Humberside Geologist no 16

The distribution of indicator glacial erratics in eastern Yorkshire

By Mike Horne*

 

Read at a meeting of the Yorkshire Geological and Hull Geological Societies in January 2005

The Hull Geological Society revived the East Riding Boulder Committee in 1987 as a contribution to its Centenary activities in 1988 (Harrison and Horne 1992) and has continued to record the various glacial erratics and their location. It is an activity that all members can become involved with and contribute their findings. This data collected has been published in its journal Humberside Geologist.

When one starts paying attention to the erratics it becomes obvious that some rocks are endemic to the whole area – such as yellow quartz pebbles, some are rarer and seem to have distinct distributions and others are very scarce. The identification of the erratics can be subjective and could be more refined scientifically (Horne 2021). This paper will look at some of the distinctive erratics and consider their distribution; it is mostly based on their occurrence as pebbles on the beaches of Holderness and coastal exposures.

On the maps a red dot indicates a record of the erratic. The size of the dot is not an indicator of abundance. The erratics were mostly noted from occurrences on beaches, rather than in situ in cliffs or beach exposures of the tills. The purple stars indicate exposures of the source rocks on the Yorkshire coast.

Larvikite erratics

rhomb porphyry erratics

green conglomerate erratics

Tilberthwaite tuff erratics

Brockram erratics

Cheviot porphyry erratics

Cannonball Limestone erratics

Gryphaea erratics

Lias erratics

 Kimmeridge Clay erratics

Speeton Clay erratics

Black Flint erratics

'mucronata' erratics

There appears to be a pattern for the distribution of some erratics. Some seem to be more abundant in areas where there is Withernsea Till generally present in Holderness – Cannon Ball Limestone, Kimmeridge Clay and Speeton Clay, B. mucronata seems to have a similar distribution, but this may be due to “rafts” of soft chalk incorporated into the till, or a sub-crop beneath the Tills. Others seem to be less abundant in those areas - Cheviot Porphyry, Brockram, Green Conglomerate and Tilberthwaite Tuff.

 

References -

Harrison R & M Horne 1992, The East Riding Boulder Committee, reports for the years 1987 to 1991. Humberside Geologist 10, 18-22.

Horne M 2000. Report of the East Riding Boulder Committee 1992 to 2000. Humberside Geologist 13, 42-45.

Horne M 2004. The distribution of glacial erratics on the Holderness Coast. [abstract] Yorkshire Geological Society Circular no 519, 6.

Horne M 2006. Report of the East Riding Boulder Committee 2001 to 2004. Humberside Geologist 14, 92-97.

Horne M 2021 (in press). The scientific study of glacial erratics.

Acknowledgements – I thank David Hill and Rodger Connell for useful suggestions at the editing stage.

*Mike Horne FGS, Honorary Fellow, Department of Geography, Geology and Environment, University of Hull.

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