OR/15/045 Cross-sections

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

A series of three conceptual cross-sections (Figure 2) have been constructed for the Cardiff area. They are intended to communicate how the main superficial lithostratigraphic units might spatially relate to each other within the subsurface.

The first section is envisaged to represent what might be found beneath the centre of the city. In this area, there is a glaciofluvial terrace or fan complex exposed at the southernmost extent of the Till complex. The glaciofluvial deposits are cut into the tills and have been dissected by the modern rivers which have deposited Alluvium. To the south, the glaciofluvial gravels are overlain unconformably by Tidal Flat Deposits formed during post-glacial sea-level rise, drowning the river valleys. The rivers Rhymney and Taff locally both cut through the terrace into the bedrock beneath, creating buried valleys, much deeper than beneath the adjacent areas. The buried valleys are thought to have been incised by glacial meltwater channels with modern drainage superimposed on top. The relationship between Tidal Flat Deposits and Alluvium is poorly constrained and will largely be controlled by post-glacial fluctuation in sea-level, as such it is shown on this section as an interdigitating relationship.

The second section is for the Morganstown/Radyr area, where Hummocky Glacial Deposits were identified during mapping (BGS, 1989). Hummocky Glacial Deposits are ice contact or supraglacial deposits formed during ice marginal retreat and are likely to overlie Till which is largely deposited during advance phases in a subglacial setting. The Hummocky Glacial Deposits may be coevol or predate deposition of glaciofluvial fans, however, as ice margins fluctuated these are likely to form complex interdigitating relationships in the subsurface.

The third section illustrates the likely relationship that may be encountered in Lower Rhymney, but could equally represent any of the three major rivers in Cardiff. The section shows a channel, cut into bedrock, partially lined with till or glaciofluvial deposits with the modern river locally incising down to bedrock. Modern deposits in rivers can conceal deeply buried valleys, filled with sands and gravels.

Figure 2 Conceptual Cross- sections though superficial deposits in Cardiff.