Erratics of the Holderness Coast.
[a work in progress]
by Mike Horne F.G.S.
There is a huge variety of erratic rocks minerals and fossils to be found on the coast of Holderness, East Yorkshire, U.K.
Here is a list of the names I have used for them in my fieldwork and the reports of the East Riding Boulder Committee.
Sedimentary rocks -
Chalk - a pure white bioclastic limestone. If it has been stained yellow this might indicate that it has been derived from a previous Till. Soft chalk found in 'rafts' may be younger than the Chalk found in situ in Yorkshire.
'Pink chalk' or 'red chalk' - pink or red stained chalk - not necessarily (and probably not) from the Albian Red Chalk Formation. It is easy to recognise the Red Chalk from its microfossils or the belemnite Neohibilites. There are other layers of pink coloured chalk recorded in boreholes in the North Sea.
"Chalk Pebble bore by sponges" - rounded pieces of chalk that have small 1-2mm dia holes in the surface - clear evidence that it is derived from a beach deposit.
Grey Flints as seen in the Chalk
Black Flints similar to the flints seen in Norfolk - probably of younger age than the Yorkshire flints.
Red Flints - perhaps Turonian in age (subject of ongoing research)
Brown Flints - weathered grey or black flints - rare.
Brown sandstone - pale to mid brown, or orangy brown sandstone, may have liesengang staining- probably Middle Jurassic.
Brown sandstone with fine cross lamination picked out in dark brown or red - may be Carboniferous in age.
Coarse orange-brown sandstone - perhaps Teriary in age on basis of one macrofossils that contains microfossils in the matrix.
Yellow Quartzite - hard, crystalline.
"Old Red Sandstone" (ORS) - a darker red and courser grained than the NRS.
"New Red Sandstone" (NRS) - a bright red, medium to fine grained sandstone - probably Permo-Trias in age.
"Middle Jurassic Sandstone" or "Middle Jurassic Rootlet Bed" or "Deltaic Sandstone" - medium to coarse grained, pale browny grey, containing dark lines, stripes or circles - similar to exposures north of Scarborough.
"Green Conglomerate" or "Greywacke" - course grained conglomerate - quartz, feldspars and rock fragments in a green matrix - perhaps a coarse greywacke from the Ingletonian or "Haggis Stone" from Scotalnd (according to Whitby Museum) [under investigation].
"greensand" - green sandstone - sometimes micaceous, but not the "Greensand".
Carboniferous Limestone - gery or dark grey limestone, often with ice scratch marks, containing solitary corals, colonial corals (listed as Lithostrotian), crinoids or Productid brachiopods (Gigantoproductus).
Black shelly Carboniferous Limestone - looks like coal with pale brown crushed shells, but fizzes when tested with acid.
"Frosterly Marble" - dark grey Carboniferous Limestone with numerous solitary corals, from Weardale - rare.
"Canon Ball Limestone" - a buff coloured limestone (perhaps dolomitic) containing 1 to 4 cm spheres or sometimes as large as cricket balls - possibly Permian in age.
"Lower Jurassic Shelly Limestone" - dark grey or brownish limestones containing Gryphaea, Cardinia, oysters or pectinid bivalves.
Oolitic limestone - Middle Jurassic - rare.
Other Sedimentary Rocks -
Septarian Nodules - large grey or grey-brown cementstones, with white or yellow calcite infilling internal cracks and cavities. Some can be identified as being from the Lias, Kimmeridge Clay or Speeton Clay.
'dogger' - rusty-red-brown, hard, may be septarian or containing ammonites - may be from the Jurassic Dogger.
Kimmeridge Clay - black or dark grey shales containing crushed ammonites.
Please note that the 'rafts' of soft white Chalk or red, buff or grey clay that can be seen in the cliffs or on beach exposures of the boulder clay are not listed in the boulder survey.
Igneous rocks -
Granites - various colours (pale grey, pink or red) and grain-size. Some have recognisable characteristics - such as Shap Granite - (with large domino shaped pink phenocrysts of orthoclase; from the Lake District) and Perterhead Granite (similar but the phenocrysts are not so large and domino shaped - though I may be mistaken in this identification).
Grey porphyry - zoned phenocrysts
in a grey matrix.
Red porphyry - pink and yellow phenocrysts in a red matrix.
"Cheviot Porphyry" - very hard, fine grained, phenocrysts yellow, white and pink and about 2-4 mm, in a black or dark red matrix. Very rounded and highly polished.
Volcaniclastic rocks -
"Tilberthwaite Tuff" or more generically Borrowdale Volcanic tuffs from the Lake District- green or pale green, sometimes brecciated or sometime showing sedimentary structures.
Metamorphic rocks -
There are many schists amd gneisses to be found as erratics - but I do not know the original sources of any of them. But please note that the augen gneiss (pink eye shaped crystals in a dark gneiss) has been recently introduced as part of the sea defences, and at Mappleton can be used to measure the effects of long shore drift!
"Lithostotian" - Colonial Carboniferous corals - seems to be two sizes 2-3 mm dia and 5-6 mm dia. Note - sometimes the rock has weathered to a red colour.
Belemnites - generally common - the "Devil's Toenail". You can roughly date them on colour: Jurassic ones are black or dark grey, Lower Cretaceous ones are grey or light grey and Upper Cretacesous ones are pale brown. It is quite easy to identify most of the Cretaceous ones to genus level. Interestingly I have collected numerous Belenitella mucronata but never seen a convicing Belenella lanceolata (the zonal indicator for the Lower Maastrichtian stage).
Gryphaea - very common erratic of Early Jurassic age. There is also a larger species identified by F Whitham as G. gigantia from the Oxynotum Zone - this is rare but good specimens can be found occasionally in beach exposures. [Note - I have spelt Gryphaea wrongly in early reports]
"Cardinia" - a fairly common erratic, usually flattened.