Geology of the valley
The Blackwater Valley lies to the north of the Hog’s Back - a ridge of chalk - which forms the southern rim of the London Basin.
The River Blackwater itself rises in Rowhill Nature Reserve, found at the southern end of the Valley. At the western boundary of Rowhill Reserve the sand and gravel forming Hallimore Hill stops and the clay that makes up the rest of the site starts. Water seeps out at this point as the sand and gravel is moderately free draining, forming a seepage bog, which is the source of the river.
At the northern end of the Valley the River Blackwater is joined by the Whitewater to flow into the River Loddon, which joins the Thames. The river has only a few small tributaries, the largest being Cove Brook.
Surrounding and underlying the Valley are deposits of clays, silts and sands that date to the Tertiary period which overlay the chalk. These comprise:
- London Clay
- Bagshot Formation
- Bracklesham Beds
- Barton Beds
In the distant past the watercourse that is now known as the River Wey North Branch, formed the headwater of a much larger River Blackwater. This ancient river cut its way through the Tertiary deposits and with the erosive action of flowing water, worked and reworked the sands and gravels to eventually leave a wide flood plain.
At some stage in the past, the headwaters of the ancient river were caught and diverted eastwards along the southern side of the Hog’s Back to form the modern River Wey. The history of the River has meant that the modern River Blackwater is a small river sitting in a wide flood plain, which is underlain by valuable mineral deposits.
Geological factors and how humans made use of the Valley over time are the main factors that shaped and influenced the landscape of the Valley that we can see today and also the wildlife that inhabits the Valley.