt and clay of Late Weichse- lian-Holocene age, gullies and landslide scars are the most common land- forms, besidesthe river valleys.The main areasol marine silt and clay are the lowlandsin easternNorway (around c)slo,Fig. 1) and Tr@ndelag(around Trondheim, Fig. 1). In western Norway the fiords and valleys are so steep that only small areas were inundated by the sea, despite the marine limit over large areasbeing 50-100 m a.s.l.The land areas covered by silt and clay are, therefore,less than in easternNorway and Tr@ndelag,and nearly alwaysconnected with ice-marginaldeposits, as in the presentarea' The clay slides in Norway are often connected with catastrophic rui- nation of farms and buildings,with the result that there are historical data from far back in time. Many geologicaland geotechnicalstudies have been performed to clarify the age, cause,and dynamics of the clay slides (e. g. Holmsen 1953, Rosenqvist19-53, 1955, Bjerrum & Rosenqvist1956, 1957, J@rstad1968. Bjerrurn 197 l). The formation of gullies,on the other hand, is a continuousprocess that does not causeconspicuous destruction over short periods of time. There- fore it has received less attention than landslides,but is perhaps as important an erosionalagent when consideredin the longer term. In Scandinaviathe most extensivestudies on gullies have been carried out along the river Daldlven in Sweden(von Post 1935, H@gbom1905, De Geer 1914, Caldenius1926, Frendin 1931, Wenner 1941, Lundqvist & Hjelmqvist 1941, Lundqvist 1951). 314 E. S(INSTEGAARD & J. MANCERUD Lundqvist& Hjelmqvist(1941) dated the formationof one of these,thc Norrhyttan gully, by a method which in principle is the same as the one we have used.Sand layersdescendin-e from the Norrhyttan gully were identified in cores taken from a peat bog at the gully foot. These layers were dated with the aid of pollen analysis,showing that the gully had formed during three periods(ca.3,700, 1,300,and 1,0(X)years B. C.) after the area had been raised above sea level. Lundqvist & Hjelmqvist assumedthat thc for- mation of the Norrhyttan gully was connectedwith heavy precipitation, and that the climate had been moist during the three mentioned periods. Similar studiesof fluvial sedimentsbelieved to have originated from gul- lies, in Szetersdal, indicated that these,for the most part, had formed ca. 400 yearsA. D. (Lundqvist & Hjelmqvist 1941). We have investigateda systemof gullies in Os, Hordaland (Fig. 1 and 2), putting main emphasison the sedimentsoriginating from the gullies, and on age of gully formation. These gullies end in the lake Banktj0rn (Fig. 2)' which receivesvirtually no mineroger.ricsediments from sourcesother than the gullies and a little brook. This situation offers a unique possibility ttr identify and date the gully sediments. The content of organic matter irr these sedimentswas too low for radio- carbon dating. We have therefore constructedpollen diagrams, and dated the sedimentsthrough correlationwith radiocarbon-datedpollen boundarics from nearby localities.The pollen analysisalso gave some resultsconcerning distribution of pollen within the basin, and concerningthe developmentot the vegetation. ' -ll l A \' .car.ea^d:,a'r? ] 'lo[ ,.%'t\n{^\4 i! 0,,, "i?!/"i"_h,f"Io / ) (.j; :'a'ql(---?(, v i --...ii* , lJ-,0 i )fi A sEA Y/o-'t i ,11: \.,,ri,"l K***)oz\".",,.y? il:r \*u11, ,li QL-=\+/ --Rtn\ JpJ' A' /.r"L' ...+j\ati\ \" , ) q""! I ..Jv fr'/uYi _ Fig. 1. Map of the Bergen-Os area. lnsertcd a key nlap of southern Norway HOI-OCENE GULLY SEDIMENTS IN OS, W. NORWAY 315 Methods Coring A pistan corer with diameter54 mm (Geonor A/S, Grini M@lle,Box 99 Rpa, Oslo 7, Norway) was used for all sampling.We did, however, increasethe length of the samplingtube from the standard 88 cm to 208 cm. The lami- nation of the sedimentswas virtually undisturbed(Figs. 7 and 8), indicating that the constructionof the sampier is very good. The compressionof the studied silty sedimentsvery seldom exceeded5 96. About 70 m cores werc sampledwith this equipmentin and around the lake Banktjprn. Core I (Fig. 2) was taken through the lake ice during winter. The coring frame was placed on two 4 m long wooden beams 4" x 4" , and fastenedto the ice by means of chains. The sevencoring localities are shown in Figs. 2 and 3. Cores 1-4 cover the entire investigatedSequence and a pollen diagram was constructedfor each (Figs. 15-18). From cores 5-7, sampleswere taken only at selected clepthsto locate important lithostratigraphicalboundaries. For calculationof sedimentvolumes, some supplementary motor soundings were performed. Grain-size analysis Grain-size analysiswas performed only on sedimentswith loss on ignition o/c' below 10 '/c. Organic matter was removed with l0 H2O2 solution. The sampleswere first washedthrough a 0.063 mm screen.The coarsermaterial was sieved,while the fines were analysedby the pipette method (Krumbein & Petrijohn 1938). The fines were not dried before the pipette analysis, '0-sample' and the total weight was found by means of (Aarseth 1911). Loss on ignition The loss on ignition (Figs. 15-18) was determinedby ignition at 750oc for one hour. This has been the standardprocedure in Geologisk institutt, uni- versiteteti Bergen (Mangerud 1970), although most other laboratoriesuse 550.C (Birks 1970, Saarnisto1970, Berglund & Malmer 1977). To test the differing significanceof these two ignition temperatures,153 sampleswere igniteclat 550'c and thereafterat 750o c. The mean differencein loss on ok ignition between these two temperaturesfor all sampleswas 0.90 with a standard deviation of 0.44 Vo. To show how much of this difference is due to further combustion of organic matter, the sampleswere grouped in o/o, three categories,those with loss on ignition values(750'C) between0-5 5-50 ck, and 50-100 % (Table 1). The resultsgiven in Table 1 show that the differencein loss on ignition is least for the group containing most organic material. This indicates that the latter is almost completely combustedat 550'C. Consequentlythe in- creasein loss on ignition at 750'C is essentiallydue to loss of water from clay minerals (Ekstrpm 1927) and possibly some releasedCO, from car- 316 E. SONSTEGAARD & J. MANGERUD Table.l . Mean differences with one standard deviation between loss on ignition at 950'C. 750'C, and 550"C. Mean difference of Mean difference of Loss on loss on ignition loss on ignition r,.--L^- ^r Numbe,r ol Number or ignition at 750'C and at 950"i and samples samPles. 750" 550"C, with one 750"C. with one analysed analysed c/o standard deviation srandlrtl deviation c/o % 0-5 0.77 + 0.36 73 0.67 + 0.t7 8l 5-50 1.12 + 0.40 50 0.90 + 0.33 ]I 50-100 0.64 + 0.31 30 0.78 + 0.40 7 0-100 0.90 + 0.44 153 0.76 + 0.28 139 bonates and S from ferrosulfides(Andersen 1961, Digerfeldt 1975). It is concluded that an ignition temperatureof 550'C is preferable. 139 sampleswere also ignited at 950'C. The mean difference between loss on ignition at 950"C and 750'C, 0.'76 o/b,is probably due to chemical reactions other than combustion of organic matter. Standard deviation in the last casewas 0.28 c/o. Preparation of pollen All samplescontained minerogenic particles which were removed according to the HF method (Fregri & Iversen 1'966), the heavy liquid (bromoform) method (Frey 1955, Vorren 1972), or a combination of these two. The re- maining material was treated with the acetolysismethod (Fregri & Iversen 1,e66). Most of the samplestreated with the HF method were stored severaldays in cold 35 o/a HF, and thereafter treated with 10 7a HCL etc' In the clay- rich samples it was, however, often difficult to remove all the particles (fluorides?)which were precipitated during the treatment. During the last few years, the bromoform method has been used with successin Geologiskinstitutt, Universiteteti Bergen (Vorren 1972, S@nste- gaard 1974, Skir 1975, Mangerud 1977). Vorren (1972) compared the re- sults of the HF method and the bromoform method, and found no signif- icant differencesin pollen frequency. Hystrix was, however, under-repre- sented in samplestreated with the bromoform method. Skir (1975) found that Hystrix often lost the characteristicspines during the bromoform sep- aration. Other systematicdifferences may also result from the preparation tech- niques, and the techniquesused are therefore indicated in the last column in the pollen diagrams. PoIIen identil i cat io ns The pollen analyseswere performed in Botanisk Museum, universitetet i Bergen, and the identificationsrefer to the referencecollection in that de- W' NORWAY 3L'I HOLOCENE GULLY SEDIMENTS IN OS' 9t 4,rc \-t r)riliiL il/ ', / '?' \$\) )'l t-.. ill,,{(rq( :-\' 5!-i,' 1r; 18) r :ir K :--r/ (l* -.:\ / / )\ <;sv (\\l'rj,7\ ,-i - \-7 \\Y,2,, N (- Locuehine s€dim€nB, Pdrll/ covere-d by peot, indicoring infilled Portsol I lok€ Bonktitrn --- Woter5h€dof the Bonktiorncdrchment ,-%H 3. Corinslocoliry $$ ------Brookin funnel 1' ocalisation of the map is marked at Fig' f use of phasecontrast and objec- partment. Routine counting was done with determinations'immersion oil ob- tives 2510.45or 40/0.6S.io' difficult jective100/1.30 was used' 'uniaEntified mainly of grains unidentifiable The group pollen' consists 3I13 E. SONSTECAARD& J. MANCERUD due to corrosion, etc., with addition of a few unknowns. High percentages of unidentified pollen can result in considerableerrors in the diagram, be- causethe preservationof pollen variesbetween the species(Andersen 1966: 269, Cushing 1967, Birks 1970). In the diagramsfrorn Banktj@rnthe cate- 'unidentified gory pollen' constitutes 1-30 % of the total pollen, but in most of the spectrait is below 15 o/o. The group cf. Populus may contain some pollen of Taxus. In several samplesit was very difficult to distinguishbetween the two species.Taxu^s occurs quite seldom in fossiliferousmaterial in western Norvay and then only in small amounts (Hafsten 1965: 46-41). On the basis of the rela- tively high percentages,e. g. in Fig. 17, it is most likeiy that the pollen grains in question are Populus. Descriptionof the area Os is located on the outermostpart of Fusafjorden,20 km south of Bergen (Fig. 1). The investigationswere carried out around the lake Banktjgrn, 1-2 km inside the outermost depositsof the Herdla Moraines (Aarseth & Mangerud 1974).The marine limit, of Younger Dryas age,is ca. 60 m above the present sea level, while Banktjdrn lies 2I m a. s. 1. The river Oselva flows in the same main valley as Banktj@rnis found, but the river does not flow into the lake (Fig. 2). The lowest pass point between Oselva and Banktj@rnis 31 m a. s. 1., i. e. only 4 m above the river at the actual site. Nevertheless,no evidencehas been found to suggestthat the river has flowcd over the passand into Banktj@rnafter the passemerged from the sea. Banktj@rnoccupies an area of 0.12 km2, and its catchment area is about t 1-z (Fig. 2). Above the marine limit there is mainly bare bedrock with a thin discontinuouscover of soil, mainly weatheringproducts and raw humus. In lower lying parts, there occur essentiallyglaciomarine silt and postglacial marine and lacustrinesediments (Fig. 2). The vegetationconsists of pine forest on the bare hills surrounding the lake. On the silt and bog along the lake shore are copsesof grey alder (Al- nus incana), black alder (Alnus glutinosa), and birch (Betula pubescens). Most of the silt area is. however.cultivated. Guliy erosionprocesses In the glaciomarinesilt there are severalgullies. Some of them are connected and form systemsof gullies,all of which empty into Banktj@rnor into parts of the lake that are now filled in with lacustrine sediments(Fig. 2). The largest systemof gullies,named the Flfiten gully, ends in the southern part of the basin. It is 450 m long and up to 7 m deep (Figs. 2-5). Long profiles of the gullies are graded and the cross profiles are U-shaped(Fig. 5), indi- cating a mature stageof development. Detailed investigationsof the processeswhich have formed thc gullies HOLOCENE GULLY SEDIMENTS IN OS, W. NORWAY 3t9 Fig. 3. Air photo of the Fliten area seen towards SW (compare Fig. 2). Coring sites L to 7 are indicated. The Flaten gully is marked by a stippled black and white line. In the background lake ulvenvatn to the right and the sea (Fusafjorden) to the left. May 1975. have not been undertaken, but some conclusions can be drawn from the forms and the redeposited sediments. In the gullies no springshave been observed,and ground water erosion is believed,therefore, to have been negligiblein the formation of thesegullies. Fig. 4. Stereophoto of the Flfrten gully. The gully ends at coring site 3. Photo Wider0e/Fjellanger 1966. 320 E. SONSTEGAARD & J. N4ANGERUD 'J":'ir trm lss .1 "* ,v ,/--. \i ,/d.ai,x ls d -.r'/ -'-./ j'o t" r3s :l!/- x:"9r 4-v!!r-:g 9 a-*g!otgslr!! ' F'9ed:tnl .*F' -----'ei"*",;ii'g,N-ulr* , 8800 3oom 4oom -Io right is indicated the position Fig. 5. Long and crossprofiles of the Flaten gully' the cxaggerated. oflhe shorelineat differenttimes. The vertical scaleis 5 times fine grained (Fig' This is also consistentwith the glaciomarinesilt being so water will run off 6) and impermeablethat the predominantamount of rain on the surface. to the gully Small landslidescan, on the other hand, have contributed with an average formation. The sensitivityof the glaciomarinesilt is high' fresh' landslide value of 60 for five measurements.Two small, relatively probably small slides scarsare observedon the east side of Banktj@rn,and however' no per- took place during the entire postglacialperiod. There are, derived from the cepti;le scarsfrom landslideswithin the gullies. The silt siltl it giti", (lacustrinesilt, Fig.6) is better sorted than the glaciomarine (Figs' 16, 1'7)' hus un even lamination and a fairly constantloss on ignition not observed. All and structures indicating larger landslide activity were suspensionduring these observations imply a continuous sedimentation from the entire Period. in easternNorway Nordseth (1974 and pers.comm.1975) emphasizesthat while the main the main gully erosioniakes place as lateral channel erosion movement' On part of the sedimentis delivered to the river by slow mass this is also the case the basis of the given observationswe conclude that for the Flflten gullY. LithostratigraPhY (informal) lithostrati- The sedimentsin Banktjorn can be sub-dividedinto six Iacustrine silt' graphic units (Fig. 11): till, glaciomarine silt, marine silt' lacustrinegYttja, and Peat. Tiu which we obtained The lowest bed consistsof a compact gravelly silt, of onlvsmallsampleswiththeboringequipmentused.Thethicknessofthis HOLOCENE GULLY SEDIMENTS IN OS, W. NORWAY 32I 9876543210-l-2-3phi 14rz 1/tz 1/to t/s V4 V2 | 2 a 8 mm 2 I I 16 31 63 IOOO & micron Unbroken line: Lacustrine samples. Stippled line: Glaciomarine samples. Fig. 6. Grain-size distribution of sediments from Banktj@rn. Curves from the most fine- grained and coarse-grained samples analysed from each of the two beds are shown. bed is therefore unknown. The sedimentcontained shell fragments and we assumethat it is a shell-bearingtill. Similar shell-bearingtills have been observedin several excavationsin the surrounding area (Undas 1963, H. Holtedahl 1964, Mangerud 1970, Aarseth & Mangerud 1974, S@nstegaard 1914). Glaciomarine silt Above the till is a bed of laminated sediments(Fig. 7). The predominant grain sizeis silt, the microfossilsare of marine origin, and we assumethat ii was depositedjust beyond the front of a glacier. Therefore we have named it glaciomarinesilt. The clay content in the glaciomarinesilt is up to 26 o/o(Fig. 6). Coarsc silt and sand occur in the lower part, thus causinga generalfining-up within the unit. The lamination is due to alternating grain sizes(mainly clay/silt) and some graded laminae,frequently ranging from sand to fine silt (Fig. 7). 322 E. SONSTEGAARD& J. MANGERUD Depth (m) r0.10 r0.20 Fig. 7. Glaciomarine silt from core 4. Some of the laminae are sraded. The lamination is distinct in the lower part of the unit but becomesgradually less obvious upwards. Small shell fragments of sand size occur in the layers of sand. The amount of other organic material in the silt is insignificant. Black zones can be found spread throughout the whole unit. These are most likely the result of finely distributed iron (II) sulphide. The glaciomarine silt has a thickness of 2.5 to 8 m in Banktj/rn, and the bed continues above the lake shore almost up to the marine limit (Fig. 2). This bed is the parent material into which the gullies are eroded. The pollen spectra from this silt (Figs. 15 and 17) show that it was deposited before the expansion of the birch, and that the vegetation in the vicinity was a nearly tree-less tundra vegetation. This indicates that it was deposited during, or very soon after, the deglaciation of the area. The rami- nation and the obviously very high sedimentation rates indicate a deposition immediately in front of the glacier. Marine silt The lamination in the glaciomarine silt disappears gradually upwards with an even transition to a non-laminated silt with a thickness of 2-3 m. The micro-fossils and shells indicate that this silt was deposited in a non-glacial marine environment, and it is therefore named marine silt. The sediment HOLOCENEGULT-Y SEDIMENTS IN OS,W. NORWAY 323 is slightly coarserthan the main part of the glaciomarinesilt and the sorting is somewhat poorer in the coarsest fractions. The continuous transition from laminated to non-laminatedsilt is probably due to the gradual change in the environmentwhich took place as the glacier retreatedand the supply of the glacial detritus diminished. Lacustrine silt The most distinct lithological boundary is between the marine silt and the next bed, called lacustrinesilt. The microfossilsshow that this is the bound- ary between marine and lacustrine sediments.All sediments below this boundary have greyish or greenishcolours, while all sedimentsabove have brownish colours due to content of organic matter. In the lacustrinesilt the content of organic matter is, however, generally low, the loss on ignition being below 10 o/c. The bed termed lacustrinesilt consistsessentially of laminated clay, silt, and gyttja silt, including a few layers of sand and gyttja' The lamination is most distinct just above the lower boundary (Fig. 8). No laminae are clearly graded.There seemsto be a coarsening-upwithin the unit (Fig. 17). Similar black zonesto those in the glaciomarinesilt occur in this bed' Lacustrine gYttia The boundary betweenthe lacustrinesilt and the next bed, named lacustrine gyttja, is not distinct.Within the lacustrinesilt the content of organic matter, and thereby the brown colour, generallyincreases upwards. we have placed the boundary at a marked increaseof the brown colour, which appearswhere the loss on ignition exceeds10 %. For the later discussionswe will stress that also in lacustrinegyttja, silt is a major constituent.In the ccntral part of the lake (Fig. 15) the boundary is least clear and the loss on ignition is only 10-15 7o inthe lacustrinegyttja. Outsidethe mouth of the Flaten gully the loss on ignition is usually 20-40/c (Figs.16 and 18). Also in the lacustrinegyttja lamination occurs,but is less distinct than in the lacustrine silt below. Peat A thin layer of peat covers most of the lacustrinesilt and gyttja south o1' Banktj@rn(Fig. 11), but is missing along the shore of the lake. The peat indicates that no sedimentationof silt occurs in the southern part of the area today. The gully sediments The silt which has been eroded from the gullies must have been deposited in Banktj@rn,and we refer to these sedimentsas the gully sediments.It is suggestedthat the identification and dating of these sedimentsalso dates the sullv formation. 324 E. SONSTEGAARD& J. MANGERUD F J a lrj = E F a :) (J ) F J a LU z E 6.90 Fig. 8. The sharp boundary between the non-laminated marine silt and the laminated lacustrinesilt in core 3. The gullies were eroded into the glaciomarine silt; therefore the gully sedimentsmust lie stratigraphicallyabove this unit, that is, in the marine silt and/or in the lacustrinesilt and gyttja (Fig. 11). Four argumentsindicate that the gully sedimentsoccur almost exclusivelyin the lacustrinesediments: The elevation of the gullies. - At the end of Younger Dryas Chronozone (10,000 years B. P.) the shorelinewas 60 m higher than at present in Os (Aarseth & Mangerud L974).The isolation of Banktj@n (21' m a. s. l.) from the sea is dated to 8,800 years before present by meansof pollen analysis' In Fig. 5 the elevation of the shoreline at different times is indicated, as- suming an even emergencebetween the two given points. The gullies lie between2l and,45 m a. s.1. (Fig. 5), and therefore the formation cannot have begun until ca. 9,500 years B. P. However, the main part of the gully volume lies within the lowermost 1-0-12m, and was therefore only exposed for some 300 years before Banktj/rn was isolated from the sea.It is there- fore reasonableto concludethat the erosionof the gullies started during the 325 HOLOCENEGULI-Y SEDIMENTSIN OS,W. NORWAY have been completeddurir-rg marine sedimentationphase, but could scarcely this period. (no' 3) was taken from imme- Grain-size distribution. - One of the cores diatelyoutsidethemouthoftheFlltengully.Thelacustrinesiltiscoarser herethanintheothercores(Fig.9),indicatingthatthesitewascloseto The marine silt' on the the sediment source during the time of deposition' than further north and south' other hand, is finer outside the Fliten gully sediment' indicating that the gully was not the source of this Sedimentdistribution'-Themarinesiltbedandthelacustrinesiltbed patterns' The thickness of the have very different thickness distribution of gullies (Fig' 11)' sug- lacustrine silt is especiallygreat at the mouths marine silt, on the other gestingthat it was depositedfrom the gullies.The basin (Fig' 11)' At the gully f,and, has an even thicknessover the entire outletsintheNWthethicknessofthemarinesiltisevenlessthaninthe centralParts of the basin' - third paragraphsabove' it Estimation of the volttmes. In the secor.rdand m rcron LACUSTRINE SILT ! OJ tn + -ru 20 qr c = rg C z m OT 20 MARINESILT Corenumber samplesfrom eachcore of lacustrine Fig. 9. Histogramsshowing the meanof the Md for in bracketsgive the number of samples silt and postglacial-or,ri" .itt. The figures analysedin eachcase 326 E. SONSTEGAARD & J. I4ANGERUD IOTALAREA FLAT E N GULTY l--J-] o tr n9 ;. q) E) = 6 I v7?vv7 Votume ( m of guttieseroded sitt ) Votumeof minerocenicIacustrrne sediments Fig. 10. Comparisons of the estimated volumes of gullies and of minerogenic lacustrine sediments. is indicated that all of the gully sedimentslie in the lacustrine sediments. During the lacustrinephase Banktj@rnreceived minerogenicsediments al- most exclusivelyfrom the gulliesand a small creek in the northern part. Out of Banktjdrn there is practically no sediment transport. The volume of eroded silt in the creek valley and the gullies must therefore be equal to, or slightly larger than, the volume of silt in the lacustrine sediments in Banktj@rn. The presumed silt surface before the erosion started has been recon- structed through field work and the study of aerial photographs.The vol- ume of eroded silt has then been calculatedas the differencebetween the present surface and the primary surfaceby means of maps with a scale of 1:1,000with equidistancebeing 1 m. This volume is calculatedto be 310,000 ms (Fig. 10). The volume of silt in the lacustrine sediments,480,000 m3, was calculatedon the basis of sevenbore holes (Fig. 2) and has been cor- rected for loss on ignition and density in relation to the glaciomarine silt in which the gullies are formed. The calculationsare, of course, only rough estimates,but indicate (Fig. 10) that the lacustrinesilt and gyttja are able to contain all the gully sedi- ments. Correspondingcalculations are carried out separatelyfor the FlAten gully. These calculations are more precise since the gully is more well- defined and more coring was performed in the sedimentsassumed to be derived from this gully, i. e. the sedimentssouth of the lake outlet (Fig. 2). The estimatedvolumes of silt eroded(130,000 m3) and accumulated(140,000 m") from the Fliten gully are almost identical (Fig. 10). HOLOCENE GULLY SEDIMENTS IN OS, W. NORWAY JLI The conclusionis that all the gully sedimentslie within the lacustrine silt and gyttja in Banktj/rn. For the Fliten gully one can conclude even more preciselythat the minerogenicpart of the lacustrine silt and gytlja is iden- tical to the gully sediments. Dating of the erosion of the Fliten gully The content of organic matter in the sedimentsof Banktj@rnis too low to determinetheir ageswith the 1aCmethod. Dating was therefore carried out by meansof pollen analysis.Well-defined pollen boundarieswhich are dated by the laC method of lacustrine gyttja from nearby areas are used. The dating and the pollen diagramsare discussedin further detail in the chapters on biostratigraphy. Four pollen diagramshave been constructed,three of which encompass the Fliten gully sediments(Figs. 16-18). Since all of the diagramsare from the samelake, the pollen boundariesare synchronous(Fig. 11). The main resultsare given in Figs. l-1 and t2. The depositionof the Fliten gully sedimentsbegan 8,800 years B. P. At the mouth of the gully (core 3, Fig. 11) the sedimentswere depositedvery quickly (4.3-6.L mm/year) and the sedimentsurface reached the water level alreadyby 7,700 years B.P. The content of organic matter (loss on ignition, Fig. 17) in the sediments at the mouth of the gully is very low. Thereafter the sedimentationtook Elmtifi Fig. 11. Stratigraphic correlations between cores 1-5 in Banktjprn. The lithostratigraph- ical units are indicated with different signatures. Time synchronous pollen levels are indicated by lines with age. Co : The Corylus rise, Cpl*. : The Corylus maximum, Ao : The Alnus rise, Qo : The Quercus rise. E. SONSTECAARD& J. MANGERUD - s-tooo . 5 ?29_aa p 8t00BP 88c0BP Fig. 12. Sedimentation rates of the minerogenic component of the lacustrine sediments in Banktj@rn during different periods. The values are thus corrected for contents of water and organic matter. The lines connecting the boring sites are the synchronous levels given in Fig. 11. place towards the north (core 2, Fig. 11) - where the content of organic matter soon after increased(Fig. 16) and the sedimentis termed lacustrine gyttja - and towards the south (core 4), where the content of organic mat- ter (Fig. 18) was high right from the start. Thus the depositionof the gully sedimentscan be describedas a buildins out of a delta outside the sullv mouth. The stratigraphicalposition of the Corylus maximum (Fig. 13) - and thereforethe 8,400 isochroneline (Fig. 11) - is uncertain. Accordingly the sedimentationrates (Fig. 12) based on this age can be slightly changed. Nevertheless,the sedimentationrate in core 3 must have been lower during the first part of the lacustrinephase than during the following period (Fig. 12). The explanationis probably that the erosion increasedgradually as the side branchesof the gully were developed. In cores 2 and 4 deposition was naturally much slower during the first period than in core 3 (Fig. 12). It is, however,very striking that the rate of depositionof minerogenicparticles at these two sites did not increaseafter the sedimentswere built up to the surface at core 3 either, even though it is at these sites that the material from the Fliten gully was now deposited (Fig. 11). Therefore it is quite clear that the erosion in the gully diminished stronglyafter ca. 7,700 B.P. -= Denudationrates Accepting the results above, the sediment transport and denudation rates for the drainagearea to the Fliten gully can be calculated(Table 2). Prac- tically all erosion has taken place in the silt area, and the erosion rates within this area are therefore calculatedseparately (Table 2). Measurementsof sediment transport in several Norwegian rivers have HOLOCENE GULLY SEDIMENTS IN OS. W. NORWAY 32e Table 2. Erosion rates calculated for the Flflten gully, compared with present-day erosion rates in Mpnsterelva, eastern Norway (Nordseth 1974). The asterisksdenote valuescalculated from informationin Nordseth(1974). SPecific Drainasc Denud- " seclment Fluvial system area auon rate transport (km2) (mm/Year) tr.i*t lv."tl FlAten gully, 0.19 975 0.65 8,800-7,700B. P. total area Fliten gully, 0.085 2r75 1.45 8,800-7,700B. P. silt and clay area M/nsterelva E +W, 27.2 420* 0.28* 1970 total area M@nsterelvaE +W, 24.5 /14* 0.31* L970 silt and clay area been carried out during the last few years (Ziegler 1974, Nordseth 1,974). Of these,the two brooks M@nsterelvaEast (E) and M@nsterelvaWest (W) (Nordseth 1974)in a clay area east of Oslo (Fig. 1) are most comparableto the Fliten gully. In Nordseth (1974, Table 48) only specific suspensiontrans- port is listed. To obtain the total sediment transport in M@nsterelva,we have added 10 o/oto the suspensiontransport values due to bedload (cf. Nord- seth 1974: 156). Between 8,800-7,700 B. P. the specific sediment transport in the Fliten gully, 975 t/kme year, was about twice the value measuredin M@nsterelva E and W (total areas, Table 2). However, according to Nordseth (pers. comm. 1975) the mean erosion rate in M@nsterelvaduring the postglacial time is also estimatedto be twice the value measuredin 1970, and therefore of the same order of magnitude as we have calculated for the Fliten gully. A more precisecomparison of the given erosion rates has little value, be- cause of the different sizes(Table 2) and topographic features of the catch- ment areas, different periods of time to which the denudation rates refer (Table 2), and the different climate between easternand western Norway. Biostratigraphy Studies of pollen and other microfossilshave been made with a view to solving stratigraphic problems. The results provided information too about the lateral distribution of fossil pollen in lacustrine sediments, and the development of the vegetation during the Holocene. The material is not ideal for solvingthese problems, the main weaknessesbeing the high minero- genic content of the sediments,and the large distancesbetween the pollen spectra in the diagrams. Ifowever, some results have been obtained. Radio- carbon dates of the Corylus rise and the Alnus rise were available from two other pollen diagrams from the Os area (Fig. 14), and these diagrams are thereforeincluded in the discussion. 330 E. SONSTEGAARD& J. MANGERUD The Holocene history of the vegetationon the coast of Hordaland is rel- atively well known (Fagri 1944 a and b, 1950, 1954, Hafsten 1965,Hagebg 1967, Mamakowa 1968, Mangerud 1970, Bakka & Kaland 1971, Karand 1974, skin 1975). The pollen diagrams from os (Figs. 14-1g) display the same main featuresas describedby the mentioned authors. Pollen zones and chrctnozones The classification and terminology of pollen zones follows the proposals given in the International Subcommission on Stratigraphic classification (1971) and Mangerud et al. (1974). we will stressthat the namesare strictly local and informal. The chronozones are according to the definitions in Mangerud et al. (r974:119). The boundariesare identified on the basis of the following radiocarbon dated pollen levels: p., The Corylus rise, 8,800 B. based on a date giving g,960 _+ 220 (T _ 1491) iust below the corylus rise in Haukelandsvatnet,Arna (Fig. 1) (skar 197s). The Corylus maximum, 8,400 B. p., based on the date from lake Leps6y_ vatn (Figs. 1 and 14). The Alnus rise, 7,700 B. p., based on the date from lake Lekvenvatn (Figs. 1 and 14). The Querqus rise, 5,700 B. p., based on several dates from Austrheim, 70 km north of BanktjOrn (Kaland 1974 and pers. comm.). Comments on the localities LepsQyvatn and Lekvenvatn Both diagrams were worked out several years ago in order to solve other problems. The lake Leps@yvatnhas a size of ca. 170 X g0 m, and the elevation is 20 m a. s' l. It is surrounded by peat, which has grown into former parts of the lake. Samples for pollen analysis were collected with a Hiller sampler in September 1,965, on the south-eastern shore of the lake. At the coring site' brown lacustrine gyttja was found to a depth of 963 cm (Fig. 14). At this depth there was a sharp boundary, with bluish-grey marine silt. pollen samples 9 and 10 (Fig. 1a) are taken respectively just above and below the boundary, and they indicate that the lake was isolatedfrom the sea contem- porarily with the immigration of Corylus. 1ac Samplesfor dating were collectedin 1966 with a Hiller sampler.care was taken to sample only the central part of the core, and it was therefore necessaryto collect samples from seven corings. Each sample covered the lowermost 5 cm of the gyttja, just above the sharp boundary to the silt. Even though the samples were collected with a Hiller sampler, the quality is as- sumed to be good, due to careful sampling and favourable stratigraphy. tnc Pollen-stratigraphically, the date covers the lower part of the Corylus maximum (Fig. 1a). The result was (T-580) 8,3g0 -+ 190 B. p. (Nydal et al. 1970:2ll\. HOLOCENE GULLY SEDIMENTS IN OS. W. NORWAY 331 The pollen of Ephedra distachya from Leps@yvatn,discussed by Mangerud (1970: 129), was found in sample 8. The lake Lekvenvatn (Fig. 1) is ca. 130 X 100 m, and the elevation is 37 m a.s.l.Parts of the former lake are filled in with gyttja and peat, espe- cially along the southern shore. Samplesfor pollen analysiswere collected with a Hiller sampler in September1965, at the southern shore. Below the peat at the coring site there was brown lacustrine gyttja to a depth of 722 cm (Fig. 14), and below the gyttja bluish grey marine silt. The pollen diagram indicatesthat a short hiatus exists in the bottom of the lacustrine gyttja, since Lekvenvatn emergedearlier from the sea than Leps@yvatn,while the bottom of the lacustrine gyttja in Lekvenvatn seems to be younger than in Leps@yvatn. Samples for 1aC dating were collected with a 54 mm piston sampler (Geonor) in October 1971. Some simple pollen counts were performed in this core, and the very well-defited Alnus rise (Fig. 14) from ca. 3 Vo to 40 o/cwas identified.The dated samplewas taken from the beginningof this rise and 5 cm upwards,which pollen-analyticallycorresponds to the depth 680-685 cm in the pollen diagram (Fig. 1a). The result was (T-1160) 1,1L0 -+ 100 B. P. (Gulliksen et al. 1975: 366). Redeposited pollen Redepositedpollen is usually an important problem in sedimentsas rich in minerogenicmaterial as the lacustrinesediments in [email protected] pollen content in the laminated glaciomarinesilt, which is the source for the mi- nerogeniccomponent is, however, extremely low. Therefore the content of redepositedpollen is also low. The marine Hystrix, being resting cysts of dinoflagellates,are the most frequent microfossilsin the glaciomarinesilt. Hystrix also occur sporadically in the lacustrinesilt, where most of it is believed to have been redeposited togetherwith the glaciomarinesilt from the gullies.Some of the Hystrix oc- curring just above the isolation contact may, however, be connectedwith inwashedsea water during high tidesand storms,or lowering of the threshold which consistsof silt and sand. The samplesfrom the glaciomarine silt in cores 1 and 3 contain on aver- age 130 o/oHystrix, calculatedon ) (AP + NAP), while those in the lacu- strine silt between the Coryltts and the Alnus rise, from all cores, contain 1.8 oft,Hystrix. If we supposethat all these Hystrix are redeposited,it fol- lows that secondarypollen from the glaciomarinesilt constitutesless than 7.4 o/oof the pollen depositedduring the first 1,100 years of the lacustrine phase,which was the most active period of gully erosion. Lateral variation of the pollen content In pollen analysisusually one diagram only is worked out from a lake or bog. Thus the question has often arisen to what degree a pollen diagram is representativeof the entire basin. Some studieshave been conducted on 332 E. S@NSTEGAARD& J. MANGERUD this particular problem (e.g. M. B. Davis 7967, 1968, M. B. Davis et al. 1971, R. B. Davis et al. 1969, Horowitz 1969, Berglund 7973, Pennington 1973), and the conclusion appears to be that within a pond or lake there are small lateral variations in the composition of pollen, especiallyin the caseof small and medium-sizedlakes. However, as a rule, there is a differ- ence between shallow, onshore, and deeper areas. This may be due to a differential supply of pollen, different floating characteristicsof the pollen grains,wind, flow of water, etc. From lake Tveitavatn,Stord, Hafsten (1965) tml;iJ-"i..i..-".;,*;l;i*d c o RY . u !l P N!!l NAp I Fig. 13. The variation of the content of Coryltrs, Pintts, and NAP pollen between the cores 1, 2, 3, and 4 in lake Banktjflrn. The comparison is only carried out for the Corylus zone. The vertical scale is changed, assuming a constant rate of sedimentation within this zone in each core. The numbers of the pollen samples (see Figs. 15-18) are given. HOLOCENE GUI-IJ SEDIMENTS IN OS, W. NORWAY 333 has two pollen diagrams,also discussedin Bakka & Kaland (1971:20). Haf- sten explainsthe small differencesbetween the diagramsin terms of greater influence of local vegetationin the diagram nearestto the shore. The pollen diagrams fromBanktj@rn,which from a pollen-analyticalview is a medium-sized lake, make it possible to compare the composition of ancient pollen in different parts of the basin. Since the boundaries of the Corylus zone are most distinct, it is easiestto make the comparison within this zone. In Fig. 13 the vertical scale is changed assuming a constant rate of sedi- mentation between the Corylus rise and the Alnus rise for each single core. Far too large distancesbetween pollen spectra, especially in core 4, reduce the value of the comparisons. The NAP valuesare in most levels somewhatlower in the central part of the basin (core 1) than in the narrow bay in the south (cores 2, 3, and 4). A similar distributionwas found by M. B. Davis et al. (1971). In the caseof Banktj@rnthe reasonmay be that the narrow bay receivedmore local pollen from the field vegetation and the shore, whereas in the central part the tree pollen from more extensiveupland dominated. The Corylus curves are similar in the main shapes,except that the pro- nouncedpeak fails to appearin core 4, certainly due to lack of spectra.The bare rock hills surroundingBanktjgrn were probably at that time, as they are at present, dominated by pine forests. Hazel was probably the most important specieson the silt area.This may explain the high Corylus values in core 3 where local hazel pollen could be supplied both directly through air and by the brook in the Fliten gully. Pinus pollen is sometimesfound to accumulatemore in the shallowsand along the shores(R. B. Davis et al. 1969), sometimesin the central part of the lake (Pennington 1947). The latter is the case for Banktj@rn, where the explanationmay be that the pollen content in the central core reflects the regional vegetation,whereas the pollen composition in the bay sediments (cores 2, 3, and 4) is more influenced by both the pollen supply from the local shore vegetation(birch, alder, herbs, etc.), and, as mentioned, from hazel forestson the silt area. In their broad featuresthe pollen curvesin the four diagramsare similar in shape.The pollen compositionseems, however, to have been slightly in- fluenced by vegetationnearest to the individual boring localities.This is in accordancewith the conclusionsof Berglund (\973) concerningrecent pollen sedimentationin a lake which both in shapeand size (500 da) can be com- pared with Banktj@rn(120 da). The NAP assemblagezone The definition of the zone is that NAP constitutesmore than 60 7o of the total pollen sum. The main taxa of the herbs are Gramineae, Rumexf Oxyria, and Artemisia (Figs. 15 and 17). The vegetation must have been very open, with a few stands of Salix, luniperus, and Betula (possibly only B. nana). The Pinus pollen is certainly 334 E, SONSTEGAARD& J. MANGERUD other exotic long-distancetransported, and the low percentageof this, and pollen, indicates that the pollen composition reflects the local vegetation. delayed im- The deficiency of trees could be due either to cold climate, it has generally migration, or edaphic conditions.During the last few years the time interval been assumedin Scandinaviathat the open vegetationin was not between the deglaciation and the establishmentof the birch forests belt of heath/ climatically determined. Probably the NAP zone reflects a the zone is time tundra vegetation along the retreating ice front, and thus transgressivein a profile along the direction of deglaciation' already during Tree birches (Betula pubescensagg.) grew in Hordaland theAllerpdChronozone(Mangerud7910,L9l'7,Skir1975)'However'we or any place do not know whether it survived along the coast of Hordaland, Chronozone in Norway at all, during the following cold Younger Dryas According to (Fagri 1.936, 7944a, Hafsten 1963, Mangerud 1970, 1'917)' Swedeneither. i"rgtuno (1g66:gg)it possiblydid not survivein southern Atpresentitisunknownhowfarthetreebirchhadtospreadduringthe be- preboreal chronozone before it reached the os area. The time interval of birch forests) tween the deglaciationand the Betula rise (establishment isprobablyonly100-200years,whichmayindicatethatthebirchdidnot the Younger have to spread far and that some birch trees indeed survived Dryas in Hordaland. The Betula-NAP assemblagezone rise and the cor- The lower boundary of this zone is defined by the Betula Corylus rise' respondingdecrease of NAP. The upper boundary is the this zone' The obviously real forests of birch establish themselvesduring transition beginning of this is contemporary with the lithostratigraphical 100-200 years from glaciomarine to postglacial marine silt, probably only after the deglaciation. ThecontentofPirutsisstillbelowl0To,butslightlyhigherthaninthe NAPzone,Astheproductionoflocalpollenmusthaveincreased,thisindi- cates that Pintrs grew closer' but certainly not in the Os area' close .fhe Junipems curve has a distinct maximum, before it disappears, this corresponds to the upper boundary of the zone. chronostratigraphically tothebeglnningoftheBorealChronozone(9,000B'P.).Skir(1975)found Arna (Fig' 1)' a similar maximum in a diagram from Haukelandsvatnet, (Fig' 7)' a Juni- Also in a diagram (Mamakowa 1968: pl' II) from Lerly (1968: perus maximum appearsin the same stratigraphical level. Mamakowa referring to the 20) assumes,however, an older age for this maximum' III/IV (Younger Juniperusexpansion as characteristicfor the zone transition Dryas/Preboreal). In Denmark(e. g. Iversen1960) and southern Sweden (e' g. Berglund |.966:134) a similarJuniperus maximum is indeed tyPical for the Younger Hor- Dryas/Preborealboundary, as referredto by Mamakowa. However, in that dalandthis maximumappears 1,000 years later. The reasonis ProbablY HOLOCENE GULLY SEDIMENTS IN OS, W' NORWAY 335 Resolved AP-drogrffi (% of >AP) q l5 P l- Fig. 14. Pollen diagrams from tho lakes Lekvenvatn and Lepspyvatn (Fig' 1)' the light-demandingjuniper was shadedout by the birch forestsin Denmark and southernSweden already early in the PreborealChronozone. In Horda- land the vegetationwas more mosaic, and probably Juniperus stands re- mained important, especiallyon areas with thin soil cover, until the pine immigrated and shadedit out. Hazel immigrated nearly contemporaneously with pine, and may also have contributed to the supersedingof the juniper, especiallyon more fertile ground. Tfte Corylus assernblagezone The lower boundary of this zone is defined by the corylus rise, and the upper by the Alnus ise. Pinus increasesat the samelevel as CoryIus, while nitutostrongly decreases.The ageof this local zone is from 8,800 yearsB. P. to 7,700B. P. The shapesof the corylus- and Pinus curves during the Boreal chrono- zone afe often discussed.A double Corylus maximum has been regarded as typical for westernNorway (Fagri !944:28, Hafsten L965: 3940). This, however, is seenneither in our diagrams,except possibly Lekvenvatn (Fig. JJO E. SONSTEGAARD& J. MANGERUD Banktjarn,core1 LITHOSTRATIGRAPHY I -l ii s le r3!l! d13 FOR SEDIMENT CLASSIFICATION Fig. 15. Stratigraphy and pollen diagram for the coring 1 t:l in Banktjgrn. t- JPeot Gyttio (Losson ignition> 10"/.) ffiGyttio silt {3"/"< ross on ignition<10%) f;!Ll Lominoted silt (Losson ignition<3%) Nonlominotedsilt ( -"-) ffi Sond F] Til I L]Locking somples 14), nor in other diagramsfrom northern Hordaland (Frgri 1954, Hagebe L967,Mamakowa 1968, Aa 1974). In Denmark the Pinus maximum appearsbefore the Corylus maximum, in easternNorway the two maxima are simultaneous,while along the coast of western Norway the Corylus maximum is usually before the Pinus max- imum. The latter is also the casein the diagram from Leps@yvatn(Fig. 14), while the diagram from Lekvenvatn is indistinct in this respect. The four diagramsfrom lake Banktjlrn (Figs. 15-18) exhibit a few small differences concerningthe Corylus and Pinus maxima. The main picture is nevertheless clearly as shown in the diagram from the centre of the lake (Fig. 15); there HOLOCENE GULLY SEDIMENTS IN OS, W. NORWAY J) I STRATIGRAPHY -. Colculoted on t(AP+NAP) appears onePinus maximum on each side of the Corylus peak' This means ttrat ttre Pinus and Corylus maxima are more or less simultaneous,as in easternNorwaY. The explanationof the double Pinus maximum may be that the pine im- the migratedtefore the hazel; it colonizedfirst the fertile silt area,but there the mJre shadow-toleratinghazel soon after ousted the pine. Laler, when pine forestswere well establishedon the hills with too poor edaphiccon- iition, for the hazel, the percentagesof pine pollen again reachedthe same values as before the hazel Peak. von Post (Ig24) wasthe first to point out that the sizeof the coryIus max- imum increasedfrom continentalto oceanicregions, and this conclusionhas in later been confirmed by severalinvestigations. This appearsalso clearly of a cross section of the Hordaland county. The maximum percentages (Frgri 1950) hazel, calculatedon sum AP, are in the two easternsites Voss and Kinsarvik (Anundsen & simonsen t967) below 15 Vo; at Eidslandet (Aa I97\ further west 31 Vo; and.at Stord (Hafsten 1-965)and around Ber- ca' 50 % ' Os has a localization ien (HagebO Lg67), close to the coast, ,i-ilu, to Stord and Bergen, and the hazel maximum has also a similar 338 E. S@NSTEGAARD& J. I\{ANCERUD Banktjarn,core2 LITHOSTRATIGRAPHY BIOST RA] 1i i'*] j-al i*]..l'*i Fig. 16. Stratigraphy and pollen diagram for the coring 2 in Banktjprn. size,being indeedonly 28 Vo inLekvenvatn, but 42 /e inLeps4yvatn,60 %o in the centre of Banktjrrn, and more than 80 vo in the diagram (Fig. 17) from the bay where we have postulated a strong local influence from the hazelforest. The Alnus assemblagezone The lower boundary of this zone is defined by the very distinct Alnus rise, which is unambiguousin all diagrams. Due to the few samples analysed higher up, we have so far not defined any upper boundary, and in the present diagramsthe zone consequentlycontinues to the surface. 339 HOLOCENE GULLY SEDIMENTS IN OS, W. NORWAY 3RAPHY + f (AP+NAPlrporei) +<-T+ !d on '(AP+NAP) : (AP+NAP) NearthelowerboundaryfirstCoryltlsandthereafterPinusdecrease markedly,andlaterneverreachthepeakvaluesinthezonebelow'The present diagrams. Atnus li usually above 30 vo throtghout the zone in the curve has a tail far In other diagrams'from western Norway, the Alnus the boundary' Similar below the increasewhich we usedfor the definition of Leps@yvatn,but are tails are found in the diagramsfrom Lekvenvatn and increaseof Alnus is, lacking in all the diagramsfrom Banktjorn. The drastic (cf' 1973)' and the no doubt, due to Alnus glutinosa (black alder) Vorren exceptionallyhighvaluesaretheresultofabeltofAlnusglutinosagrowing increasealso indicates aton! the lake shores.We believe that the discussed 340 E. SONSTEGAARD& J. MANGERUD Banktjarn,core 3 L ITHOSTRATIGRAPHY BIOSTRATIGRI I -l . Banktj@rn' Fig. 17. Stratigraphy and pollen didgram for the coring 3 in that the immigration of Alnus glutinosa. Hageb1 G967) assumes,however' the tail indicates that Alnus glutinosa immigrated simultaneouslywith the we corylus maximum, and that the increaseis due to increasedhumidity. 341 HOLOCENE GULLY SEDIMENTS IN OS, W. NORWAY E (AP+NAP+spores)-+ +-1+ t (AP+NAP) of Alnus find it more probable that the tails are due to ear$ immigration be solved incana (grey alder) (Tallentire 7973,7974), but this problem must by by means tf macrofossils,as the two speciescannot be distinguished 342 E. SONSTEGAARD& J. MANGERUD Bankljarn, core4 LITHOSTRATIGRAPHY BI OSTR E € 6 .9 ' E .9 = o -9 -l m m roiro 30 50 Fig. 18. Stratigraphy and pollen diagram for the coring 4 in Banktj/rn. meansof pollen morphology.It is worth noting that Mangerud (1977) found up to 11-Vo of Alnus in Late Weichseliansediments in Hordaland. The sum of the constituentsof the mixed oak forests (QM) increased soon after the Alnus rise. However, the Ulmus and Quercus curves have long tails, which go through the entire Corylus zone. Characteristic is a small peak in the lower part of the tail (Figs. 14 and 15). Both Hageb| (L967) and Mamakowa (1968) assumedthat a few trees of Quercus and Ulmus grew in Hordaland during the Boreal Chronozone.Such an assump- tion can explain the tails, but the delayed main expansionis in that case surprising. The further course of the QM curves is similar to previous results from westernNorway: Ulmus decreased,while Tilia increasedand reacheda peak value approximatelysimultaneous with Quercus. Conclusions The sedimentsfrom the Fliten gully were depositedin lake Banktjfl,rn di- rectly outside the gully, more or less like a delta. This is evident from the lateral thinning of the bed and the fining of the lacustrine silt away from the gully mouth (Fig. 11). The erosion of the gullies in the Os area started about 8,800 years B. P., shortly after the sedimentshad been raised above sea level, and continued NORWAY 343 HOLOCENE GULLY SEDIMENTS IN OS, W' loted on :(AP+NAP) 5 (AP+NAP+spore$ 5(AP+NAP) about 7,700 yearsB' P' until the gulliesreached bed rock in their back ends, modefate, possibly due During tie first 400 years the denudation rate was yet. The erosion was at a to the fact that side brancheshad not developed B' P'' and we assumethat the maximum in the period 8,400-7,700 years entire gully systemwas active at this time' Thedenudationratesduringthetimeofgullyerosionhavebeencalcu- latedto|.45mm/yearwithinthesilt-coveredareaand0.65mm/yearforthe whole catchmentarea (Table 2)' have been dependenton The formation of the gullies do not appear to Lundqvist & Hjelmqvist climate, in contrast to the assumptionmade by (lg4:)forthegulliesinDalarneinSweden.Thisdifferenceisreasonable, and heavy precipitation' as Banktjorn lies in an area with oceanic climate whileDalarnehascontinentalclimatewithrelativelylittleprecipitation. for the gully formation Thus precipitation would never be a limiting factor periods in Dalarne' at Banktj/rn, as it was during relatively warmldry within the lake sediments The lateral variation of the pollen composition is, however, slightly in- in Banktj/rn is found to be small. The composition boring localities' fluencedby the vegetationnearest to the individual mainly similar The pollen diagrams display a vegetational development Norway' A maximum to that concluded trom previous studies in western pinus and corylus is discovered, of.Iuniperus just before tire immigration of andcomparedtotheL,000yearsolderJuniperusmaximuminSwedenand 344 E. SONSTEGAARD& J. MANGERUD Denmark. The Boreal Pinus and Corylus maxima are found to be nearly simultaneous,as opposed to the general view that the Corylus maximum should be first along the coast of Norway. Acknowledgernents. - Mr. Kjell Spgnen, Mr. Harry Isachsen, Mr. Terje Seb@ and cand. real. Odd Pedersen provided assistance with coring. Mr. Johan Lund has per- formed some of the pollen preparations. ldentifications of pollen were discussed with cand. real. Peter Emil Kaland, cand. mag. Jan Berge, and cand. real. Dagfinn Moe. Peter Emil Kaland kindly permitted us to use unpublished t+C dates from Austrheim. The figures were drawn by Miss Ellen lrgens and Mr. Masaoki Adachi. Dr. Ron J. Steel, cand. real. Tore O. Vorren, cand. real. Kjell Nordseth, and cand. real. Kjell R. Bj/rklund kindly read through the manuscript critically. Dr. Steel also corrected the English language. To all these persons we proffer our sinsere thanks. We also thank the Norwegian Research Council for Science and Humanities (NAVF) for financiai support. These investigations were started by Mangerud. The field work and laboratory analysis were carried out by S@nstegaardin his cand. real. thesis under Mangerud's guidance. We have written the present article together. April 1976 REFERENCES Andersen, S. T. 1961: Vegetation and its environment in Denmark in the Early Weich- selian Glacial (Last Glacial). Dan. Geol. (Jnders. Il Rekke, Nr. 75, 175 pp. Andersen, S. T. 1966: Tree-pollen rain in a mixed deciduous forest in South Jutland (Denmark). Rev. Palaeobot. Palynol. 3, 263-275. 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