OR/15/045 Overview of Superficial Geology

Kendall, R S. 2015. Conceptual cross-sections of superficial deposits in Cardiff . British Geological Survey Internal Report, OR/15/045.

During the Late Devensian age, glaciers are thought to have originated in the Brecon Beacons and Fforest Fawr uplands and flowed south through the South Wales Coalfield to finally emerge onto the Vale of Glamorgan (David, 1883, Lewis, 1970). The glaciers, at their bedrock-ice interface, deposited well consolidated, lodgement Tills. The composition of the Till reflects the bedrock geology over which the glacier has flowed, namely the Old Red Sandstone Supergroup, the Marros Group, and Pennant Sandstone Formation. These rock units lithologies have imparted a sandy and gravelly character to the tills of the Cardiff area.

During its retreat, morainic Hummocky Glacial Deposits were left behind at the ice margins, and are indicative of glacier ice stagnation. Hummocky Glacial Deposits have a distinctive egg box like topography, with very a chaotic lithology and structure. Fans, terraces and spreads of Glaciofluvial Sand and Gravel were deposited from meltwater streams which reworked and redeposited the Tills during deglaciation. Small lenses of Glaciolacustrine Deposits locally formed in ice dammed lakes and ponds.

Within the Cardiff area, the surface extent of Devensian deposits is limited to the north of a line running from St Nicholas, close to Culverhouse Cross, north of Leckwith extending south- eastwards to the coast (Figure 1). The deposits are part of a larger area of gravelly glacial drift, that skirts the rim of the coalfield from Swansea Bay to west of Newport, defining the southern limit of the Late Devensian Glaciation (Charlesworth, 1929) in the region.

During the Holocene, Alluvium and Tidal Flat Deposits were deposited in the river valleys and Severn Estuary as modern river systems including the Ely, Taff and Rhymney became superimposed upon the pre-existing relief and rapid post-glacial sea-level rise led to the drowning of the lower reaches of river valleys. Peat accumulated in hollows with restricted drainage and head formed in response to downslope movement of eroded material under the influence of periglacial climatic conditions.

The following deposits are also found within the Cardiff district but are not found within the area of interest for the City Region project. These include: deposits of calcareous tufa are locally preserved where limestone of the Porthkerry Formation, has been subject to dissolution. Storm beach gravels and coastal dunes that have accumulated on the coast.

Figure 1 Principal superficial deposits in and around Cardiff. © Crown copyright and database rights [2015] Ordnance Survey. Use of this data is subject to terms and conditions.

Summaries of the main superficial deposits

Till. In the Cardiff district, the dominant type of glacial drift is gravelly till with lenses of sand and gravel. The Till has a variable composition ranging from stiff, stony, silty clay to clayey gravel. Gravel grade material is usually pebble to cobble sized. The matrix consists of variable mixtures of sand silt and clay. Lenses of sand and gravel comprise grey to buff sands and sandy pebble-cobble gravels. The sands are commonly laminated or cross bedded when seen in exposures.

Hummocky (Moundy) Glacial Deposits. Glacial sand and gravel deposits of the Cardiff memoir are here referred to as Hummocky Glacial Deposits. These comprise a heterogeneous, largely unbedded admixture of sandy pebble-cobble gravel with lenses of locally pebbly sand, laminated clay and gravelly till.

Glaciofluvial Sheet Deposits. Clayey sandy pebble-cobble gravels with thin beds of gravel which are matrix free. Glaciofluivial sheet deposits are often overlain by very fine clayey sand/silt and may be cryoturbated in their upper parts. Glaciofluvial deposits can be bedded or cross bedded, Glaciolacustrine Deposits. Laminated clays occur in lenses throughout the Till and Hummocky Glacial Deposits. Glaciolacustrine deposits also typically contain silts.

Tidal Flat Deposits. In the memoir these deposits are referred to as Estuarine Alluvium. The deposit mainly comprises blue clay with very subordinate silts, sands and gravels. A peat is commonly present at the base and scattered thin peats are found throughout. Interbeds of sand and gravel are noted in borehole logs.

Alluvium. This is a variable deposit, associated with the major streams and rivers. It commonly comprises clay, sand and gravel. Typically, alluvial deposits can be parallel and cross stratified.

Peat. A diachronous basal peat is commonly present at the base of the Tidal Flat Deposits as well as occurring as scattered thin layers. The basal peat is well developed beneath the former East Moors Steelworks site but rarely present beneath the estuaries. Two other higher peats are also recorded from boreholes. Peat is also recorded as accumulating in kettle-holes within areas of Till deposits.