The distribution of some indicator glacial erratics in eastern Yorkshire

By Mike Horne*

Read at a meeting of the Yorkshire Geological and Hull Geological Societies in Hull 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. The 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, whereas 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.

The erratics selected for this report are –

• Larvikite – a dark coarse grained igneous rock from southern Norway; the large feldspar crystals give off a blue interference colour.

• Rhomb Porphyry – large pale crystals in a fine grained reddish purple matrix from southern Norway.

• Green Conglomerate – well rounded clasts of quartz and rock fragments in a fine grained green matrix probably from the Ingleton area.

• Tilberthwiate Tuff – a green slate from the southern Lake District.

• Brockram – a Permian breccia of large clasts of grey limestone in a pale red matrix from the Appleby area.

• Cheviot Porphyry – medium sized phenocrysts in a black, brown or reddish matrix; these erratics are shiny as if they have been polished.

• Cannon Ball Limestone – a buff limestone made of large spherical concretions of Permian age from the Sunderland area.

• Gryphaea – the “Devil’s Toenail” oyster from the early Jurassic of North Yorkshire and offshore exposures in the bed of the North Sea during periods of low sea levels.

• “Lias” – a grey shelly limestone from the early Jurassic of North Yorkshire and offshore exposures.

• Kimmeridge Clay – a dark shale from the late Jurassic, sometimes containing distinctive crushed ammonites. This rock is exposed in Filey Bay and also occurs in the Vale of Pickering, but the erratics probably also came from the bed of the North Sea.

• Speeton Clay – fossils and nodules from the early Cretaceous. This rock is exposed in Filey Bay and also occurs on the norther edge of the Yorkshire Wolds, but the erratics probably also came from the bed of the North Sea.

• Black Flint – black flints from the latest Cretaceous (not found in the onshore Yorkshire Chalk).

• Belemnitella mucronata – a distinctive butterscotch coloured belemnite from the latest Cretaceous (not found in the onshore Yorkshire Chalk).

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. A list of the sites is given in the appendix to this article.

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, such as 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, Mary Howard, Paul Hildreth and Rodger Connell for useful suggestions at the editing stage.

*Mike Horne FGS, Honorary Fellow, Department of Geography, Geology and Environment, University of Hull, Cottingham Road, Hull, HU6 7RX.

Appendix –

Burniston

Scalby Bay

Scalby

South Bay Scarborough

Carnelian Bay north

Carnelian Bay south

Osgodby Point

Cayton Bay north

Cayton Bay south

Filey Bay

Speeton

Thornwick Bay

North Landing

Stottle Bank

Selwicks Bay

High Stacks

South Landing to High Stacks

South Landing

South Landing to Danes Dyke

Danes Dyke

Danes Dyke to Sewerby

Sewerby Steps

Sewerby to Bridlington

South Bay Bridlington

Fraisthorpe

Barmston going north

Barmston

Barmston going south

Ulrome

Skipsea

Skirlington

Atwick going north

Atwick

Hornsea going north

Hornsea

Hornsea to Mappleton

Mappleton

Cowden

Aldborough going north

Aldborough

Aldborough going south

Ringborough going north

Withernsea

Holmpton

Dimlington

Easington

Kilnsea

Spurn

Spurn Point

Hessle foreshore

North Ferriby foreshore

South Ferriby foreshore

copyright Hull Geological Society 2021