Swallow Hole viewed from SW

Location Grid Reference TL 207246, on private land.  No public access.

This is a small seasonal bourne leading to a large swallow hole where floodwater percolates through gravels into the Chalk bedrock. The swallow hole area and a blind valley to the south-east flood occasionally.

The Totternhoe Stone here was formerly quarried for building stone. It is a Hertfordshire and Middlesex Wildlife Trust Nature Reserve. Only a few small exposures of Chalk can now be seen.

Location Grid Reference: TL 381 366
This is a small disused chalk pit which lies 0.7km to the north of the village of Barkway and 4km south of Royston, near the top of the north-facing chalk scarp slopes.  As well as a RIGS, it is a Hertfordshire and Middlesex Wildlife Trust Nature Reserve.

The site is of geological interest due to the chalk exposure, which was pushed over boulder clay by the Anglian Stage Glacier over 400,000 years ago.  The face is a section through two glaciotectonically moved blocks of chalk containing the chalk rock and beds immediately above and below. The succession in the larger block extends down to the sub-chalk rock (Reed marl). This block is traversed by a fault that repeats the higher part of the succession. The two blocks are separated by Anglian till and are faulted against till. This is one of a group of pits (including Reed Chalk Pit) where ‘out of position’ chalk is exposed as a result of being displaced by the Anglian glacier impinging on the Chalk scarp.

Take a virtual tour with Dr Haydon Bailey of the HGS, filmed for the 2020 Geologists’ Association virtual Festival of Geology (vFoG).

The Hertfordshire Bourne (Bourne Gutter) was made famous by John (later Sir John) Evans, who was the first in 1878 to describe its intermittent flow. Later, whenever the bourne flowed, it was visited by members of the Hertfordshire Natural History Society, often under the leadership of John Hopkinson. An early myth was that the bourne was a woe water which flowed at times of war or natural disasters – typically every seven years! But the Victorian capacity to observe and measure the records by Evans, Hopkinson and others indicated whenever rainfall in the previous calendar year exceeded 32 inches, flow occurred for a period between January and June.

The precise extent of the flow is less predictable as various parts of the valley can be occupied by the surface stream in different years or even the same year. Very often flow starts at a series of springs just above White Hill (SP 991052), and sometimes a small lake accumulates here beside the road, but at other times it has started a short distance downstream below Mounts Hill (Mounts Rise). Sometimes the flow terminates in a swallow hole just upstream from Bottom Farm, but more often it flows through the garden of Bottom Farm and across the meadow to the east, then terminating either in a large gravel pit (TL 005061), which acts as another swallow hole or, when this overflows, the stream can extend through a culvert under the new A41 to the appropriately named hamlet of Bourne End, where the bourne joins the River Bulbourne

You can find a virtual guided tour on the HGS Active Sites page,

Car Parking  On or near the bridge where Swing Gate Lane crosses the new A41 on the southern side of Berkhamsted (SP 997065).

Location From the parking area you can walk down the lane to Bottom Farm on the floor of the Hertfordshire Bourne valley.

It is possible to follow public footpaths and roads from Bottom Farm to Bourne End. Please respect – access to fields where there are no public footpaths requires land owner consent 

Catering options include numerous cafes and pubs in central or canal side in Berkhamsted. Or the White Horse pub in Bourne End.

Hertfordshire Geology & Landscape p. 271 – 272

Pierpoint, N. (2013). ‘Stream flow in the Bourne Gutter near Berkhamsted – March-April 2013’. Hertfordshire Naturalist: Trans. Herts. Nat. Hist. Soc., 45 (2): 140-144

Pierpoint, N, (2014). Observations on the Bourne Gutter 2014 Hertfordshire Naturalist: Trans. Herts. Nat. Hist. Soc., 46 (2): 136-140

Nearby Attraction Berkhamsted Castle (HP4 1LJ) is a Norman motte-and-bailey castle in central Berkhamsted. The castle was built to obtain control of a key route between London and the Midlands during the Norman conquest of England in the 11th century.

Puddingstone on roadside verge

Location Grid Reference – SP 993 085,  Berkhamsted HP4 1HE

Perhaps the most enigmatic rock found in the county is Hertfordshire  Puddingstone, many fine examples can be seen in church buildings or as
free-standing boulders on the roadside. Several large blocks of Hertfordshire Puddingstone are accessible for inspection on the grass verge outside the house No. 13 Castle Hill.  The largest block is partly sarsenstone (silcreted sandstone usually devoid of flint pebbles), though the sarsen part contained a line of dispersed pebbles. The importance of this specimen, demonstrating the close relationship between puddingstone and sarsen, has been recognised in designation of the group as a Hertfordshire RIGS site.

Puddingstone conglomerate

The puddingstone and sarsen  originated by silicification of pebble beds or sand in either the Upnor Formation (late Palaeocene) or Reading Formation (early Eocene). As these deposits extended up the sub-Palaeogene erosion surface now exposed as the 6 dipslope of the Chilterns, the blocks had moved down the slope of Castle Hill from the plateau surface north of the Bourne Valley. The silcretes had probably formed within soil profiles developed in humid subtropical conditions on these formations in the Palaeogene, i.e. soon after their deposition.

Location Address – Castle Hill, Berkhamsted, HP4 1HE Hertfordshire

Car Parking – roadside parking possible

Catering – options include numerous cafes and pubs in central or canal side in Berkhamsted

Nearby Attraction – Berkhamsted Castle (HP4 1LJ) is a Norman motte-and-bailey castle in central Berkhamsted. The castle was built to obtain control of a key route between London and the Midlands during the Norman conquest of England in the 11th century. Can you find the dozen or so blocks of puddingstone incorporated in the castle walls?

Take a virtual tour with Nick Pierpoint (HGS) .

For more information take a look at:

• Hertfordshire Geology & Landscape p. 108 – 117
• Our Puddingstone page
• East Herts Geology Club’s webpage on Puddingstone

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Hill End Chalk Pit is a small, disused chalk pit located 3km to the west of Stevenage.

Location: Grid Reference TL 197 239.
The B651 road passes the site; on the opposite side of the road is a car park that serves both the chalk pit and the adjacent Hitch Wood.

The area owned by NHDC contains the disused pit, a small area of chalk grassland and associated scrub, with mature woodland around the peripheries.  The quarry is the type locality of the Hitch Wood Hardground at the top of the Chalk Rock. This hardground is exceptionally fossiliferous here, and has probably yielded more fossils of all groups (notably ammonites) than any other Chalk Rock locality apart from the Kensworth Chalk Pit GCR site, Bedfordshire.

Hill End Chalk Pit is Registered Common Land and a Local Wildlife Site due to its chalk grassland habitat. 

A brief history of the site and it’s geological importance is available via a pdf which can be downloaded here.

Take a virtual tour with Dr Haydon Bailey of the HGS, filmed for the 2020 Geologists’ Association virtual Festival of Geology (vFoG).

TL 196295
Access restricted private ownership.
Chalk faces above Hitchin railway station car park. Middle Chalk above Melbourn rock. Southern wall stratigraphically important. Former SSSI.
This geosite has not been visited in the recent past due to Lockdown and Covid considerations – the status reflects the situation in 2008. Once circumstances permit the site will assessed and relevant details provided here.

Geomorphological site interpreted as naleds or icings (depressions where large masses of ice accumulated on the valley floor because water from a spring froze semi-permanently in the Late Devensian).

Sedge vegetation indicating different soil conditions in a depression.
The site slopes gently down towards the railway embankment on R.

These are a scattering of subtle depressions in ancient meadows along the Bulbourne valley SW of Hemel Hempstead.  They are located on the valley floor or the lowest part of the side slopes, and can be recognised in dry weather by different vegetation such as sedges, and during wet periods by the fact that they get marshy or sometimes hold standing water.  During the Devensian (most recent cold period of the current Ice Age) the ice did not reach this far south, but the conditions here were periglacial, with permafrost below the surface and an active layer above subject to annual freeze-thaw.   Depending on the hydrology (surface and sub-surface water flow, springs etc) a lens of ice might grow below the surface forming a bulging pingo, or a semi-permanent body of ice known as a naled or icing might form on the surface.   Both kinds of ice accumulation can leave an impression on the ground when they eventually melt, which initially forms a pond then gradually silts up.  See diagrams below.    When the A41 was built through one side of the meadows in 1992, one of the features was trenched and soil samples analysed.  This confirmed the existence of a subsurface depression in the chalk bedrock roughly 3m deep and 12m in radius, with the lower 2m of fill consisting of peat.  Palynology and related analyses showed aquatic species near the bottom, and a sequence of colonising woodland species such as birch, pine and oak in the upper portion.  This feature was described as ‘the Boxmoor Pingo’ in the report.

Naled lifecycle – formation to relict

Pingo lifecycle – formation to relict

Relict pingos and icings are sometimes hard to tell apart without very detailed subsurface investigation, and the features at Boxmoor are likely to include both types. Pingos tend to be circular, while naleds are elongated along the direction of flow.  Pingos are expected to have a raised circular rim (as can be seen dramatically in examples such as East Walton Common in Norfolk), but the Boxmoor features are small and any rampart would be unlikely to survive the erosion of several thousand years.

Similar features can be seen in rough meadows on the upper reaches of several other chalk streams, notably the Gade and Ver valleys. 

Location Grid Reference TL037060 – the series of meadows or ‘moors’ collectively known as Moorend Farm Meadows or Boxmoor Common, located between the A41 and the old London Road. The meadows are now cut off from the river Bulbourne not only by the A41 (on an embankment screened by trees) but also the railway line (1842) and the Grand Union Canal (1819).  The features can be seen most clearly in the eastern section known as Herdsmans Moor, with the ‘Boxmoor Pingo’ at TL038059 in the small triangular section to the east of the access lane Old Fishery Lane that passes under the A41.  Open access throughout; there may be grazing animals. The area is managed by the Boxmoor Trust (see below), and maps can be found on their website. 

Car Parking Limited roadside parking in side roads off London Road, with some parking restrictions.  

Catering options ???

Herts Geology & Landscape p. 176

Additional notes: the Boxmoor Trust

The fore-runner of the Boxmoor Trust originated in 1594, when the Boxmoor Estate was created from the meadow lands around the Rivers Bulbourne and Gade, for the benefit of inhabitants of the Parish of Hemel Hempstead and Bovingdon.  Over the centuries the Estate purchased or was bequeathed further tracts of land, including chalk downland as well as water-meadows. The present Boxmoor Trust is a thriving community resource and environmental asset.  Their web site gives details of the various meadows, environmental projects, walks and trails, and much more.

TL 116046
Access restricted private land enclosed by fence.
Paramoudra and marl seams in Upper Chalk.
This geosite has not been visited in the recent past due to Lockdown and Covid considerations – the status reflects the situation in 2008. Once circumstances permit the site will assessed and relevant details provided here.

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Looking up Clay Pit Hole.

Geomophological Site: Chalk escarpment with steep-sided coombes and chalk springs. 

Location: Grid Reference TL 104293. Ravensburgh Castle Hill Fort lies on a spur of the Barton Hills, one-mile South West of Hexton, and occupies the West half of a plateau surrounded by deep coombes on every side but the North West.

Important: This RIGS site is on private land and requires the landowner’s consent to access the site – a public path (TL 106302) cuts through the eastern flank of the valley providing glimpses of the steep sided coombe valleys.

The morphology of the valleys on the scarp face slopes contrast with characteristics of those on the dip slope. In this part of the NE Chilterns scarp face valleys are typically steep sided slopes, usually short, blunt ended; often have a flat valley floor marked right angle bends. Their ‘youthful’ appearance suggests they may result from a later stage of erosion. 

The striking feature of the coombes are the distinctive right-angle bends which may reflect some structural control on erosion. Possibly from deep basement faults re activated during the Alpine orogeny or some workers have suggested they may be attributed to a rectangular jointing pattern.

The exact mode of origin of scarp face dry valleys of the Chilterns is ambiguous but much of their development can be attributed to late Devensian gelifluction, but this followed earlier nivation, incision by meltwater from snow and ice or headward erosion by spring sapping. Which continues today where the valley floor intersects the water table.

Within the valley network a small spring emerges at Bur-well springs at the point where Claypit Hole meets the main valley and disappears at approx. TL 105300 dependant on the state of the aquifer. The springs emerge over the less permeable layers such as the West Melbury Marly Chalk Formation.

Car Parking: street parking is possible in the village of Hexton.

Catering: Local pubs include the The Raven in Hexton and The View in Pegsdon. More options including shops can be found in Barton-le-Clay.

Hertfordshire Geology & Landscape p.179-180 synthesis of the potential processes resulting in the formation of the distinctive scarp slope dry valleys within the Chilterns.

Nearby Attractions: Barton Hills and Deacon Hills.

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TL 359370
Within HMWT Nature Reserve. Former SSSI.
Glaciotectonically disturbed, highly fossiliferous chalk pit.
This geosite has not been visited in the recent past due to Lockdown and Covid considerations – the status reflects the situation in 2008. Once circumstances permit the site will assessed and relevant details provided here.

TL 361403
This Nature Reserve is a bat hibernaculum.
Middle Chalk with exposure of Reed Marl.
This geosite has not been visited in the recent past due to Lockdown and Covid considerations – the status reflects the situation in 2008. Once circumstances permit the site will assessed and relevant details provided here.

This is a glacial erratic boulder of Millstone Grit (Carboniferous sandstone), brought by the Anglian ice sheet probably from Derbyshire or Yorkshire.  Although described on the nearby information board as a ‘pebble’, it is roughly a meter across. Also known as the Royse Stone or Roisia’s Cross, it has stood as a landmark at or near this location for more than 900 years.  It was used as the base of a cross set up at the crossroads of Ermine Street and the Icknield Way soon after the Norman conquest, when the town of Royston did not yet exist.  The cross itself has long disappeared, but the square socket-hole can be seen clearly.  The stone has been moved slightly to avoid inconvenience to modern traffic.

Location Grid Reference TL356407, in Royston town centre, in a small pedestrian area on the south side of Baldock Street where it becomes Melbourn Street.

Car Parking Roadside parking is limited, but there are carparks nearby.

Herts Geology & Landscape  p.157

Nearby Attractions

The Royston Cave is a circular, bell-shaped chamber cut into the chalk bedrock below the same crossroads in the centre of Royston. It is 8 metres high and 5 metres in diameter, with numerous medieval carvings on the walls.  The origin of this chamber is unknown, but the numerous carvings around the walls have suggested various users, including  Knights Templar and Freemasons.  Consult the Royston Cave web site for details, including opening times.

Therfield Heath SSSI, just outside Royston, is an area of unimproved chalk grassland habitat with a rich chalk plant community, including the rare pasque flower.

Chalk was frequently quarried in Hertfordshire and across the Chilterns , but besides being quarried it was often more convenient to sink shafts to mine chalk from just below the Lower London Tertiaries.  The ‘Chalk Drawers Arms’ pub at nearby Colney Heath (TL 208060) is a reminder of this activity in which small groups of men sank shafts, excavated the chalk and spread it on the land as a primitive manure.   The Shenley Chalk Mine is one of the few places where the surface features of this activity can be examined by geologists.

All that can be seen on the surface are three entranced shafts into this underground complex of excavated chambers and tunnels in the Upper (or White) Chalk.  After the removal of an estimated 23,000 tons of chalk the chalk mine was closed on the 12^th of April 1913 as evidenced by a soot graffito deep underground.  One of the entrance shafts is now a ‘wishing’ well in the garden of the first cottage, the second is a brick plinth with a securely locked grill and the third is a ‘blockhouse’ again with a locked grill.  These are marked 1 -3 on the plan of the underground workings.

With the aid of a powerful torch the second shaft (shaft 2 on the plan) can be examined.  The shaft penetrates the Reading Beds, which being friable are lined with brick, before entering the solid Upper (or White) Chalk.   A cone of debris can be made out at the bottom of the shaft as the shafts had been used for the disposal of rubbish since they were abandoned. The underground network of dome-shaped excavations is linked by galleries at a depth of 20+ metres.  All are excavated within the solid chalk except for one face of a chamber which is predominantly of clay.   This marks the position of a fourth, infilled, shaft which coincides with the deep pool on the southern side of the trackway opposite the cottages.   There is no evidence of water ingress into the Chalk mine, either at the present or in the past.   The area around Pinks cottages and the former Pinks hotel bears the marks of previous industrial activity including chalk quarries, brick pits and sand pits.  No archaeological remains connected with these activities have been identified.

Nearby geological features

In the field to the north-west of the cottages, which is accessible by a stile, there is a distinct circular depression.  This probably marks a collapsing doline.

About a mile away along a footpath which follows the Catherine Bourne, is a large swallow hole where the bourne disappears ( TL 214015 ).

Access: The chalk mines are on private land and permission of the owners who live in the first cottage is mandatory to visit the surface features of the site.  The underground mines are now a bat hibernaculum which is visited regularly by the Hertfordshire Bat Group.  The mine complex can only be entered by special arrangement with the Hertfordshire Bat Group on one of their visits.  The visit would involve abseiling down the 20 m deep shaft and climbing back up again on a caving ladder, so it should only be attempted by seriously fit persons with some caving experience.

Location: Grid Reference  TL 203013;  SatNav  WD7 9AW . Vehicular access is down an unmade road off Rectory Lane, Radlett which leads to two cottages and the entrance to a farmyard / industrial area.  The locality can also be accessed via several public paths leading from Ridge, Shenley and South Mimms where public houses can be found which offer food and drink.

Car Parking: Two to three vehicles can be tucked in along the track just before the farm yard entrance.  Larger groups need to ask at the first cottage before a visit is planned, when a field at the end of the track can sometimes be made available for additional parking. 

Nearby Attractions:  The de Havilland Aircraft Heritage Centre, which is currently open from March to October on Tuesday, Thursday, Saturday and Sunday, is two miles away by road (SatNav AL2 1BU).  Visit www.dehavillandmuseum.co.uk    for up to date details.

Additional notes

The mines were almost certainly associated with the brickmaking industry which has been on this site since at least the mid-eighteenth century.  The ‘Topographical map of Hartford-Shire’ issued by Dury & Andrews in 1766 notes a ‘brick kiln’ as being precisely on this site while in some of the issues of William Smith’s famous geological map of 1815 he marked this site with a blue dot, which was probably his convention for a limestone quarry or brick pit.   The chalk was also used for liming the land as J. Harris, in his privately published book The Parish of Erith dated 1885, states  “at Shenley in Hertfordshire chalk is being used for agricultural purposes and for making lime.  It is not taken from the surface of the ground, but excavated about 90 ft. below the top by means of a shaft and windlass.”

   I examined the clay plug which is situated at the end of the westerly gallery of the chalk mine (shaft 4 on the plan produced by the Chelsea Speleological Society – Fig 1 ) in 2018.  The plug is of compacted clay with an admixture of black, well rounded ‘Reading Beds’ pebbles, chalk fragments and broken bricks.  This mix of materials is consistent with it being waste from the brickworks which had been disposed of down an abandoned chalk mine shaft.  This must have taken place prior to the excavation of the currently accessible network of chalk mines.

There are distinctive linear extrusion marks on the clay where it abuts against the Chalk bedrock indicating that the plastic clay plug is being slowly extruded due to the weight of the infill of the 20 m. of clay and other debris in the shaft. These are very like slickenside striations found on fault planes which are evidence of differential earth movement.

REFERENCES

Catt, J (ed), (2010)  Hertfordshire Geology and Landscape. pgs 201 & 303-4

Flood, S. & Ruston, A . (2004).  Dury & Andrews map of Hertfordshire 1766.  Hertfordshire Record Society.

Howgate, M.E. (2019) Hertfordshire Naturalist 51. pgs 54-5

A plan of the Shenley chalk mine simplified from an original survey by Rod LeGear, Harry Pearman and Terry Reeve. 
Reproduced by permission of the Chelsea Speleological Society.

                                                                         Michael E. Howgate 

Tring Park is a 132.9-hectare site to the south of Tring, halfway between Aylesbury and Hemel Hempstead. The site runs along the Chiltern ridge and sits within the Chilterns Area of Outstanding Natural Beauty (AONB).

At Tring Park we can see a prominent and dramatic section of the Chilterns scarp slope.  The escarpment folds and twists along its length and forms a natural amphitheatre around Tring Park. The chalk scarp is a marked geomorphological feature, formed by Pleistocene ice erosion. The dominant rock underlying Tring Park is chalk which extends throughout most of Hertfordshire providing much of the relief to our landscape. The crest of the scarp marks the line of the Upper Chalk, while the middle and lower slopes (and the hollows of Tring Park below) are formed on Middle Chalk. Under a new nomenclature the Upper and Middle chalk are both called the White Chalk.

The highest part of Tring Park is capped by a red/brown clay deposit termed ‘Clay with Flints’. This constitutes a weathered residue the Lambeth Group (56-59 mya) of clays, sands and flint cobbles by the Pleistocene ices ages in particular the Anglian Ice Age about 450,000 years ago.

The site was been designated a RIG’s (2009) given the well-defined Dendritic pattern of 5 shallow dry valleys cut in extensive bench at the base of the main chalk scarp on the southern side of Tring Gap. It represents a fossilised drainage pattern created during permafrost conditions. The bench is formed of the Middle Chalk with no hard band to explain its origin. Thought to have originated by glacial erosion in the Anglian Stage – If this is the case then the Anglian limit lies further south in the vale of Aylesbury than previously thought.

The dry valley pattern results from stream erosion in post – Anglian periglacial environment with the streams draining into the Tring Gap.

Location – Grid Reference SP927105

Car Parking – within Tring or near the Natural History Museum (HP23 6AP) in Tring. The free, joint Woodland Trust and Natural History Museum at Tring car park can accommodate 30 cars, with four spaces designated for blue badge holders.  There are no time restrictions on length of parking for visitors to the site, but the car park is closed at 5pm.

Catering – options include numerous cafes and pubs in Tring. Why not try the Kings Arms.

Nearby Attraction –  Natural History Museum (HP23 6AP) in Tring.

For more information take a look at:

• Hertfordshire Geology & Landscape p. 181