BLAKENEY ESKER AND HOW IT FORMED Introduction The Blakeney Esker is a ridge, around 3.5 km in length, which runs southeastwards from west of Blakeney, to Wiveton Downs, north-west of Glandford, in north Norfolk, UK (Figure 1). Figure 1 The location of Blakeney. There are several near right-angle bends in the esker. The northern section of the ridge between Morston Downs and Kettlehill Plantation has a sharp, well-defined shape. It is between 40 and 100 m wide and, in places, rises to around 20 m above the surrounding topography. Further south, the ridge is lower for about 1 km and the sharp ridge is not as obvious. This is due to the removal of aggregate during the early 1940s, used in the construction of nearby Langham Airfield. The ridge forms a prominent feature again towards its southern end at Wiveton Downs (Figure 2), where the ridge surface rises up to 15 m above the surrounding topography, the name Downs is a name often given to ridges or hills. Several small isolated hills, located between Wiveton Downs and 1
Glandford, are in line with the esker and may represent the final eroded remnants of the ridge. Figure 2 Sketch map showing the location of Blakeney Esker. The shape of the esker has been simplified. Internal composition of the ridge The name esker, originates form the Irish word eiscir, meaning sandy ridge. An understanding of what the ridge is made of helps us to understand how it formed. The ridge is composed of two units, and these can be seen in some of the quarries that exist along its length. Unit B a sand and gravel, with occasional lenses of till, making up the ridge. Unit A a chalky till that occurs beneath, and adjacent to, the ridge margins. 2
Unit A lies beneath the ridge and the surrounding fields. It is a compact, cream-coloured very chalky till (boulder clay) (Explanation Box 1) that contains numerous clasts (pebbles) of chalk and flint. The texture of the till matrix (the material between the clasts that holds the unit together) is very silty, and feels smooth when rubbed between the fingers. This unit is a subglacial lodgement till (Explanation Box 2) which was deposited beneath a glacier. Beneath the ridge, the surface of this unit is not flat, but is cut by a series of channels that run in the same direction as the ridge. These channels have steep sides, are between 15 and 25 m wide, and can be traced for up to 400 m. These channels, which formed beneath the glacier due to the flow of water, are called Nye channels (Explanation Box 3). Unit B infills the Nye channels cut into chalky till (Unit A) and forms most of the ridge. It can have a thickness of up to 15 m. The unit consists of clast-supported (Explanation Box 4) cobble gravels separated by pockets of yellowish orange sand. The main constituent of the cobble gravels is flint and individual clasts are usually very round in shape. This shape suggests that very high-energy turbulent flow conditions existed, causing the edges of the rough cobbles to be broken off and smoothed as they bumped into each another. This process is known as abrasion (Explanation Box 5). The gravels also commonly have a structure called imbrication (Explanation Box 6). This is a useful feature, used by geologists, to understand the direction of water flow. In this case, flow was broadly from the north-west but tends to lie parallel to the trend of 3
the landform. Some of the gravel and sand layers, referred to by geologists as beds, also become finer upwards. This means that within an individual bed, the average size of the gravel clasts becomes smaller upwards; likewise within sand beds, the coarseness of the sand reduces upwards. This fining-upward suggests that there were frequent changes in flow velocity and therefore, the flows ability to entrain (carry) and transport different types of material. This is because a reduction in flow velocity means only the lighter material can be carried by the water, causing the heavier material to be deposited. There are occasional lenses of flinty till within Unit B. How the esker formed When we look at the shape (morphology) of the ridge and its internal composition, we can build up a model of how the ridge formed. This helps us to decide whether or not the ridge actually is an esker. Five stages within the ridge s evolution can be identified: 4
Stage 1: This stage coincides with the movement of ice across the Blakeney area and the deposition of the chalky till (Unit A). Stage 2: Modification of drainage beneath the glacier and the erosion of Nye channels onto the upper surface of the chalky till by meltwater. Movement of ice is towards the reader. 5
Stage 3: Transportation and deposition of coarse sands and gravels within these Nye channels by meltwater (Unit B). Because the channels are beneath the ice they are operating within an enclosed space, which greatly increases the pressure and flow velocity. This explains why the water had the energy to erode channels into the till, and transport high quantities of gravel and cobbles. Stage 4: Enlargement of the Nye channels into a tunnel and its subsequent infilling by meltwater sand, gravel and cobbles (Unit B). The enlargement from channels to a single tunnel can be explained by the frictional melting of ice, driven by the flowing meltwater. This is an example of a positive feedback process. As frictional melting of the ice creates a larger channel that can transport more water, this drives greater frictional melting due to increased water flow and greater surface area of the channel margins. 6
The origin of the rare lenses of till within Unit B is not clear, but the most likely explanation is that it was eroded from upstream and carried by the meltwater. Stage 5: Meltwater flow and the deposition of the sands and gravels would have continued until either the source of the meltwater was exhausted, or the drainage pathway was diverted. Later, northwards retreat of the ice margin revealed the ridge. Is the ridge an esker? The shape and internal composition of the ridge reveal that the landform formed by meltwater flow beneath a glacier and is therefore an esker. However, the shape of the esker is slightly different to that of many others, as some sections don t appear to be inline with one another. This offset pattern is commonly associated with crevassing (Explanation Box 7) within the ice. This suggests that during formation, the course of the water was strongly influenced by crevasse patterns that extended through the glacier. 7
THE BIGGER PICTURE How other north Norfolk glacial landmarks are related Cromer Ridge The Cromer Ridge (Figure 1) is one of the most prominent landforms in north Norfolk. It forms some of the highest ground within the county and in places rises to over 50 m above surrounding land (over 100 m OD). Figure 1 Cromer ridge from Beeston Bump, just east of Sheringham, looking east. The ridge intersects the east coast between Trimingham and Overstrand, where it can be identified in the cliffs, and traced westwards for 15 km to Sheringham. From Sheringham, the ridge bends towards the southwest and extends for 20 km towards Thursford. A small gap, known to geologists as the Briston Gap, occurs where the ridge is absent (Figure 2). Figure 2 Sketch map showing approximate extent of Cromer Ridge. The coastal cliffs around Trimingham, and several quarries situated on the ridge itself, provide us with a valuable insight into what it is made of 8
and its structure. This has shown that the ridge has a very complex history and was formed during several different phases. The bottom 60 m of the ridge comprise beds of pre-existing till and sand, silt and clay, which have been highly deformed when they were pushed southwards by a glacier. Effectively, this bulldozing process is similar to pushing a heavy book over a tablecloth it causes the tablecloth to ruck and fold over upon itself. Smaller scale examples of this occurring in front of glaciers can be found in Iceland today (Explanation Box 8). This is exactly what happened with the glacier pushing into the preexisting sediment pile. An example of this deformation can be seen in the cliffs near Overstrand where blocks of chalk and layers of sand and till have been bulldozed and stacked on top of one-another (Figure 3). Figure 3 The Cromer ridge reaches the coast in the cliffs near Overstrand. The arrow shows the direction in which the glacier pushed the sediment. This part of the ridge is called a push moraine, and it is also a terminal moraine because it marks the maximum extent of the ice. Examples of these can be found at many modern day glaciers, such as in Iceland (Explanation Box 9). The top 35 40 m of the ridge, overlying the moraine, is composed of sands and gravels and these were laid down as an extensive glacial outwash fan during a later advance. 9
Kelling and Salthouse outwash plains An outwash plain or sandur occurs where fans of sediment deposited by meltwater from a glacier margin coalesce to form a braided river channel system that extends across a broad, gently sloping plain (Explanation Box 10). Such river systems can deposit vast quantities of sand, gravel and cobbles. Two outwash plains are located close to the Blakeney Esker. The first of these is called the Salthouse outwash plain and this forms the ground around Salthouse Heath (Figure 4). Just to the east of Salthouse Heath is Kelling Heath, which forms the second, and larger of the outwash plains. Figure 4 The old outwash plain at Salthouse Health today. Kames and terraces Adjacent to the Blakeney Esker and on both sides of the Glaven Valley, are a series of small hills and conical-shaped mounds. These range in 10
diameter from 20 to 400 m and can be up to 20 m in height, although most are less than about 10 m. Geologists have interpreted these features as kames and kame terraces. Kames are piles of sand and gravel, which have been laid down at the front of a melting glacier. They grew as the melting glacier deposited sediment. Secondly, a succession of flat terrace features can be found on the western and eastern sides of the Glaven Valley near Glandford. These terraces comprise up to 10 m of outwash sand and gravel with thin layers of chalky till. Geologists have interpreted these features as kame terraces (Explanation Box 12). These features form in contact with, or close to, the sides of a glacier, where glacial meltwater has deposited sand and gravel against the valley sides, forming a series of terrace features when the ice melts. HOW ALL THE LANDFORMS FIT TOGETHER By fitting all of this information together we discover the order in which the features formed, and build a picture of how the geography of the area has evolved in relation to an active ice margin. Five stages of evolution can be recognised, and each stage is illustrated in a series of schematic cartoons showing a planview and cross-sectional view. Stage 1 formation of the Cromer Ridge push moraine (Figure 5) The Cromer Ridge is the first feature that was formed, created by the pushing and stacking of pre-existing sediments at the ice margin to form a push moraine. 11
Stage 2 the Cromer Ridge outwash fan (Figure 6) Draped over the top of the Cromer Ridge push moraine is a thick sequence of sands and gravels that form the Cromer Ridge outwash fan. The fact that this fan is superimposed upon the moraine demonstrates that the formation of the outwash fan was after the moraine. The outwash fan itself developed by braided meltwater streams, coming from the ice margin to the north, depositing vast quantities of sand and gravel. The position of individual drainage channels is not known, but examples of modern outwash fans from places such as Iceland, suggest that there were likely to have been a series of smaller fans coming together from drainage points along the ice margin. Stage 3 Kelling outwash plain (Figure 7) Following the deposition of the Cromer Ridge outwash fan, there was a small north-westwards retreat of the glacier margin. The ice margin at this time probably lay along the present northern and western flanks of Kelling Heath as suggested by steep ice-contact slopes. Meltwater draining from the glacier deposited extensive sheets of sand and gravel as part of the Kelling outwash plain, in the present area of Kelling Heath. This constrained meltwater drainage both to the north and south between the glacier and the Cromer Ridge respectively. At some point during this stage, the Cromer Ridge was breached by meltwater in the Briston area forming a drainage outlet to the south, called the Briston Gap. 12
Stage 4 Salthouse outwash plain and Blakeney Esker (Figure 8) A further phase of glacier retreat occurred with the ice margin lying along the northern and western edges of Salthouse Heath as indicated by the heath s steep northern and western ice contact slopes. The Blakeney Esker was formed during this stage within a sub-glacial drainage channel that was feeding the Salthouse outwash plain. The kames and kame terraces within the Glaven Valley were probably also formed at this time. This new drainage pattern creates a geomorphological problem that is currently unresolved. The Salthouse outwash plain appears to have been bounded on all sides, either by glacier ice, the Cromer Ridge or the Kelling outwash plain, which is 11 m higher than the Salthouse outwash plain, so where did the outwash drain to? Did it form a lake infront of the glacier? Or perhaps drain through the older permeable Kelling outwash sediments? Stage 5 present day (Figure 9) Following complete deglaciation the landforms of the Blakeney area have experienced surface weathering for several hundred thousand years, during which at least two interglacial warm stages (including the present day) and one further glacial episode are known to have occurred. This, together with gravel quarrying, has resulted in the partial erosion of the landforms, although their surface expression is still evident today and is testament to a very dramatic and significant climatic episode within Norfolk s past. 13
Figure 5 Stage 1 formation of the Cromer Ridge push moraine. 14
Figure 6 Stage 2 formation of the Cromer Ridge outwash fan. 15
Figure 7 Stage 3 Formation of the Kelling outwash plain. 16
Figure 8 Stage 4 Formation of Salthouse outwash plain and Blakeney Esker (K = kames and kame terraces). 17
Figure 9 Stage 5 present day. 18
EXPLANATION BOXES Explanation Box 1 Explanation Box 2 Explanation Box 3 19
Explanation Box 4 Explanation Box 5 Explanation Box 6 20
Explanation Box 7 Explanation Box 8 21
Explanation Box 9 Explanation Box 10 22
Explanation Box 11 Explanation Box 12 All figures featured are BGS NERC 2006 23
Discussion points / homework topics 1. Geologists have only just agreed that the ridge is actually an esker. Other ideas included: a. open crevasse filling (filling in of crevasses in the glaciers surface then when ice melts it leaves a ridge of sediment) b. a linear remnant of a larger mass of sand and gravel that has been eroded away c. formation in an ice-marginal environment. How did geologists discount these theories and conclude that it was an esker? ANSWER: The quarrying that revealed the presence of Nye channels in the till surface, is vital. The Nye channels are aligned with the ridge, strongly suggesting that they have a close association in terms of origin, suggesting that the ridge formed by streams flowing along its length. This means it was not deposited onto the surface of the ice in crevasses or as a large sheet, as suggested in theories A and B. Analysis of the ridge sediments has demonstrated pipe-flow conditions, which means they were deposited in a subsurface channel, meaning it formed subglacially (underneath the ice) rather than in an ice-marginal environment (in front of the glacier). 2. Why do you think the chalky till is so chalky? Where does the chalk come from? 24
ANSWER: Till is made up of all the rock types that the glacier moves over as it progresses over the land (including land that is now the sea bed). Geologists know that the glaciers flowed from the north. Looking to the north, the area near the east coast from Flamborough Head down to Norfolk is the only region with chalk as bedrock that the glacier could have come travelled over, suggesting the chalk has been carried from here. See BGS geological maps for more information and to support this explanation. 3. Suggestions on teaching this topic: Pose the question how did our local landscape form? and brainstorm the possible suggestions, perhaps burial grounds, something crashing from outer space, did the ridge grow from the ground, is it the remains of a older higher land surface etc. Following this, looking at the possibility that ice was involved, provide the Icelandic analogues as described in this document and assess the similarities. 25