The natural history around Water End

The following text was written for a geological walk around North Mymms in October 2004. It's been included in the North Mymms History Project because of its natural historical value. This site thanks Mike Howgate M.Sc, who is the recorder for geology for Hertfordshire, for giving us permission to reproduce his work.

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London Region of the W.E.A

Geology and landscape around Water End

Sunday 17th October 2004

Leader Mike Howgate M.Sc.

Note: Walking distance is about 4 miles with one short uphill section to begin with. The walk can be muddy in places, especially after rain, so boots are advisable. Please keep out of the woods which are private game reserves.

Introductory Notes

Solid Geology: The chalk which forms the Chiltern Hills to the north and west dips towards London at a shallow angle of 2-3 degrees beneath the rocks, actually sands, pebbles and clays, of the London Basin. The general dip is interrupted by a slight anticline which brings chalk to the surface in the valleys of the Cuffley Brook north of Northaw and the Mimmshall Brook west of Potters Bar. This can be considered the most northerly ripple of the Alpine mountain building movement, caused by the collision of Africa with Europe about 25 million years ago.

The chalk here is the upper chalk, which is characterised by abundant layers of nodular flints. The top of the chalk is irregular and often has solution pipes filled with younger (tertiary or quaternary) sediments. The first layer of the tertiary sediments is a bed of massive unworn flints which are often coated with green glauconite - the Bullhead Bed.

Above this are about 60 feet of sands with layers of well-rounded black flint pebbles - the Reading Beds. Where these are cemented by silica they are transformed into the well known Sarsen stones and Hertfordshire puddingstone. The upper parts of the Reading Beds become increasingly silty and are transformed eventually to a uniformly dark grey, sticky clay with occasional layers of septarian nodules - the London Clay. These tertiary beds make up the area between North Mymms Park and the Lower Lea Valley, an area known as the South Hertfordshire Plateau.

Drift Geology: The surface of the South Hertfordshire Plateau is capped by the plateau gravels which are made up predominantly of well-worn flint pebbles many of which are covered with chatter marks indicative of a beach deposit. However there are spreads of the gravels which contain sub-angular pieces of 'pin hole chert' which could only have come from the Lower Greensand beds of the northern fringes of the Weald. They were therefore deposited before the Thames cut its present course and when it flowed in the Vale of St. Albans. In the low lying area to the north, and in places overlying the plateau gravels are patches of chalky boulder clay which was deposited by the most southerly extension of the Anglian glaciation (500,000 years ago).

Outwash gravels produced by the melting of the ice sheet and the transportation and sorting of the boulder clay are the basis of a lot of the gravel extraction in the area. These gravels can be identified by the abundant 'erratic' pebbles of vein quartz and tough quartzite transported here by the ice from the Triassic 'Bunter Pebble Beds' of the Midlands.

The main lobe of the ice sheet filled the Vale of St. Albans and almost reached Watford. This blocked the valley of the Mimmshall Brook just south of Hatfield at Welham Green and diverted the course of the stream into the Colne Valley to the west.

The Walk

St Mary's Church, North Mymms
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

Park in St. Mary's Church Road in North Mymms Park, not in the church car park. This can be reached from the A 1000 from Potters Bar by turning left into Dixons Hill Road and going through Welham Green. Once over the Al(M) turn sharp left and go straight ahead through the gateway.

Map of walk created by the North Mymms History Project
Using MapHub and elements © Thunderforest © OpenStreetMap contributors
A larger, more detailed, interactive map is embedded at the foot of this piece

Locality 1. St. Mary's Church, North Mymms Park (O.S. TL 222045)

This mainly flint church has 'clunch' dressed stone at the comers (often replaced by Portland Stone or Cotswold Oolite). The flints are mainly knapped, though a part of the north wall is mainly of freshly dug chalk-coated flint nodules. The tower also has several other distinctive rock types. In particular look out for blocks of clunch, Hertfordshire puddingstone, 'false' puddingstone, carstone and sarsen stone.

Hertfordshire puddingstone is very tough and occurs as irregular 'blocks' with distinctive well-rounded flint pebbles scattered through them. The pebbles are cemented with silica which is of the same hardness as the flint and so the fractured faces of the Hertfordshire puddingstone block often breaks evenly through the pebbles and the matrix. The blocks often look either dark grey with black pebbles or fawn coloured with brown or reddish pebbles. Hertfordshire puddingstone was formed when Britain was subjected to 'desert-like' conditions during the Palaeocene period some 55 million years ago.

Silica rich water, which percolated through the sandy pebble beds of the Reading Beds, lost their water as they neared the hot surface. The silica left behind as the water evaporated cemented the loose sediments into a tough 'silcrete'. Where there are no pebbles to be cemented the silicified sand is known as sarsen. A few small pieces of this tough orange/fawn rock can be found with a bit of diligent searching. Sarsen stone is more common in Wiltshire, where the megalithic monuments at Avebury and Stonehenge (except the inner circle) are made of it.

'False' puddingstone can be seen as large flattish blocks in which the generally rounded flint pebbles, which are of all shapes and sizes, are often mixed in with more angular pieces. The 'pebbles' stand proud of the softer matrix which is of iron oxides and hydroxides. They can be seen at distinct levels of the tower and may have acted as a string course to bind the flint rubble core of the tower wall.

Carstone occurs as irregular masses of reddish and brownish rock, often with darker veins and patches cutting through the lighter material. Like the 'false puddingstone' it is of relatively recent origin and formed as an iron oxide rich hard pan within glacial sands and gravels.

Clunch is a hard variety of chalk, often used for corners, doorways and internal arches in flint churches in the area. The two varieties most often encountered are 'Tottenhoe Stone' and 'Melborne Rock'. The relative toughness is due to the presence of small amounts of calcium phosphate mixed in with the softer calcium carbonate. However it is easily scratched (look for a carved 'mass dial' and graffiti of several different dates) and after several hundreds of years of exposure to the weather it is badly corroded, appearing soft and flaky. Often the most badly weathered pieces are replaced by blocks of more resistant Cotswold limestone (yellow to honey coloured and of Middle Jurassic age) or Portland limestone (white, sometimes with shells and burrows and of Upper Jurassic age).

You could try finding other pieces of Hertfordshire Puddingstone and 'false puddingstone' in the walls of the rest of the church. Make a sketch of each wall and plot your finds. On your way out of the churchyard inspect any molehills looking out for black, very smooth flint pebbles which are indicative of the underlying Reading Beds.

We now walk through the church car park and turn right to go up the road that heads south from St Mary's Church Road.

The road south from St Mary's Church, North Mymms
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

Do not follow the road round to the right, instead take the bridleway straight ahead which soon goes uphill through woodland. On reaching open ground at the top of the hill follow the edge of the wood round to the right.

The path referred to above is not official; instead you can turn right further down Potwells Valley
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

After going above a small stream, which follows a hedge, head for the highest point where a track goes through a gap in the trees leading into a flat field. Face east and you are looking down Potwells Valley where you can see two streams which join on the other side of the track.

The view as described, looking east across Potwells Valley
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

Can you see where the combined stream disappears? You get a good impression here of the flat South Hertfordshire Plateau with the valleys cut into it.

(Locality 2.) Potwells Valley (O.S. TL 218031)

Now go through the gap and follow the left hand edge of the field, examining the pebbles as you go, until you have a view of the Vale of Saint Albans; which is the former course of the pre-Ice Age 'Proto-Thames' (Online editor's note: the Thames at an early stage of its development as a river). The best view is down a short tractor track on your right as the woodland ends.

The path and landscape to the west of Potwells Valley
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

Now retrace you steps back to where you entered the field. The wooded ridge you are on is a part of the South Hertfordshire Plateau, which is capped by up to 15 feet of plateau gravels. The field you have crossed can look like Brighton Beach when clear of crops.

As the cross-section (below) indicates, there is a spring line at the base of the gravel, where it meets the London Clay. The small streams flow down Potwells Valley across the London Clay outcrop before disappearing in a series of swallow holes as the Reading Beds / chalk outcrop is encountered.





Follow the edge of the wood round to the east crossing a second stream by a wooden bridge and cut across the field ahead diagonally (rushes and boggy patches indicate that you are now crossing the impermeable London Clay). Cross the bridleway and follow the path around the edge of the field with woodland to your right, after about 20 yards there is a convenient log, which makes a good coffee stop for a couple or family group.

You will soon cross a small bridge over the now united streams we crossed previously.

 (Online editor's note: The next three paragraphs describe a short diversion slightly off the public right-of-way. Please see inserted coloured map above where an alternative route on public footpaths is marked with a white star on a red icon).

Follow the track alongside and to the left of the stream downhill and try and find the series of swallow holes that the stream disappears into.

The final swallow hole, often dry because the flow now disappears down an active swallow hole higher up the stream bed, is near the prominent oak tree near the far end of a 'basin like' hollow. In very wet weather this area becomes a small lake (note the change in vegetation even in dry weather) with an overspill at the shallow col to the east. A complex of inactive higher level swallow holes can be examined just inside the nearby Hawkshead Wood ahead on the right.

Go straight ahead through a hedge keeping the open field to your left and the woodland to your right as you swing round to the right you enter another field continue alongside the field hedge heading east until you reach a prominent track-way (Byway 5).

Turn right here and follow the track over a slight rise and then down alongside fields to your right which contain abundant flints and pieces of chalk. Cross the A1(M) by the footbridge and loop back under it, cross Swanland Road with extreme care then walk straight ahead down the short cut-through lane with a high new wall on your left.

Locality 3 Warrengate Road (O.S. TL231032 -TL228043)

You are now in the valley of the Mimmshall Brook which rises near High Barnet. Following a series of floods which affected the southern end of Warrengate Lane, the Environment Agency proposed a major flood alleviation scheme to protect the 13 bungalows and an electricity substation deemed to be at risk.

The flood defences on Warrengate Road
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

The scheme, costing about 1.5 million pounds, involved installing a piled floodwall along the roadside fronting the Mimmshall Brook between its junction with Hawkshead Lane to near the pumping station where the road was also raised and a bund constructed in the field to the left. Opposite the bridge at the junction with Hawkshead Lane a brick-clad floodwall surrounds no. 84 and forms the western abutment of a heavy steel double-vee floodgate which can seal off the section of road and the buildings liable to flooding.

The flood hazard is mainly due to the ponding effect of the lake which forms at Water End when the swallow holes can no longer take the flow of the Mimmshall Brook and Potterells Stream. According to the Environment Agency the work was necessary as the flooding had a frequency of greater than once in 50 years. I have twice seen floods here in the last 25 years, with sandbags in the driveways of the most southerly bungalows. It is amazing that anyone ever thought of building housing in an obvious flood hazard area let alone sighting an electricity sub-station there.

Although the floodgate was officially opened (or rather closed) on the 5th of March it is still not fully operational. Complaints from local horse riders about the metallic sealing strip upsetting their steeds means that a new horse-friendly strip must be fitted and the gate height adjusted accordingly. In the event of a flood alert someone has to come and close the gated manually, cover the roadside drain holes, set out warning signs on the approach roads, and put warning lights on the floodgate. Let's hope Mother Nature can wait. The effects of the most recent flood on Monday 30th October 2000 can be seen here.

As you walk up Warrengate Road you can see a prominent swallow hole on the far bank. This is dry except during flood conditions. Further along there is the option of visiting The Woodman Inn.

(Online editor's note: The original text also mentioned the now closed Old Maypole pub and the Water End Cafe).

Continue along Warrengate Road, then onto the pathway alongside an industrial area, where it joins Swanland Road. Cross the bridge over the overflow channel, ignoring the steep way marked footpath by the bridge abutment. About 20 yards further on there is a footpath sign pointing right info a scrub-filled area, follow the path ahead through the thorn scrub until you reach the Pottrells Stream (often a dry stream bed). You are now at ...

Locality 4. Water End (O.S. TL 231045)

Screen grab from Google maps of an aerial view of the Water End swallow holes

This is an SSSI (Site of Special Scientific Interest) because it is where a group of swallow holes takes the entire flow of the Mimmshall Brook and the southwards draining Potterells Stream. That is except at times of very heavy rainfall, when the whole area can become a lake, and then it can overflow along the channel through North Mymms Park to become the headwaters of the River Colne.

Take great care if you wish to go exploring for there is an active swallow hole. The areas around the swallow holes is often composed of unstable muds and clays and the whole area can be littered with debris dumped here by the streams. It is advisable to only attempt this with an organised group and in winter when the vegetation is down.

You can see a swallow hole to the right of the path just before the Potterells Stream crossing. At different times of year the stream can be a trickle, totally dried up or totally impassable. A good idea is to follow up the streambed of the Potterells Stream using the track-way to the left to see if you can find where it disappears. Again winter is best, when the nettles are down.

A swallow hole at Water End
Image by the North Mymms History Project
Released under Creative Commons BY-NC-SA 4.0

Cross the streambed and follow the footpath, turning right into the wood. An idea of the maximum extent of the lake can be estimated from the 'strand line' of the mainly plastic debris in among the trees. When the lake is dry there is a slightly raised track across the middle of the lake which leads to the car park of The Woodman Inn. Exploration to the south of this track in amongst a network of channels and swallow holes should reveal the currently active swallow hole.

History of the Mimmshall Brook

Before the Ice Age the Mimmshall Brook was established as a south bank tributary of the pre-glacial Proto-Thames which flowed eastwards through the vale of St. Albans. Rising on the high ground of High Barnet it flowed north to join the proto-Thames at Hatfield. It gradually cut its was down through the London Clay until it reached the sands of the Reading Beds, at which point some of the water could channel down into the soluble chalk.

At the maximum extent of the glaciation the Mimmshall Brook's northward flow was blocked by ice and glacial debris and it was diverted to the west alongside the ice sheet into a glacial lake at Bricket Wood and thence into the Colne system which then may have been the 'diverted' Thames. There would be no underground flow as the area would be permafrost.

After the Ice Sheet melted, the northward flow remained blocked by glacial debris. However the melting of the permafrost allowed the underground flow to be established in the soluble chalk, mainly along widened cracks and fissures. Today the flow is mainly underground into the River Lea between Essendon and Ware, where it emerges as a series of riverbed springs. Only after prolonged rain is the old overland course to the Colne re-established. At this time the stream flows both east (underground) and west (over ground).

On the way back you can turn left where there is a new signpost (set up by East Herts. Ramblers and the East Herts. Footpath Society). This new path takes you past a couple of swallow holes and alongside the overflow channel. You can ascend the slope by the bridge and turn left. You should now retrace your steps back to Warrengate Road and Swanland Road - cross both roads with care. Then ascend the steep path to cross the A1(M) by the footbridge, noting the culverted course of the overflow channel beneath the motorway. You can now follow the road, with the overflow channel on your left back to the entrance gateway of North Mymms Park and your parked vehicles. Just inside the park a bridge crosses the overflow channel. The bed and sides are well 'armoured' as during periods when the overflow is active there is an impressive, if small, waterfall here. The channel here has been artificially over deepened.

You can, if you like, now follow the course of the overflow channel through North Mymms Park by following the footpath to Colney Heath which starts at the bottom of the churchyard. After crossing the footbridge over the overflow channel you can follow it until you reach a farm bridge just before some woodland the channel is now generally occupied by the headwaters of the river Colne.

References

• Ordnance Survey 1:25,000 Explorer 182 (St. Albans and Hatfield)
• Geologists' Association guide 30a The London Region (North of the Thames) 1967
• British Regional Geology: London and the Thames Valley HMSO 1996

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Online Editor's Notes: This piece has been edited slightly to reflect changes since the text was written. For example, the original mentions the Water End Cafe and the Old Maypole which are both now closed. Links have been added where needed. A link to the London Region W.E.A, referred to at the top of the piece, can be found here. Sketches were supplied by the author, Mike Howgate M.Sc. , who is the recorder for geology in Hertfordshire.  Photographs are by the North Mymms History Project and are released under Creative Commons BY-NC-SA 4.0.

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