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Indlæg fra arbejdsseminar d. 25. -26. september:


The identification of navigable sailing channels within Vålse Vig
Shaun Rømer and Henrik Breuning-Madsen 

Vålse Vig is the 5km2 diked bay situated on the north east coast of Falster. The Kippinge church in the bottom of the bay was one of Denmark's most well known pilgrim churches and accessible to the sea via navigable sailing channels through the bay. It is also thought that during the Viking age the bay was used as a refuge during the winter storms and Wendish / or local pirate raids on the Danish coasts. There is some evidence to suggest (Thorsen, 1999) that there was an escape route dug across the moraine hill at Grøftestykkerne (the south west basin of the vig) to the Guldborg Sound, to facilitate the escape of the boats moored in the bay. 

To identify the location of the sailing channels within the vig and determine if the proposed escape route existed, a soil survey of the vig was carried out in the spring of 2000. A soil map was created of the vig by identifying and mapping the boundaries of the moraine clay surrounding the vig, as well as the marsh sediments within the vig. The subsequent identification of the location of the sand and gyttja deposits, a sediment that is usually deposited in low lying areas, under low energy conditions of any inter tidal marsh area, would then form the basis for the location of the possible navigable sailing channels in the vig. To substantiate the location of these low lying areas, an accurate altitude model using a DGPS (Differential Global Positioning System) of the same area was created. An accurate representation of the navigable channels in the vig would then be possible by comparing the positions of the low lying areas depicted in both of the maps.

The same techniques were used to identify the location of the proposed escape route, from the church to the Grøftestykkerne basin, although a more careful note was taken of any disturbances found in the expected natural sedimentary layers and soil development around the suggested site of the escape route in the south western part of the Grøftestykkerne basin, which might indicate the presence of a man-made ditch or channel.


The soil survey and GPS field work

Due to modern farming techniques, the process of erosion and the decomposition of the organic material in reclaimed marshes, it can be difficult to map the recent (Holocene) landscape or man- made channels and ditches of the past. To do this one has to 'remove' the upper one metre of the landscape, which has been recently manipulated, and map the underlying soil types (Jacobsen 1964). 


Soil survey

The aim of this soil survey was then to bore beneath the upper 1m of the landscape, using a 1.20m auger, and map the moraine clay boundaries of and within the vig and the extent of the various limnic sediments within found in the vig, all at a depth of - 1m. 
A preliminary investigation was undertaken to define the predominant soil and sedimentary characteristics in the vig and create a unique soil key used to simplify the actual survey. This initial phase comprised drawing up 4 profile lines transecting the vig in such a way as to represent all the possible morphological features and sediment types likely to be found within the vig. 
The 4 profiles were orientated east-west across the vig and situated south of the dam, south of the vig road, in the middle of the and the southern profile from Klintgård in the east to Skomagerkrog in the west. The borings of the profile lines were to be spaced at approximately 100m intervals and stop in the mineral soil.

Map 1: The reclaimed area of Vålse Vig and the profile lines across the bay.
Source: KMS 1995


The upper profile borings were made using standard 5cm diameter auger with a 1m extension. The continuation of the deeper borings in the deeper soft sediment was made using a one meter long, 2cm in diameter half cylinder bore with 1m extensions. 
The depth of the borings, as well as, horizon soil texture, colour, presence of and gley (presence of visible iron in the soil) features were all noted.

To be able to map the soil features quickly and accurately in the field, unique soil mapping units needed to be created that are specific to the sediment profiles of Vålse Vig. From the observations of the soil characteristics in the upper 1m of the profile borings, the soil key or legend shown below was created. It was found that the vig below 0m DNN is made up of moraine clay with deposits of gyttja, sand and peat. These soil textures were then used to create the soil key. 


foto. Soil key

Table 1: Soil key used to create the soil map.

S - sand, T - peat, W - gytje, Mc - moraine clay, Ap - 30cm plow layer, g - gravel 

The textural combinations in the key were the only combinations expected to be found while mapping the upper 1m of the vig. 
Ap in the key donates the upper 30cm soil layer, which is the plough layer. A plough layer covers the whole of the vig as the vig is intensively farmed. The texture of this layer varies with the dominating soil type, clayey on the high lying moraine hills, peat in the bogs but mostly fine sand in the low lying areas. Mc is yellowish moraine clay with small stones and flint, Sf is the fine sand fraction, Sm medium to coarse sand and Gravel (after Madsen 1988,   Greve and  Sørensen 1992). T is peat or tørv and W is gyttja. 
The texture of gyttja is marzipan and no difference was made between the different peat types.
The layers in the profiles of the key have a minimum thickness of 10cm with the lowest layer defined between 100 - 120cm. It is the combination of layers rather than their absolute thickness that determined their classification. 
Given the 4 textures, there are 16 possible combinations, all of which were observed in the preliminary borings. Only the classification number was noted on the 1: 8 500 topographical map during the boring campaign. Specific observations and details were also noted to correct the final map. 

The soil survey was then conducted to map the vig area. A 1.20m Eikelkamp auger was used to bore through the upper soil layer, identifying the soil characteristics and classifying according to the key. The position of the individual borings was estimated in the field using the features in the landscape for orientation with the aerial photos as backup. The free surveying method was chosen so that the landscape features could be interpreted while in the field and the required number of borings taken to represent the feature. 
The scale and detail of the map plus the large number of borings 1 per 10m2 were conducive to the accurate representation of the actual soil layout within the vig. Subsequent simplification of the key and interpretation of the boundaries during the creation of the final soil map has decreased this accuracy but still within acceptable limits (+/- 20m) for the identification of the navigable channels in the vig.

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(pga. detaljerede kort, kan downloadtiden blive 1 - 2 min)

 



Redigeret  17/10/2007