In this month’s blog from our Community Geologist, Dr Ian Kille we delve into the world of Carboniferous coal swamps and fossil plants to explore Mystery Rock 6, which featured in last month’s WallCAP newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
Breaking the soil open with my hand-fork and gently, slowly, pulling each of the mini-minimalist Christmas trees that were the horsetails invading my allotment, I found their roots. The long thin segmented rhizome penetrated the soil like lightning: dark brown with patches of shiny white, with wriggly roots radiating from the joint of each segment. Frustratingly frequently the top broke off at one of the segment joints leaving more rhizome and root to keep the insurgence going. This gardeners’ nightmare was well designed, simple, and effective at maintaining its presence in the soil and fast to invade with its network of rhizomes. Not much wonder then that it has survived as a group of plants (the Equisetidae) from the Devonian Period over 360 million years ago, when plants were becoming a major part of the landscape.
They thrived in the Carboniferous coal swamps too, becoming one of the most common understory plants, preferring wet environments as indeed they do now. Carboniferous gardeners would not have been happy. There were many more groups of these plants then, from the elegant Sphenophyllum to the more familiar looking Calamites. Calamites had a similar looking form to modern horsetails, with needle-like leaves radiating 
from the stem nodes. It was, however, much larger, growing up to 10m tall with secondary growth giving it a woody stem with the requisite strength to support its stature.
Calamites weren’t the only familiar plants to be found in the coal swamps. Ferns which wouldn’t look out of place in a modern shade garden could be found in amongst the undergrowth. However, alongside them would have been two groups of plants, one that is now extinct and the other which is uncommon and markedly different from its ancient ancestor.
One of the problems in identifying fossil plants is that they are quite fragile and break up and decay readily – consider looking at your compost heap and trying to visualise the constituent plants from the prize cottage garden that was degraded to make this organic matter. What is preserved tends to be fragmented so that it is difficult to tell which bits belong to each other, much like trying to do a jigsaw puzzle with most of the pieces missing and no picture on the box lid. Early in the research history of Carboniferous plants, the Carboniferous became known as the period of ferns. It wasn’t until more fossils were discovered and with more detailed observations that it became clear that a number of the ferns were a completely different group which produced seeds. These seed ferns (Pteridosperms) as they became known have fern-like leaves but are not ferns. The pteridosperms are now extinct, appearing with the Equisitidae (horsetails) in the Devonian Period; by the end of the Cretaceous Period they had mostly disappeared.
The other group is now represented by club-mosses, quillworts and firmosses; large by moss standards (albeit they aren’t mosses) but diminutive by general plant standards, with the largest reaching circa 2m in height. 
These are the lycopods and in the Carboniferous, just like Calamites, were much bigger than their modern relations (this gigantism and its cause is another story for another blog). Lycopods formed the canopy in the Carboniferous swamp forests reaching over 30m in height with trunks 2 metres wide at the base. As with the seed-ferns, palaeobotanists initially identified different parts of the same tree as different species. So it is that the bark of one group of these giant lycopods, or scale-trees as they are also known, are given the species name of Lepidodendron. On the other hand, the root structures of these lycopods – both for Lepidodendron and Sigillaria (the other common Carboniferous lycopod) – have been given the species name of Stigmaria.
The giant lycopods, unlike modern trees, are tubular with a simpler vascular structure (the veins that carry the water and nutrients around the plant). They do, however, share the same remarkable polymer used to give plants strength, lignin. Evolving the ability to make lignin was one of the most important steps to enable plants to colonise the land. Lignin’s strength and resistance to decomposition had major consequences as the mass-trespass of the land-surface got underway in the Devonian Period and expanded into the Carboniferous Period. The plants grew and then died, and the presence of lignin meant that the carbon in the ex-plants was held within the sediments.
When the layers of decaying plant matter were sufficiently thick, as in the Carboniferous deltaic swamps, this resulted in coal formation. This in turn meant that by the early Carboniferous the plant proliferation had reduced the CO2 content of the atmosphere, in turn moving the global climate to something more temperate (complete with fresh new ice-caps) as this greenhouse gas was removed.
Coal is an extraordinary material and there is much more to say about it. Having explored its formation rooted in the plants it was made from as well as its time and place in our evolutionary and geological history, what else is there to look at? A more detailed exploration of the process of turning plants into coal, the sediments that are associated with the coal seams and what this tells us about these ancient environments (including the animals that cohabited with the plants) would all be interesting narratives to follow. So too is an exploration of the way that coal has been exploited, not least by the Romans, and the consequences this exploitation has had on human development and the environment we live in. But these are all stories for another day.
