8 Palaeosols


The fascinating thing about geological research is not just finding fossils like dinosaurs, but finding the soil that their food grew in!   Geologists have now recognised “fossil” soils, called palaeosols (= ‘old soils’) in several rock formations in Wales.  Like all fossils, these have changed slightly, mainly in colour, since they were real soils. Neither geologists nor soil scientists are experienced in studying these soils, but we know enough to begin to understand them.

Palaeosols can contain almost any of the features of modern soils, including evidence of roots or burrows, soil structure, horizonation and movement of clay, iron or salts through the profile.

Devonian Palaeosols in Anglesey. The oldest abundant land plants date from the Devonian Period, and their soils are preserved among the Devonian sandstones  –  the“cornstones”  are an example of calcretes.

To see these, take the unclassified road sign-posted to Lligwy off the roundabout near Moelfre on the A5025, and park at Lligwy (toilets and tea shack available).  Cross the beach to the north side at SH493876 and examine the sequence of rocks in the low cliffs and on the foreshore.  A middle to low tide is best.

(The sediments forming the upper part of the cliffs are Irish Sea glacial deposits (till) with profiles of the Salop (occasionally Flint) Series)

During the Carboniferous Period, parts of Wales resembled tropical rainforest, leading to coal deposits;  again the associated soils are preserved within the Coal Measures.  Not easy to see, but there are root channels within this sandstone, formerly a sandy soil, at Malltraeth.

More recently, the Ice Age led to development of artic soil features which have been preserved in some soils.

Location:  Criccieth, Gwynedd – located on a low sea cliff eroded into  glacial debris east of the beach.  Park in the beach front car park (at SH505380) and walk east along the coast for 100m.

This site displays a common feature of older glacial deposits that have undergone soil formation in peri- or early post-glacial times.  The deep, vertical, paler features are ice-wedge casts, the infilling of cracks formed by ice lenses within the frozen soil material.   Often there is an orange-brown, iron-stained margin to the soil around these features.  There are two glacial sediments here, the lower greyish clay is the Criccieth Till, with a weathered surface and ice-wedge casts, the upper yellowish-brown stony material is the Llanystumdwy Till (illustrated here), also with cryoturbation features (~churned up by freezing).   These features must be about 14,000 years old.

The soil profile at the surface is that of a brown earth  This site is protected as a SSSI. please do not damage it.

Patterned Ground features: ground freezing produces features by separating stones from the finer soil.  It would seem that on the mountain tops these processes are still active.  On the Carneddau summit ridge, stone polygons are still visible, whilst on  Moelwyn Mawr, the same process has produced linear features called stone stripes  due to the greater slope.

And at Lleiniog, the foreshore preserves a submerged peat / forest bed now exposed only at low tide but presumablydating back to before the 8000 BP sea level rise that drowned large areas around our coastline

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