Glaciofluvial Pebbles and Cobbles

Brancliffe Grange pebbles and 'Bunter' pebbles

 

In the drainage ditch at Brancliffe Grange, all of the pebbles and cobbles that I saw were of the same yellow/brown colour and these were sporadically scattered throughout the yellow/red soil horizon and the brown loam above the unweathered red marl of the Edlington Formation. The samples that I collected for examination from a heap at the edge of the field, therefore, are representaive of the exposure as a whole.

 

The drainage ditch at Brancliffe Grange

I don’t own a microscope or sawing and polishing machinery, and I am not affiliated with any academic institutions or testing laboratories, so I have to rely on my eyes, a hand lens, a steel knife and a bottle of hydrochloric as my tools - and British Geological Survey maps and memoirs as my principal sources of reference.

 

The geological tools at my disposal

Although none of the geological maps that I have access to show any Quaternary deposits in the immediate area around Brancliffe Grange, the Sheffield memoir mentions that a scattering of glaciofluvial pebbles and cobbles throughout the district show where more substantial deposits must have been.

 

Quaternary deposits between Shireoaks and Kiveton Park

 

To the east of Doncaster, there are vast spreads of both Quaternary glaciofluvial and river terrace deposits, which contain pebbles that mirror the bedrock over which the glaciers and rivers flowed. Although I didn’t collect specimens, I visited many quarries where these are exposed, when undertaking survey work for the Doncaster Geodiversity Assessment.

 

Exposures of Quaternary sand and gravel in Doncaster

In the river terrace deposits, those to the north are predominantly composed of pebbles of Carboniferous sandstones and Permian limestones, over which the precursor to the River Don flowed. In the south, the pebbles are mainly of the 'Bunter type' deposited by the River Idle, which flowed from the south across the Triassic sandstones of the East Midlands, where these pebbles are very abundant.

 

Glaciofluvial pebbles at Cedar Road Quarry

The dispersed and relatively infrequent Brancliffe Grange pebbles, however, are much more like those that I saw in the glaciofluvial deposits above the Chester Formation at Cedar Road Quarry in Doncaster and I would therefore assign a similar origin to them.

 

A comparison with 'Bunter' pebbles

Examining the Brancliffe Grange pebbles closely, they consist of hard sandstone with a siliceous cement that has been polished to the extent that the individual grains can barely be determined and they are quite quartzitic in appearance.

 

A comparison with 'Bunter' pebbles

 

Compared to well rounded ‘Bunter’ pebbles of white vein quartz and quartzite that I took directly from the Chester Formation at Alton Towers, they are also distinctly angular. Although not as well developed as the dreikanter that I collected at Common Lane Quarry in Doncaster, their faceted form identifies them as ventifacts – pebbles that have been polished by wind driven sand or ice crystals in a periglacial environment.

 

A dreikanter from Common Lane Quarry

The coarser grained sandstone cobble that I also collected is much larger than those typically seen in the river terrace deposits and, although not faceted, it has also been polished. In parts of Doncaster that are near to glaciofluvial deposits such as these, cobbles form the oldest parts of the churches at Kirk Sandall, Fishlake, Hatfield and Thorne.

 

A sandstone cobble