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Fisher Brook and our Local Geology BACKGROUND Summarizing, the local geology is based upon a very thick layer of water non-permeable clay that is topped with various sedimentary layers which are generally water permeable; some sandy, some essentially limestone; either of these types bearing large quantities of iron as oxides (eg. rust). Owing to the erosion from stream and rivers, the various layers can be exposed to the ground surface in various places. So in Rothersthorpe Vale and in the base of our small valley that the canal follows, the subsoil is clay. The surface soil does not necessarily look like clay because of a covering with a thin layer of glacial or river drift, silt, sand or gravel etc. In the map these areas are shown coloured light green. The slopes of the deep excavation near the north tunnel mouth were deliberately covered with good soil so as to support grass and trees (but in places this cover has been removed). On the other hand, gentle mounds can be be found to the north of Candle Bridge, created c1795 as the canal was dug out and consisting of nothing but blue clay. Those areas support little more than moss and briars to this day. Because this is mostly about water we can simplify the situation by saying that the next layer, exposed at slightly higher ground levels, is a semi-permeable (and variably permeable) limestone which generally but not always carries iron. In places this sometimes un-workable ground is covered with drift which has been worked up to an excellent growing soil, such as in the Stoke Road Allotment field and in the neighbouring field immediately to the east. Distribution of this layer is shown as a white area in the map. Finally, another major layer lies on top, cream colour in the map. This is a particularly permeable limestone and is the main substrate across Blisworth Hill but it is again covered with glacial drift so that normal cultivation is possible. In the schematic inset on the map the layers are drawn with a tilt. This is because the two major layers are tilted downwards towards the southeast. This tilt causes any transitions, facing north or eastwards, from high ground to low to be quite abrupt. For example, note the sharp fall in levels just north of Gayton Manor House and, on a gentler scale, the sharp drop just west of the Baptist Chapel and west of the Old Rectory. Across the little valley from these last two are the relatively gentle slopes of Gayton Hill. These are all ironstone edges that resisted erosion by water and may have been rocky outcrops 1000s years ago.
SPRINGS & FISHER BROOK Having briefly described the geology we can now concentrate on water. In the map the major springs are shown by red arrows. Springs tend to emerge from the ground where there is a transition from one ground permeability to another. Water accumulated in the semi-permeable layer will not sink through a less permeable layer below it. Hence the water is likely to emerge as a spring. In the inset schematic the little red arrows indicate two "spring lines". The locations of these spring lines across our parish are indicated by the yellow and the orange borders in the map. Note how the wavy spring line in the village has encouraged the placement of the High Street and Chapel Lane so that houses along these streets would have been able to sink none-too-deep wells for their water. Nearby, a major spring labelled W was in evidence up until the 1960s when it was diverted into a culvert and the street built above it was named Westbrook. Another spring is labelled R and is recorded as far back as 1718 as Ramwell Spring, having a notably purity. In around 1900 there was a tank established near the outflow of Ramwell and the collected spring water was piped down to Grafton House. Before the canal was brought through, the main stream (called at various times South Gutter, Winter Brook, Wash Brook and Fishweir or Fisher Brook) powered a water mill when the flow was sufficient. The brook is the result of two major springs labelled F rising just to the north of Nunn Wood, which lies on the watershed between the Nene and Ouse river systems. With these springs, combined with the lateral run off from fields, there has been a substantial flow at times. At the time of building the canal this flow was actually quite a nuisance and was channelled into a 900mm diameter culvert which discharges into the canal near the tunnel mouth - on occasions the flow has been quite impressive). The culvert is shown in the map with a dotted blue line. Its southern end lies in a hollow that was once twice the area it is today (it had an earth bank built across it for the 1800s horse railway). This was the location of a fish pool, probably called Young's Pool, and when drained some time before 1700 the field there was named "Fish Weir Close"; the name degenerating later to Fisher Close by 1900. DROUGHT CONDITIONS Fisher Brook is not* how it used to be! The years 2010 and 2011 have been very light of rain. The permeable layers have probably largely dried and so, when it rains, most rain runs into the ground rather than runs over the ground to join the visible stream. The stream joining the canal has been noted at times to be near zero and this has been of some concern to British Waterways who require water to operate the locks at Stoke Bruerne. When surveying Fisher Brook in early August 2011, some unusual conditions were observed; flow rates measured crudely in litres per minute were noted at various points and are shown in magenta-coloured figures near the stream line in the map. As one sampled further to the north, the flow rates become larger, as expected, but just as the stream is leaving the permeable substrate the flow rate drops to some very low level as the stream degenerates into a series of small near-stagnant pools. A little further on, at the culvert entrance, the flow is zero. However, at the culvert exit, by the canal, it is measurable once again. Clearly the culvert is taking spring water from other places and we should not be at all surprised by the Georgian engineers wanting to collect any water that threatens to be a nuisance to them. The probable spring is indicated by a dotted red arrow to summarise the situation.
Note that there is the possibility of water flowing underground towards the south, because of the tilt of layers. Water running south might enter instead the drainage ducts that were built under the tunnel (to keep it dry whilst building was in progress) and which drain into the basins below the Stoke Bruerne locks. We have no record of how far towards the north end of the tunnel those drainage ducts were dug (ie. whether they are even relevant). In the event of the ducts taking water then that flow would constitute an effective loss to British Waterways for the upper locks at Stoke Bruerne. * I am indebted to Jan and Alan Andrews for first raising concerns about Fisher Brook in 2010 and to Steve Jowers (Tiffield, Geographer) for his explanations on our Geology. The geological detail, ie. spring lines, was obtained by freehand copying from an Ordnance Survey mapping of groundwater. By March 2012 one farmer was moved to state that "the national rainfall's line of credit for farmers has literally run dry.." as he is obliged to cultivate the surface 4" of soil and plant his corn in that, it becomes moist during the winter whilst deeper layers remain dry (hence the lack of water in the Brook!). The corn throws roots into the deep and the shallow soil but only the latter do any good, waiting for the vital rescue in late Spring. Tony Marsh August 2011. |
The dwindled flow of the stream must be attributed to an
inclination for the water to again flow underground and, maybe, even descend through
fissures in the otherwise non-permeable clay to emerge through the walls of the
tunnel - see the detailed inset diagram here. Evidently
there is a considerable flow through the tunnel walls near the Blisworth
end and this must be "Fisher Brook" water. It is hard to
say whether the water must have passed through fissures in the
non-permeable clay or has instead found sneak routes to the tunnel roof
through layers near one of the ventilation chimneys. That
British Waterways is receiving what water there is will remain the case
even when the walls are repaired because, at 89 metres, the canal is the lowest
conduit to act as a receiver.