By Alan Fyfe
In the middle of the North Sea, about 200km east of Dundee, there are a number of deep trenches known collectively as the Devil's Hole. Figure 1 is a map of the Central North Sea and shows the location of the deeps. The north-south trending depressions which make up the feature were noted as long ago as 1931 by J.W. Gregory in the Geographical Journal (vol. 77, p. 548). They were first charted by H.M. Survey Ship Fitzroy with soundings taken at regular intervals along straight-line traverses. The method of fixing the positions of the soundings employed a series of beacons whose positions were known from star-sitings and from taut-wire runs from Aberdeen and Souter Point. Bearings were taken on the beacons and the depths plotted at the appropriate positions on the chart.
Figure 1 - Location mapModern position-fixing methods rely on radio waves sent out from stations on the mainland or from satellites. The Marine Geology Unit of the I.G.S. use the Decca Main-chain system of navigation for their sampling work. A console on the ship displays three coordinates calculated from shore-based radio-transmitting stations and converts these to a latitude and longitude position. This can be plotted on a chart and in the Central North Sea gives a position accurate to about 200m.
Traverses run by the Marine Geophysics Unit of I.G.S. use a system of satellite navigation whereby fixes and updates are received every few hours, intermediate positions being determined by Doppler sonar off the sea-bed. This system gives a position accuracy to about 100m.
The presence of the Devil's Hole was known long ago to fishermen, some of whom managed to lose their trawl nets on the steep sides of the trenches and thence gave it its name. The average water depth over most of the surrounding area is between 80m and 90m but reaches over 230m in the deepest part of the trench. The sides are steep - up to ten degrees. This may not sound very great at first but for a seabed slope it is quite considerable. As a comparison, the Continental Slope northwest of Britain has an average gradient of around one degree, a point worth recalling when you next see a diagram of the Continental Shelf, Slope and Ocean floor.
Figure 2 - Bathymetric contour map of the Devil's Hole area (contours in metres)
Figure 2 shows a bathymetric map of the area of the Devil's Hole. The narrowness of the features and their approximate north-south trend can be clearly seen. There has been some debate as to what these features should be called - the words channels, valleys and trenches have all been used at various times in the literature. Channels and valleys both imply an origin which may not be appropriate. For the time-being I shall refer to them as trenches. These are not to be confused with deep sea trenches which are a product of plate tectonics! They are on average 1 to 2 kilometres wide, 20 to 30 kilometres long and at their maximum development 120m deep (below the seabed). The relief along the bottom of the trenches is sometimes quite dramatic when seen on echo-sounder recorders (Figure 3).
Figure 3 - Echo sounder profile across part of the Devil's Hole area showing irregular nature of the base of the trenches (vertical exaggeration x 25)
As well as echo-sounder profiles of the seabed, the geophysical records include shallow seismic lines. Differences in the frequency of the sound source mean that in addition to a surface trace, these show reflections from below the seabed. The best type of equipment to show the nature of the deposits in the Devil's Hole is a high resolution 'boomer' seismic source. Figure 4 shows a 'boomer' record across a part of the Devil's Hole. It can be seen that below the present-day seabed surface there are a number of conformable reflectors. This apparent lamination is known as seismic layering and is caused by changes of the density in the sediment. These changes can be interpreted as representing variations in the nature of the sedimentary deposits. The 'multiple reflections' shown on the section are a function of the seismic process and are related to secondary reflections of sound waves from the seabed and sea-surface.
The scale along the top of the section is drawn by measuring the distance between fixes (heavy vertical lines) which are taken every 10 minutes. The thin horizontal lines across the section represent time measured in tens of milliseconds. This is the time taken for the sound wave to travel to a reflector and return to the receiver towed behind the ship. It is known as the 'two way time'. It is not a direct measurement of depth, the exact time being a function of the velocity with which sound travels through the sediments. Examples of some velocities are given below in metres per second:
|Soft clay||1600 m/s|
If we take an average value of 1700 m/s, the 10 millisecond scale mark on the side of the section can be seen to represent 8.5m. It is clear therefore that there is a great deal of vertical exaggeration in this section (approx. 17x)
Figure 4 - 'Boomer' record across a trench in the Devil's Hole area
In 1981 the Marine Geology Unit drilled a borehole on the flank of this trench, shown in Figure 4, through the underlying sediments. The core returned showed that the top 11 m consisted of sand which proved to be very fine to fine grained with a variable amount of silt. The sand grains were dominantly quartzose with a subsidiary quantity of metamorphic rock fragments and heavy minerals. Angular shell fragments were also recovered. Below the sand, the sediments comprised very soft silty clays with occasional interbedded sands. Shell fragments and small scattered pebbles were found through this unit. These soft clays were recovered down to 53m where the drill penetrated a 7m thick bed of slightly gravelly shelly sand. At 60m the drill encountered stiff clay and sand, which was taken to represent the base of the trench infill. From this information, we are able to construct a cross section of the trench and its infill (Figure 5).
Figure 5 - Geological cross-section across a trench in the Devil's Hole area (vertical exaggeration x5)We can see from this geological cross section that the soft clay makes up the greatest thickness of sediment in the trench. The seismic layering also suggests that the clay has been deposited in a more or less symmetrical distribution, although the axis of symmetry is not the same as the present axis of the trench. The basal gravelly shelly sand layer also has a generally symmetrical distribution. In contrast, the upper fine grained sand layer is markedly asymmetrical, occupying only the western side of the trench. Looking at other trenches in the Devil's Hole, this is found to be the case over much of the rest of the area as well. Locally there is also some seismic layering which resembles large scale cross bedding within the upper sand, the sediment transport appearing to have been from the west. Near the Scottish coast, fine grained sands very similar to those in the upper part of the Devil's Hole sediments appear to be related to river outwash fans.
Several types of microfossils have been recovered from the sediments in this borehole. These include foraminifera and dinoflagellate cysts which have been analysed by the Palaeontology Unit of I.G.S. The fossils from the basal gravelly sands and from the soft clays are commonly arctic species while in the sand at the top of the borehole, several temperate and boreal forms are found.lifestyles skyn original condoms reviews hair salon near me open on sunday rift zone coffee bean and tea leaf nutrition 3rd avenue studio denver science or science and data buzztrend trends this year trend it coffee price its fashion google astronomy he did this for science coffee monday morning books astronomy new astronomy dogs haircut trend at science 2015 science 20 4 haircut lifestyle home shoes with top coffee 14 coffee maker amazon science power 2018 spring hair trends season science fashion to some trends trends app lulus return policy indian astrologers near me astronomy ii part haircut coffee i own you trend price how to observe the night sky coffee mugs clip art life trends astronomical league members astronomy articles for students york lifestyle list of hairstyles lifestyle church coffee de astronomy binoculars where to find cheap cute clothes for winter buzz lifestyle keto pills my first coffee 10 dash one haircuts 2019 cos astrology color correction in chelsea nyc daily life hair and beauty place science's coffee music fashion yourself cute women hairstyles game for science lifestyle police dc haircut passive margin science fair project ideas for 8th grade science buddies she's so fashion my own science science in 2014 abc haircut before coffee unique winter street style lifestyles unlimited expo 2019 size trends sciencedirect error scientific method definitions why do stars revolve around polaris child astronomy astronomy god anderson bernstein gray young woman shopping online men's hair cuts near me astronomy list lifestyle lyrics rap genius where is fashion thiel dress skirts popular nail colors 2019 instant gratification ave p salon for party polyvore clothing mobile astronomy recent posts hair revelation what are astronomy 3 lifestyle scientific where's waldo scientific method prediction definition coffee shop downtown plano who what where fashion lifestyle policy science 11 family and lifestyle lifestyle room coffee state trend video names of stars in space astronomy words trending bracelets god science 12 science lifestyle movie all you need is coffee short straight hair flat trend email all about science best science coffee maker k cup brakeman's charlotte m and m coffee lifestyle 28 do your own fashion lifestyle sex astronomy program in fashion now create a fashion board fashion for me science fiction lifestyle lift asia science center strapless ribbed dress lifestyle keto 700mg lifestyle problem fashion on stars in the sky lyrics best in coffee astrology planets chart from science coffee 12 trending news sites coffee time pretty dress mode be on trend science god coffee out uranographia be the trend trending news google business lifestyle fashion my hottest nail colors right now describe the scientific process that's science my lifestyle group christmas lifestyle astronomy song coffee and grey decor natal chart basics urban coffee dallas going really short hair science buddies biology projects shavers for haircuts coffee by me lifestyles of the rich and famous robin leach quote the sun shop melbourne fashion name coffee houses nevada city how to look stylish in the snow just in trend astronomy places cute and sporty sweatshirts star astronomy tv show lifestyles of the rich and famous