‘The route: ‘down the Valley of the Great Ouse and across its floodplain‘

6.1 miles (9.8 km)  Walking guide time 3hrs plus stops

Starting on the iconic limestone bridge in its attractive riverside setting, the Trail turns east to travel down the valley and across flood meadows (providing they are not flooded!) and through the villages of Fenstanton and Fen Drayton  Once past the marina near St Ives, you descend below the 5 metre contour for the rest of the walk, except where River Terrace sands and gravels have created higher, and therefore drier, land where the settlements are located. Flood waters continue to create fertile pastures in the valley, although these are now replaced in many areas by lakes formed from previously excavated gravel pits. In the RSPB Fen Drayton Lakes nature reserve, a large area is being transformed into a mosaic of wetland habitats reminiscent of the once extensive, wildlife-rich marshes and wet meadows that once covered the river’s floodplain and adjacent fenland.

Landscape and Geology

The bedrock in this part of the Fens was formed in Jurassic times. Oxford Clay, known for its use in making bricks (including those at the How brickworks in St Ives), underlies most of the area. The younger bedrock of the West Walton Formation lies to the south and east of the Oxford Clay and is at the surface in the southern part of Swavesey. It mainly comprises Ampthill Clay but it also contains small bands of limestone that are known by their local names such as ‘St Ives Rock’ (which occurs at the surface in small patches to the south of Fen Drayton and to the north of the river).

The younger deposits cover the Jurassic bedrock along the whole of this walk. These are either the River Terrace sands and gravels, deposited during the Pleistocene Ice Age, or more recent Alluvium, left by the river during the warmer conditions of the Holocene. The walk starts on the bridge over the river channel, which is infilled with Alluvium. It then crosses the 1st/2nd Terrace (it is not possible to distinguish between the two here) sands and gravels on which most of St Ives sits. After returning to the Alluvium nearer the river, the walk reaches Fenstanton, which is located on the older and higher 3rd Terrace, with the highest point of the walk being reached at the Parish Church (16m). Fen Drayton is also on the 3rd Terrace, although not as high as Fenstanton. Descending once more onto the Alluvium of the floodplain you reach the lowest points of the walk. The elevation profile (below) shows the route as at or above 5 metres in the floodplain due to the banks built up along the river, tracks, footpaths and guided busway, although the adjacent land is below 5 metres. The walk finishes back on the younger (1st/2nd) Terrace on which the northern part of Swavesey sits.

Landscape and Geology

The bedrock in the area is mostly clay (Ampthill Clay). It dates from the Jurassic period when the ‘mud’ on the floor of a shallow sea became compressed enough to form a rather sticky and relatively soft rock. This is now overlain by much younger deposits the oldest being Glacial Till (Boulder Clay), material left by glaciers during the Anglian Glaciation (c. 425,000 years ago). There are also patches of sand and gravel from Glacial Rivers. These glacial deposits now only remain on the higher land, any left in the river valley having been eroded by powerful rivers of meltwater during subsequent glaciations which also brought the substantial deposits of sand and gravel that form ’Terraces’. These can be aged by the fossils they contain – the younger 1st/2nd terraces are difficult to differentiate but the older 3rd and 4th terraces are more distinctive. Along the Ouse are extensive deposits of Alluvium, a finer material (covering hundreds of hectares in this area) that formed in more recent times and is still being left by the river, especially in times of flood. In some areas Peat is at the surface (elsewhere it underlies the Alluvium). This formed during stable periods of (freshwater) waterlogging at some stage in the last few thousand years.