Welcome to the Ice Age Coast

Descriptions of distinctive erratics

(found in Yorkshire, Lincolnshire and Norfolk)

Click on the name to view picture

Minerals -

Agates - concentrically banded coloured chalcedony. Origin - Scotland.

Amber - pale toffee brown, very low density, feels warm. Shiny when wet.

Coal - black or dark grey. bedded. low density. black streak. shiny when wet. Origin - Middle Carboniferous of North Yorkshire, North Sea or Scotland

"Green Jasper"* - green, opaque, hard, Shiny when wet.

Jasper - dark blood red, opaque, hard. Shiny when wet. Origin - perhaps north-west Scotland

Jasper red pebble* - thin veins of blood red jasper in a dull red matrix.

Jet - black, low density, feels warm, brown streak. Origin - Lower Jurassic of North Yorkshire or North Sea. Warning heavier pieces of jet may contain pyrite which will rot in you collection!

Opal - pearlescent creamy-white

Pyrite - "Fools Gold" - heavy, metallic, golden-coulured, mineral, which may have red or rusty patches. It will probalby tarnish rapidly if you collect it and may need special conservation (you will regret it if you don't!). May be found gathered in hollows on a sandy beach.

Pyrite -

Quartz - probably the most common erratic because of its resistance to weathering and abrasion. Pebbles may have been recycled from earlier rocks. Origin - could be almost anywhere. Comes in a variety of colours:

Quartzite on the Wolds - the distribution on the Yorkshire Wolds from an early glaciation was studied by J W Stather and D Gobbett


Igneous Rocks -

"Cheviot Porphyry"* (Cheviot Porpyrite of old surveys) - medium grained porphyry (smaller phenocrysts than Norwegian Porphyry) in a red, brown or black matrix. Always looks glossy as if it has been polished. Origin - ?Cheviot Hills.



Grey Granite -

Larvikite (Augite-Syenite of old surveys) - coarse grained dark rock, shines blue in sunlight when wet. May have been described as Syenite in early boulder survey publications. Origin - Larvik in Norway.

Norwegian Porphyry* - yellowish phenocrysts up to 1cm in a red-brown matrix. Origin - Oslo Fjord in Norway

Peterhead Granite - looks a bit like Shap Granite but the orthoclase phenocrysts are more of a tinned-salmon pink and not as domino shaped.

Rapakivi Granite - origin Aaland

Shap Granite - pale granite containing distinctive large crystals of pale pink orthoclase (like pink dominoes), sometimes with obvious simple twinning. It is difficult to identify in small specimens because you need to see the phenocrysts, so it may be under-reported. Origin - Shap in Cumbria. There is a large boulder near the base of Sewerby Steps that can be seen when it is not covered with sand.

Tilberthwaite Tuff* - green fine grained rock, sometimes with current bedding, fining upwards bedding and volcanic bombs. This is a volcanic ash that fell into water and was later metamorphosed (so it is an igneous, sedimentary and metamorphic rock in one!) - Origin - Lake District.

"Whin Sill" * (Whinstone of older surveys) - medium grained almost black. Origin - northern England.


Sedimentary Rocks

Black Flint - hard, black, sharp if broken, may have a white cortex. Origin - Upper Campanian and Maastrictian of the North Sea and Holderness. "Rowe Formation". May containg fossils.

Black Shelly Carboniferous Limestone* - pale brown shells in a black matrix which may weather to brown. Origin - midland valley of Scotland. Matrix may be bleached or weathered to a chocolate brown colour.

Brockram - a breccia, pieces of grey limestone (usually around 25mm across)  in a dull red matrix.

Brown flint - dark gey brown, hard, breaks with a conchoidal fracture, with a paler brown cotex. Probably weathered Black Flint. Not so common in Yorkshire.

Brown ripple bedded sandstone - fairly fine grained pale sandstone with ripples picked out in darker brown

Carboniferous Limestone - dark grey limestone

Carstone - phosphatic nodules from the Aptian, occasionally steinkerns of Jurassic fossils. . Found south of the Humber.

Cementstones - large (over 30cm), grey flattened nodules, usually septarian. Origin - mostly from the North Sea or Vale of Pickering.

Chalk - hard, very fine grained, white. Origin - Yorkshire Wolds or North Sea

Dogger* - hard orange or red fine grained ironstone from the middle Jurassic often containing fossils

Flow banded porphyritic rhyolite -

Frosterly Marble - dark grey to black Carboniferous Limestone with numerous large solitary corals. May be under-reported because you need ta large enough specimen to recognise it. Origin - Lower Carboniferous of Weardale.

Green Lake District conglomerate (a.k.a. "Ingleton Granite" or Greywacke Grit of old surveys) - a conglomerate with rounded clasts of quartz and other ninerals up to 1 cm in size in a green matrix

Green sandstone - medium grained,

Grey Flint - from the Yorkshire Chalk. Pale or very pale grey, hard, foes not break with the conchoidal fracture you would normally "expect" for flint. Origin - Yorkshire Wolds or North Sea.

Jurassic plant bed (Esturine Sandstone of old surveys or Deltaic Sandstone) - pale grey, well bedded, mpstly quartz grains with some black plant remains.

Kimmeridge Clay - a dark grey shale often containing flattened white ammonites. Origin - Vale of Pickering or North Sea

Magnesian Limestone - quite hard, fine grained, creamy yellow colour.

Oxford Clay - grey or dark grey clay. Occasionally seen as "rafts" on the beach or in the cliff in the area around Aldborough. Origin - probably from the North Sea.

"New Red Sandstone" - medium to fine grained, with some mica, brick red colour. Origin - Permo-Trias of north west England.

"Old Red Sandstone" - medium to coarse grained, dull blood red colour. Origin - Devonian, perhaps Scotland.

Orange brown coarse sandstone - bright rusty brown colour, grains about 1-2mm, poorly cemented, may contain Tertiary foraminifera. Origin - probably from the North Sea.

Phosphate nodules - small pale brown nodules from the Speeton Clay, may contain fossils.

"Pink Chalk"* - hard, very fine grained, pink. It may not necessarily be from the "Red Chalk Formation", you would need to see distinctive fossils to be sure. Origin - North Sea and Yorkshire Wolds.

Red flint - hard, reddish brown, sharp if broken, may have a white cortex. Origin - perhaps the Danian of Denmark

Rhaxella Chert - found in Trent Valley and Norfolk, said to be from the Jurassic of the Howardian Hills (not recorded in Holderness)

Septarian nodules ( "turtle stones" ) - (see also Cement Stones)

Shelly Jurassic Limestone* - darkish grey limestone with fossils including Gryphaea, Lower Jurassic. Origin - North Yorkshire or the North Sea

Soft Chalk - white, putty-like. Can be seen in "rafts" in the cliffs. Origin - Upper Campanian and Maastrictian of the North Sea and Holderness. "Rowe Formation"

Speeton Clay - seen as "rafts" on top of the Chalk of the Flamborough area.

Red Flint - very hard, red-brown, sharp when broken, may have white cortex. Origin - perhaps the Danian of the North Sea and southern Scandinavia.

Tilberthwaite Tuff (see above under Igneous Rocks)


Metamorphic rocks -

Garnet mica schist - hard rock with sparkly muscovite and small red garnets. Occasionally you may find a version which is mostly biotite and garnets that crumbles when you try to collect it.



Fossils - this is not a comprehensive list but includes some of the more common fossils that can be found.

Ammonites **- a numerous variety of ammonites can be found. Some ammonites were first identified and named by C Thompson from specimens found in Holderness. Here are some common ones -

Belemnites ** - often found broken or damaged - here are a few hints towards identification -

Bivalves -

Corals - usually from the Carboniferous in a grey matrix. The matrix can also be black, red or yellow. These look really nice if you wet them.

Crinoids - these can look a bit weird when seen in cross section at strange angles. Pentacrinus* is star shaped.

Echinoids -

Fossil wood -

Jet - (see above under "minerals")

Microfossils - the boulder clays do contain microfossils but because there are so many grains of sediment in the processed sample picking them out is very tedious!

Pyritised fossils -

Trace fossils - burrows in sedimentary rocks


Sub-fossils - from the Quaternary not yet turned to stone

Corbicula fluminalis - enigmatic bivalve (found today in warm fresh water) found in the gravels of the Keyingham area.

Elephant teeth

Mammoth teeth and tusks



* Note - specific definitions used by Mike Horne for the East Yorkshire Boulder Survey

** Note - just because these can be seen in situ on the Yorkshire coast, their origin is more likely to be from the North Sea.

(c) Mike Horne & Hull Geological Society  2020

Copyright Hull Geological Society.