Sea level and coastal change
At the beginning of the post-glacial period, large volumes of water were still locked up within ice sheets, and rivers flowing across previously glaciated areas of the Vale began to adjust to new base levels through down cutting, locally to around minus 20 m OD.
Through time, sea level rise resulted in sedimentation and infilling of these incised courses. By around 6,100 years BP, mean high water of spring tides was at about minus 10 m OD and there are indications that saline waters were encroaching into the lower Ouse system, although the intercalation of freshwater peats and marine clays dating to around 5,300 years BP indicates the delicate balance of marine and freshwater processes. By around 4,400 years BP, intertidal sedimentation was occurring in all the major valleys draining the Humber Estuary and expansion of these processes continued until 2,900 years BP, after which time, sea level change has been limited.
Within this relatively stable period, two episodes of rapid coastal change have been identified; between 3,000 and 1900 years BP and 800 and 400 years BP. Whilst the former of these seems to be a localised phenomenon within the Humber, the latter can be linked to a well-documented period of storm surge activity in the North Sea region.
Upstream of tidal influence, the low gradient river systems of the Vale have been characterised during the Holocene by low rates of lateral movement and relatively high rates of fine-grained (sand, silt, clay and peat) sedimentation. Deposition probably occurred in a series of multi-channelled, anastomosed systems with documentary and cartographic evidence indicating that many of these river patterns survived in the southern part of the Vale of York and Humberhead Levels until large-scale drainage and land reclamation in the early 17th century AD.
The identification of episodes of river incision and sedimentation in the Yorkshire Ouse basin through radiocarbon dating of organic materials recovered from alluvial units has shown that variations in Holocene river activity, principally flood frequency and magnitude, can be correlated with periods of climate change and, particularly in the last 1000 years, with human impact. Since the Bronze Age, progressive deforestation associated with pastoral and arable farming, has probably enhanced both run-off and the supply of fine-grained sediments to river valley floors. Since the 11th century AD and during the last 250 years, the base-metal mining industry of the Yorkshire Dales has been a major source of fine-grained sediments for rivers of the Ouse basin, introduced into upland river channels through the erosion of mine spoil heaps or by ore processing and extraction.
In addition to natural changes in the rate and patterns of sedimentation in the tidal part of the Vale, humans have also controlled alluvial deposition through the process of warping. This technique used the natural tidal flow of the lower Ouse, Aire, Don and Trent to introduce fine-grained alluvial sediments across areas of low-lying land in need of agricultural improvement. Sediment rich waters were distributed by a number of artificial drains controlled by sluice gates and each warp flood could deposit an average of 2 mm of silt and silty clay across the landscape, with 0.3 m of material accumulating in a single warp season and up to 1.5 m being achieved by sustained warping programmes.
Warp deposits form extensive tracts along the banks of the Ouse to the north of Yokefleet, to the west of Goole and around the Ouse-Derwent confluence. The earliest known example of warping is from the 1730s near Rawcliffe on the River Aire with the last recorded evidence from land to the east of Yokecliffe in 1947.
The establishment of dense vegetation cover associated with climatic amelioration during the early Holocene led to the stabilisation of these aeolian deposits. However, radiocarbon dating of organic material within sands at East Moor, near Sutton on the Forest indicates sediment reworking, tentatively dated to episodes of Anglo Saxon and/or medieval forest clearance have documented more recent wind erosion of these fine-grained deposits to the north and east of York during the winter and spring of 1967. This recent episode of sand destabilisation was linked to a combination of unusually dry conditions, high temperatures, and changing farming practices relating to the cultivation of sandy areas, which prior to enclosure between AD 1760-1830, had served as parish commons and moors.