The Mimmshall Mystery EarthCache

The casual traveller heading north along Station Road from Brookmans Park to Welham Green probably would hardly notice that they had passed over Potteralls Stream flowing down the shallow valley and draining water from the high ground of Bell Bar to the east. The same traveller heading down Warrengate Lane, next to the old A1, might be forgiven for assuming that the larger river to their left was the same river or that Potteralls Stream flowed into it. Like going through a tunnel on a train, you see something as you enter the tunnel and then something similar when you come out the other side, and assume that this is a continuation. However in case it is not true!

The more observant would spot something strange. The Potteralls Stream flows in a South-south-westerly direction, but the river alongside Warrengate Lane flows northwards and is in fact a separate river called Mimmshall Brook. So how can that be? Two rivers flowing in opposite directions towards each other appear to meet at a place suspiciously named "Water End". Do they come to a sticky end? So where does the water go? The only other river close by is the River Colne. It has its source at a spring in North Mymms Park just 1700 metres to the west. Does the water go there? NO!!!
Potteralls Stream and the larger Mimmshall Brook together carry quite a high volume of water, particularly after heavy rain and ALL that water has to go somewhere!

In a water meadow at Water End there are some strange things happening. The ground is quite soft and in places very boggy. Both rivers flow into this meadow and then disappear. The only clue is that dotted around this meadow are funnel or bowl shaped holes - most are small, but there are at least 15 major holes and some can be quite large!! But big enough to swallow 2 rivers? YES!!

These features are called Swallow Holes and the whole of this area surrounding the holes has been designated a Site of Special Scientific Interest (SSSI). One of these holes appeared overnight on December 31, 1928 and was 8 metres in diameter and 15 metres deep! During the summer, most of the meadow is covered in a thick carpet of green vegetation and it can be difficult to see all but the biggest of the holes. Also after heavy or prolonged rain the volume of water flowing into the meadow is greater than the holes can swallow or the ground can absorb and the surrounding area floods. It is only in these circumstances that this natural basin overflows and the excess surface water passes along a channel through North Mimms Park, and eventually into the River Colne. In winter when the vegetation has died back, and particularly after light or moderate rain, it is possible to walk across the meadow and actually see quite large streams of water disappear down these holes into the ground.

For a long time it was thought that the water flowing into the holes reappeared at the spring of the River Colne to the west. However this was disproved in 1937, when researchers put fluorescein dye into the water at one of the swallow holes, expecting it to see it appear shortly afterwards in the River Colne. They waited and nothing happened. Then, 3 days later, the dyed water reappeared, nearly 16 kilometres away in the River Lea valley at the Chadwell Springs (between Hertford & Ware)! The mystery of the disappearing rivers had been solved!

Logging Requirements - The coordinates will take you a spot on the eastern edge of the water meadow. Nearby is a very strange sight - one tree growing out of the centre of another. You can't miss it and we will call it the "Tree within a Tree".

To log this cache you MUST do the following -
a) Take a picture of yourself (not just your hand!) holding your GPS receiver standing next to the Tree within a Tree, with the water meadow in the background.
b) Take a picture of a swallow hole (either full of water or empty) - there are several that can be seen without leaving the public footpath.
c) Go to the Warrengate Lane flood defences (N51° 42.829 W0° 13.220) and estimate how high the water level would have to be to breach the flood wall.
Post both pictures to the cache webpage with your log and e-mail the answer for c) to me (please don't put the answer for c) in your log). Any logs not meeting these requirements will be DELETED! - EarthCache Rules!!

Conservation and Safety The surrounding area is a Site of Special Scientific Interest (SSSI), so please don't leave the public footpath. The guiding principle of Earthcaches is Leave No Trace, so please leave this important area in as good or better condition as you find it. Unfortunately some people have dumped rubbish and unwanted items in the area (particularly when it is flooded) so you might want to take a rubbish sack with you and do a little EITO (Earthcache In - Trash Out).

As with all caches near water PLEASE take great care, particularly if you have children with you. Be careful of the swallow holes that are close to the path, don't fall in and especially keep your distance when the area is flooded.

The coordinates for the cache and the Tree within a Tree are on the eastern edge of the water meadow. The shortest route to the Earthcache site is from the west along a public footpath that starts next to the Woodman Inn on Warrengate Lane (N51° 43.376 W0° 13.315). If the water meadow is flooded it is not possible to use this footpath. If there hasn't been much rain and the water table is low it is possible to walk across the water meadow on the public footpath, but waterproof boots or wellies are advisable as it will still be soft and boggy in parts. Alternatively from the west you can use the public footpath, which starts by the bridge on Swanland Road which was the old A1 (N51° 43.484 W0° 13.390). This path goes around the northern side of the water meadow to its eastern edge and can be used even when the water meadow is flooded. A third route is from the east by the public footpath that starts on Station Road, Welham Green (N51° 43.859 W0° 12.700).

The water flowing in the 2 rivers initially stays on the surface because it is flowing over London Clay, which is fairly impenetrable to water (it is the material which was used for the lining of most early English canals!) However, the clay layer has worn thin in places and at certain locations there are outcrops of the underlying rock, which is chalk. When the river water, which is slightly acidic flows over these chalky outcrops, it gradually attacks small cracks in the chalk, called fissures. By a combination of chemical and physical erosion these fissures are enlarged becoming small underground channels and cave systems. Rivers will always take the path of least resistance to get to lower ground and eventually to the sea. If it becomes easier for the river to cut a channel through the underlying chalk layer rather than try and erode a path through the tougher more impervious surface layer it will go underground and a swallow holes will be formed.