Harpenden's 'Dry Valley'
A pictorial trail from Zouches Farm near Dunstable to the River Colne at Smallford
Bruce Campbell’s proposition that there was once a river Kin, prompted us to trace the shallow dry valley which runs through Harpenden from its source on the dip-slope below Blows Down near Dunstable to Smallford in the basin of the river Colne.
Rivers of the Chilterns
During the Ice Age, the rocks and soils of the Chiltern Hills were frozen into impermeable ‘permafrost’.
When the ice-sheet began to melt, water poured over the still-frozen ground of the dip-slopes, cutting the deep valleys of the Lea, Ver, Gade etc. Where the lobe of glacial boulder clay still blocked the the way, terraces of glacial ‘outwash’ gravel were formed along the Lea valley. Eventually the Lea overflowed and cuts its eastward course, reaaching the Thames to the east of London. The Ver and Gade joined the Colne, which flowed south-west, reaching the Thames to the west of London.
Many tributary valleys, such as the valley through the centre of Harpenden, formed later and were shallower, so when, at the end of the period of glaciation the underlying chalk became permeable again, the water-table fell below the level of these valley floors, and they became ‘dry valleys’. Some have streams which re-appear in wet years. The valley through Harpenden and Sandridge is a ‘dry valley’.
Most of the rainfall on this district soaks through the soil and the permeable chalk rock beneath to add to the water-table of the aquifer.
But during heavy rainfall there is also some surface drainage down the slopes of the valley sides. Water will flow in ditches off the patches of clay drift deposits on the chalk ridges. Storm-water runs off tarmac road surfaces and paved areas in the town, finding its way into the roadside ditch and so into the town drainage system which carries this run-off water to the Southdown storage ponds, whence it overflows into the soak-away pit to percolate into the chalk below.
The gravel-pit at the foot of Dark Lane similarly serves as a sump for the run-off from roads at Southdown.
There are other pits in the valley-gravel at intervals along the valley floor through No Mans Land and Sandridge, down to the Colne.
The dry valley in pictures
The long gallery below follows the valley, as far as it can be accessed. The northern section mainly runs through private land, and can only be viewed from footpaths which give views along the line of the shallow dip which is the valley. The sequence has been put together over several years, in varying weather, and includes some old photos at times of flood or when there was more visible water in the centre of Harpenden.
To view the pictures, click on the first picture (or wherever you want to start your exploration) to enlarge the image. Then press next, or previous, to progress on your virtual walk.