Rock strata, Haslingden Flag and fossils

Rock strata and Haslingden Flag

Whilst the general process that gave rise to sedimentary rocks was the same over the whole of the Pennines, conditions would vary from place to place giving rise to local variations in the type and sequence of rocks. Peculiar conditions in what was to become Rossendale gave rise to deep beds of hard sandstones, known geologically as Haslingden Flag. This stone has a hardness and silica content equivalent to granite and was the principal reason for the growth of quarrying in Rossendale.

Haslingden Flag is of particular interest to geologists. Detailed geological mapping shows the shapes of the Millstone Grit deltas. The two most common types are found throughout the Pennines, but a third type is recognised only in the Haslingden Flags in Rossendale. This is an elongate delta: the nearest modern equivalent is the Mississippi / Everglades ‘Birds Foot’ type delta. Because it shows evidence of this rare type of delta, Lee Quarry is designated as a Site of Special Scientific Interest (SSSI).

Two units of Haslingden Flags are recognised - Lower and Upper – of interbedded sandstones and siltstones. These delta type sandstones show elongate west to east orientation as ‘bar finger delta form’ instead of the northerly orientation of the other delta types. The lower Haslingden Flags are important in the west and north-west Rossendale from Haslingden to Crawshawbooth. The Upper Haslingden Flags dominate the southern moorlands of Rossendale from Edenfield round to Whitworth.

Above the Haslingden Flags is usually a weak layer of mudstone; then a quite dramatic geological change to the Rough Rock – a very coarse gritstone. In places the bottom beds of the Rough Rock are rich in broken fragments of plant fossils, with branches stems and roots, sometimes nicknamed ‘Log Jam Rock’. Powerful currents must have swept in these sediments and trees, possibly representing climate change. One possible theory is rapid glacial melting in the mountains near the source of the large river that created the ‘Haslingden Delta’

The Rough Rock is the most common gritstone of the Millstone Grit group, occurring right across the Pennines, with its origins as the most common ‘sheet delta sediment’. Although quarried elsewhere, it is ignored and rejected in Rossendale as inferior economically to Haslingden Flags.

Fossils In Rossendale Quarries

Over the centuries the word ‘fossil’ meant different things, now the term is restricted to ‘recognisable traces of ancient animals and plants preserved naturally in the earth’s crust’. Fossils are important because they tell us what life was like millions of years ago.

Fossilisation is a chancy process. The conditions of formation of the sediments of most of Rossendale’s quarries were not really suitable, with shallow water conditions and turbulent current action breaking up shells and plants.

The tops of the deltas of the later Carboniferous sandstones were forested by large horsetails, seed ferns and now extinct club mosses. Trees and branches, perhaps swept down in flash floods, were stranded on sand banks and are often in the Rough Rock at the tops of the quarries. Sometimes the vegetation remains are only indicated by a black carbon film.

Much more common in the Rossendale quarries, rather than the fossils themselves, are the traces of fossils including tracks, trails or burrows of invertebrate animals living on the deltas. Feeding tracks show as furrows in the sandstone, and irregular worm burrows or casts are often found. Very common are oval ‘bump marks’ indicating positions of the escape burrows of shellfish (cockle shells) (see picture). Although millions of years old these marks are mere points in time – hours or days!

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