Leader: Derek Brumhead

Derek leading a field trip at Tegg's Nose
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Photos taken at the Groups' Summer Display, Tuesday 21 August 2012

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GEOLOGICAL TRAIL OF TEGGS NOSE.

Geology 1 field trip, Monday 3 June 2013.

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Tegg's Nose Country Park is situated on the Buxton-Macclesfield Old Road at SJ 950732. The site is designated a Regionally Important Geological Site (RIGS). A geological trail leaflet is available from Tegg's Nose Country Park Visitor Centre, from which some of the following information has been derived.

The escarpment of Tegg's Nose is formed of Chatsworth Grit a formation near the top of the Millstone Grit Series. The large quarry (Location 8) provides the best exposure of this rock east of Macclesfield. The grit formation is about 200 feet thick but this includes a number of shale beds, especially in the upper part. The basal layers are a massive, tough, compact, fine-grained free-working sandstone, above are flagstones. Pebbles of quartz and feldspar are common. Ferruginous concretions occur up to 8 feet across and these soften and disintegrate on weathering leaving cavities in the rock face.

Location 1. This round boulder is a glacial erratic brought here from the Lake District by an ice sheet and deposited about 15,000-20,000 years ago when the ice melted. It is a volcanic rock and the holes in it are 'popped' bubbles of gas.

Location 2. Along the path appears a fine view to the east of Shutlingsloe, a peak 1659 feet high. It stood above the ice during the last ice age. It is a residual hill capped by an isolated outcrop of Chatsworth Grit, the same rock forming Tegg's Nose. This shows that between the two localities this formation has been worn away from the landscape over tens of millions years.

Fig. 3. The hills to the east. Shutlingsloe is centre skyline.

Location 3. This fine rock face shows layers of well-jointed gritstone in the lower half and immediately above is an excellent section of tabular cross bedding showing that the sand was deposited in a river channel flowing from left to right. Immediately above there are an increasing number of shale beds interbedded between layers of sandstone.

These rocks were deposited about 330 million years ago during the Carboniferous period. At that time the British Isles did not exist as a separate entity, since all the present-day continents were fused together into an enormous supercontinent called Pangea most of which lay in the southern hemisphere. The British Isles and western Europe lay across the equator and hence experienced a humid tropical climate .At that time, the whole of northern England, consisted of extensive water-logged plains or deltas at or just above sea level. They were fed by rivers bringing sediment from a great range of mountains which lay where Scandinavia and northern Scotland exist today in the same way today that the Ganges delta is fed by rivers from the Himalayas.

Much of the sand was deposited in a series of braided low-sinuosity river channels flowing across the top of the delta. Sedimentary structures (sand banks) typically formed in such an environment include planar (tabular) cross stratification (bedding), as shown here.

Fig. 5. Photograph from the Tegg's Nose Geological Trail, (Cheshire East Council). The trail leaflet is available from the Tegg's Nose Country Park Visitor Centre.

Location 4. The quarry machinery on display here includes a rock crusher, a crane, and a swing saw. Quarrying took place as early as the 16th Century and continued until 1955. Working of the gritstones, hardened by silification, stopped due to the steadily increasing overburden of the more shaly rock above.

Location 5. From the quarry machinery continue along the path to a right fork leading to this location with a splendid view over a sloping quarry face. This is a huge joint plane, a line of weakness which was exploited and exposed by the quarrymen as they levered the rock away in blocks. It is now used as a climbing wall. A little further along the path crags on the right show rock that is thinly bedded producing the hard, durable flags for making the roofing slabs and pavements for the Macclesfield streets that Tegg's Nose quarry was noted for (Fig 10). From here return along the path noting that in one place there is a rock slab on the ground with a good example of ripples marks, best seen when wet (Use your water bottle if it's not raining !).

Location 6. From here it can be seen that the large quarry face (Fig. 8) shows thick gritstone beds at its bottom. Noticeable are several oval or irregularly shaped depressions (Fig. 9). These are where softer concretions have been worn away. They tend to be iron-rich, probably originally composed of sandstone cemented by iron-bearing calcite, the carbonate mineral, siderite (FeCO₃) instead of silica. Removal of the calcite by solution in ground water leaves iron-stained sand behind which is easily weathered and eroded to leave the cavities in the rock face.

Location 7. From here walk further along the path and take a fairly steep grassy track into the quarry where there is a viewing point by a fence. This is a much better close-up view here of the climbing wall. The rock face on the right is a fault plane. A pair of binoculars will reveal some slickensides – scratch marks on the rock (high right to low left) where the rock has slid against other rock. There is also another worn-away concretion.

Location 8. is a good place to stop for lunch (if you have brought it!) with stunning views over Shutlingslow and the Macclesfield Forest.

Fig. 8. The large quarry face at Tegg's Nose.

Location 9. This is a fine viewpoint towards the west and south of Tegg's Nose. An information panel allows you to identify the landscape features. The reservoirs were built for Macclesfield’s water supply.

Location 10. From Tegg's Nose summit to the west there is a view of the Cheshire plain and Alderley Edge, a Triassic sandstone escarpment. The Red Rock Fault, running north-south along the edge of the high ground, brings the Carboniferous rocks and Triassic rocks next to each other probably with a throw (displacement) of at least c. 600 feet, probably much more in some localities. (Some have suggested it is an unconformity !)

Words and photographs by Derek Brumhead.

References:

• Tegg's Nose Rocks ! Cheshire East Council/Rangers. Supported by the Geologists' Association Curry Fund and Cheshire RIGS
• W B Evans, et al. Geology of the Country around Macclesfield, Congleton, Crewe and Middlewich, HMSO, London, 1968, p. 63.

THE TORRS, NEW MILLS: WOODHEAD HILL ROCK.

Geology 2 field trip.

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Introduction
The town of New Mills in stands astride the River Goyt, at its confluence with the River Sett. Both rivers are deeply incised to form an impressive gorge, 20-40 metres deep, called the Torrs. The gorge was cut at the end of the last (Devensian) glaciation about 10,000-15,000 years ago, by subglacial meltwater from a glacier which occupied the Goyt valley. When the ice melted, the River Goyt was diverted from its original course into this newly-formed gorge. The sides of the gorge are formed of sandstones, sedimentary rock made mainly of quartz grains. These sandstones are named  the Woodhead Hill Rock, deposited during the Westphalian stage of the Carboniferous period (some 313 to 303 million years ago). This is the oldest formation of the Carboniferous period recorded in north west Derbyshire.

At that time the British Isles and western Europe lay across the equator during the Carboniferous and hence endured an equatorial climate. Huge river systems fed off the uplifted highlands and flowed over what are now northern England, southern Ireland and western Europe. These rivers transported sand and mud, which were carried from the north to the south and west feeding large deltas as they entered the sea. Much of the sand was deposited in a series of braided low-sinuosity river channels flowing across the top of the deltas which gradually built out into a large marine basin. The sandstone units visible in the Torrs contain evidence of being transported by rivers. Sedimentary structures typically formed in such an environment include trough and planar (tabular) (Figure 2) cross stratification (bedding), and parallel lamination.

Trail
From the front of the bus station, turn left and take the second road on the left, Station Road, signposted to New Mills Central Station (rail). Immediately after crossing the rail bridge, take the footpath on the left and walk down onto the Millennium Walkway.

Warning. You are reminded to take care when approaching the rock faces on this trail and not to make any attempt to hammer or try to dislodge any piece or to climb on any part.

Location 1
Walk to the far end of the Millennium Walkway and bear right down the slope to the bank of the River Goyt.

On the opposite bank of the river, there is an outstanding exposure of the Woodhead Hill Rock. These sandstones were deposited in large river channels, the flow of water producing the characteristic cross bedding. The lowest portion of the cliff consists of a stack of interlocking, tight-crested trough cross stratified sandstones. This bedding type was formed by the downstream movement of large underwater (subaqueous) sand banks gradually migrating along the channel axis.

You are looking downstream at a section cut across the front of curve-crested sandbanks. The rocks above these units contain large, planar tabular cross stratified beds, produced by straight crested sandflats. These bedforms record the movement of large sandflats and bars that migrated obliquely down the channels in a zig-zag fashion, and the thickness of the bed (from base to top of individual cross beds, separated by bedding surfaces) suggest high energy deposition within deep channels of a large braided river system. Eventually, given delta conditions and the prevailing equatorial climate, luxuriant vegetation would colonise the areas between the braided river channels. It is this vegetation that, after death and burial by further sediments, formed the coal seams of the Carboniferous. Around New Mills, the Woodhead Hill Rock is capped by the Yard Seam (known as Bassy elsewhere in the Pennines), and as a result the area was important for coal mining from the eighteenth century.

Location 2
Walk up the slope, away from the river, back onto the main path and towards the impressive rock face, a former quarry.

Here you can look closely at the Woodhead Hill Rock (Figure 4). It is a coarse grained sandstone, mainly composed of sand-sized (2mm-0.062mm diameter) quartz and alkali-feldspar grains, bound by a siliceous (SiO₂) cement, a natural chemical precipitate. Quartz and alkali feldspar are minerals derived from the erosion of igneous and metamorphic rocks some distance north and transported by rivers into the deltas. Whereas the quartz grains and siliceous cement are quite stable, the feldspar grains have altered over time and now consist of whitish or cream-coloured clay minerals, with only the rectangular cross sections of the original feldspar minerals preserved today.

Figure 4.

The 30m high face consists of individual beds stacked on top of each other each about 1.5 metres thick suggesting a water depth in the river of around 4m. Each bed contains large-scale planar cross stratification, representing the down-current structure in sand banks (Figure 5), indicating that successive stream flow was maintained in the same direction, in this case from right to left. It was not a meandering flow but braided, powerful, large channels of a delta system.

At various points on the lower quarried face there are sections through what appear to be ripples (Figures 6 and 7). Close inspection will reveal that the features have no internal structure. It is not easy to be sure of the origin of these unusual structures but a possibility is that they are just the top of cross-stratified deposits which have been differentially eroded along the bedding planes forming an eroded surface which mimics ripples. Sand was then loaded down from above onto the uneven surface. These features can be seen at various scales.

Figure 6

Figure 7

Cavities up to a few metres in diameter plus patches of eroded sandstone are common at certain levels in the Woodhead Hill Rock. They mark the position of nodules and concretions (Figures 8 and 9). They tend to be iron-rich, probably originally composed of sandstone cemented by iron-bearing calcite, the carbonate mineral, siderite (FeCO₃) instead of silica. Removal of the calcite by solution in ground water leaves iron-stained sand behind, which is easily weathered and eroded to leave the cavities in the rock face. The presence of organic matter and its bacterial decay in the original sand is probably the cause. A number of such structures are seen at Location 2.

Figure 8

Figure 9

Location 3
Walk under the Union Road Bridge, built in 1884, which spans the River Goyt. On reaching the footbridge, note the impressive crag on the other side of the river with fine planar (tabular) cross bedding (Figure 10).

From here follow the path bearing round to the left.
The rock face here shows a small feature which has been described as soft sediment deformation, where the thin beds have slumped to produce an inverted vee shape (Figure 11). There are several examples of this nearby, some very small. Such structures may have been caused by the escape of water from below, during or soon after deposition, and are possibly related to some local disturbance, such as an earth tremor. Some adjacent beds also show tiny faults which may be related.

The bedding below this feature is an example of planar stratification, a sedimentary structure of parallel thin layers of sediment. The layering is picked out by changes in grain size, orientation, grain composition and colour. Traction currents move the coarser grains (sand) with suspension settling in between (clay). These beds are typical of tidal deposits within an estuarine environment.

Location 4
Continue round the corner for a few metres.
Another water escape feature is well preserved within a well cemented concretion (Figure 12) seen at the undercut base of the rock face (at waist height). This shows thin bedding or laminations in a sandstone which have been deformed (altered from their original shape) into an inverted vee-shape.

They pass through the concretion and into the rocks on either side, showing that the sedimentary feature formed just after deposition, probably as a result of water escape due to compaction of the sands, or slumping down the front of a sand bank, and the concretion is later, a post-depositional feature. As in previous examples, it is being weathered out, and the cements in the sand make this concretion slightly harder, so the feature stands proud in the rock. The ferrous iron-bearing chemical compounds are being removed by solution in the ground water producing the iron-staining.

Location 5
Continue along the path for about 15m.
At this locality, you will see the two rock faces set at right angles to each other, developed along the faces of joints and fractures (photographs). Both faces consist of thick units of stacked planar cross stratified sandstone, having a combined thickness of over 20m. Due to the advantage of seeing the rock faces at right angles, it can be noted that the beds are not of a constant thickness, some of them appear to thin out as wedges, and the direction of deposition varies, due to variations of the original bedforms (bars and sand banks deposited in river channels by fast flowing currents) (Figure 13).

Prominent at this locality are vertical, horizontal and inclined fractures. No displacement across these fractures is evident and so these features are interpreted as joints widened as a result of pressure release during uplift and removal by erosion of the overlying strata.

Before proceeding to Location 6, retrace your steps and cross the footbridge spanning the River Sett. This gives access to an excellent viewpoint opposite Location 5. From here, it is possible to determine what the former or palaeocurrent direction might have been in the braided river channel containing the sediments.

Location 6
Cross back over the river, walk past Location 5 and pass under the Midland Railway viaduct built in 1867.
At this locality the remains of a weir crossing the River Sett can be seen. It was built by mill owners in the late eighteenth century to control the flow of water along a specially-cut channel to a mill (now disappeared) a few metres downstream, just before the railway viaduct. The weir uses a natural rock step in the river bed. In the opposite river bank good examples of deformed cross bedding may be seen.

Location 7
Continue along the path to some steps. Climb the steps, pass through the gate and turn left up the path.
At the top on the right is a small, lichen-covered rock face, displaying well-bedded sandstones. Each bed consists of prominent cross stratified units (Figure 14), the boundary between each unit being an erosion surface. The bedding planes at this locality are clearly defined by the presence of lichen. The lichens have colonised the thin partings of siltstone or clay, that occur at the tops of the beds and which have been weathered out in preference to the coarser, tougher, sediments.

Adjacent to the right, the rock face at this locality was formerly the rear wall of a house or millworkers' cottage.

Location 8
To finish this geological tour, continue along the path for 10m - 15m until you come to the site of the former chain horse house. Here were stables used to house the horses which hauled the mill products out of the Torrs.
The rectangular and square blocks of rock making up the floor of the chain horse house are 'flags' or 'flagstones'. If you have walked on Kinderscout or Bleaklow, you will already be familiar with such blocks, as they have been used to protect the footpaths in these areas. The name 'flag' refers to the ease with which the material could be split into thin slabs, parallel to the fine layering or lamination in the rock. The property is due to the presence of bands rich in mica (a complex mineral, largely composed of aluminium, silicon and oxygen) in flat, sheet-like grains lying along the layered surface. In dry, sunny weather, silvery micas may be seen glinting on the surfaces of the flags. Note that weathering and erosion have removed sections of the thin laminations on the surfaces of some of the flagstones.

The surfaces of many flags appear to have a 'grain', rather like the 'grain' in woody material (Figure 15). This structure or feature is called current lineation and it was formed when elongate mineral fragments were lined up parallel to the flow of water. Like cross bedding, current lineations provide evidence of the orientation of ancient water flows (palaeocurrents).

From here walk up the setted lane into New Mills.

This trail is an abbreviated version, text and photographs by Derek Brumhead and Jonathan Redfern. With contributions by Hedley Hickling, Fred Broadhurst, Colin Jones and David Thompson.

GLOSSARY