The Geology of the Study Area
by Karen Anderson
The study area for the Anglo-Saxon project included parishes surrounding Romsey on either side of the Test valley. Most of this area lies within the Hampshire Basin, a geological depression. The down-folded strata within the basin are made up from material deposited by ancient rivers, consisting mainly of gravel, sand, silt and clay. This basin extends west beyond Poole Harbour and east into Sussex and is bordered on the north by Cretaceous chalk. Its southern rim runs east-west through the centre of the Isle of Wight where it appears as a prominent landmark,The Needles.
The underlying geology has a significant impact on the appearance of the landscape and on the lives of the people living in it. This was particularly the case in past centuries when people relied on their local environment to supply their basic needs for daily life. Our study made use of mapping software in order to investigate documentary and archaeological evidence within the landscape. By considering this additional dimension, we were able to make a significant contribution to the understanding of the lower Test valley in the Anglo-Saxon period.
A satellite image of the drought-stricken landscape in the summer of 2022. North of the Isle of Wight the colours of the parched fields have mapped the bedrock geology. The yellow-brown chalk outlines the arc at the northern edge of
the Hampshire Basin.
The chalk at the southern edge of the Hampshire Basin forms The Needles on the Isle of Wight. Less dramatically, its northern edge is visible as the higher ground on the horizon looking north from Hillier Gardens over the valley of the Fairbourne.
Photos by Karen Anderson
The parishes in the study area mapped along with the river systems. The Test with its multiple channels runs from north to south through the centre. Parishes lacking streams lie on the Chalk.
The Test divides the study area into two parts. The river has created a wide valley, appearing on the map as a complex network of channels, many of them man-made or man-managed. The water traces very different patterns either side of the Test. Two tributaries enter the Test from the west. The Blackwater, to the south running through East and West Wellow, has a large catchment area with numerous streams draining rainwater from the gravels. Further north the Dun flows through East and West Dean, Lockerley and Mottisfont. This stream flows along a small syncline, a down-folding of the underlying bedrock. This geological structure lies near the edge of the Hampshire Basin, another, larger, syncline. The two are separated by a chalk ridge which forms Dean Hill. The place-name Dean comes from the Old English word denu meaning a valley, an appropriate name for a distinctive, wide valley formed by the erosion of the softer sediments flanked by chalk bedrock. The Dun receives much of its water from springs. Chalk acts as an aquifer, a reservoir of groundwater. Rather than draining from the surface and feeding directly into streams, rainwater percolates into the chalk bedrock and is stored in a network of fractures. This water flows along the fracture lines and emerges downslope as springs. Streams fed by chalk springs tend to have a stable flow with the chalk regulating the water supply. Heavy rainfall on chalk will not result in flash flooding on its rivers. Having a consistent and reliable supply of water, chalk streams like the Dun and the Test were valuable sources of water power for driving mills. The Anglo-Saxons undertook extensive engineering projects to harness the Test for their mills.
The drainage pattern on the east side of the Test is very different from that on the opposite side of the valley. Here the Test has only two small tributaries joining it from the east. The Fairbourne runs through Michelmersh and Timsbury on the northern boundary of the parish of Romsey Extra, while the Tadburn enters the valley on the southern edge of Romsey town. Minor rivulets flowing westwards further south have been captured by the Andover to Redbridge canal. The east boundary of Romsey Extra follows a watershed. Streams further to the east drain towards the tidal estuary of the Test or flow south into the Itchen. This north-south drainage pattern is related to the formation of a step-like series of river terraces along the Test and the Itchen during the Pleistocene, the Ice Age. This climatic upheaval shaped the landscape we see today.
The superficial geology of the study area mapped with a LiDAR hillshade image and surface water.
The deposits are:
Brown - Clay-with-flints
Orange - River terrace
Yellow - Alluvium
Pink - Head
This material overlies, and is much younger than, the bedrock. It was deposited over the past 2.6 million years, from the beginning of the Pleistocene to the present.
The superficial geology deposits record the impact of the Pleistocene on the landscape. The Pleistocene began 2.6 million years ago and ended only about 11,500 years ago. The ice advanced and retreated a number of times, dividing the epoch into glacial and interglacial periods. The ice sheets did not extend into southern England. Here there would have been a periglacial environment similar to the Arctic tundra with heavy winter snow and permafrost. Warmer summer weather would thaw the surface while the subsoil remained frozen. On top of the chalk bedrock are deposits of Clay-with-flints, brown on the map. This is the residue produced by the erosion of chalk and other deposits that had previously overlain the remaining chalk. This material has been mixed and distorted by repeated freezing and thawing. The small patches of pink represent Head, a deposit that slumped downhill during the seasonal thaw to infill the valleys.
The two other main superficial deposits are related. Together they trace the evolution of our landscape through the Pleistocene to the present day. Yellow on the geological map represents alluvium, sediments consisting of gravel, sand, silt and clay deposited by a river. The presence of this alluvium indicates the extent of the current, active floodplain, the area that has been flooded by the river and is liable to future flooding. Orange represents river terrace deposits. A river terrace is a former, abandoned floodplain created when a river was flowing at a higher level. The terraces were formed by the Test and Itchen and their tributaries in response to tectonic uplift during the Pleistocene along with the fall in sea level during periods of glaciation.
The river terraces are composed mainly of flint gravel and sand derived from the chalk downs to the north, combined with reworked sediments from within the Hampshire Basin. The deposition of the large volume of material making up each terrace would have taken place at the close of a period of glaciation. Initially, meltwater formed wide, braided rivers carrying heavy loads of the sediments released from the permafrost. Rivers evolved from their braided form, with multiple small channels separated by temporary islands of gravel and sand, to a single channel. The river then cut through the sediments of the floodplain to flow at a lower level, creating a new floodplain and a terrace.
There are extensive river terrace deposits along the Test and Itchen with remnants of additional terraces between the two rivers. The terraces have been numbered from 1 to 11 according to their height, roughly correlating with their age. A staircase of terraces flanks the Test. Romsey lies on the most recent terrace, numbered 1, with a series of steps, 2 to 5, to the east, each terrace stepping back in time. A steep scarp on the west side of the Test rises to terraces 6 and 7. Romsey’s river terrace started to develop at the end of the Ice Age, 11,500 years ago. As the glaciers to the north melted, the silt incorporated in the ice was blown from the surface and deposited on the top of the terrace gravels. Known as brickearth due to its suitability for making bricks, it provided a rich and fertile soil. With good land for growing crops and water for livestock nearby, Romsey was an ideal location for settlement.