It has become known in recent years that the continents of the world do not hold the fixed positions that are seen today in an atlas. Instead, they are continually in motion. For example, the Atlantic Ocean is widening by a few centimetres each year and this process has been going on for many millions of years. The evidence for this is obtained by studying the magnetic properties of rocks in the Earth’s crust.
When rocks are formed they become magnetized in the Earth’s magnetic field. If they are subsequently moved out of their original position their magnetic properties no longer ‘fit’ their new position. They can however be theoretically ‘moved’ back to their position of formation and maps of continental arrangements can thus be drawn up for past geological times.
To describe the scientific work involved in this in just a few sentences represents an enormous simplification of what actually needs the use of the most powerful computers available. Nevertheless, it remains true that what happens in this kind of work results in our being able to see whereabouts on the Earth’s surface fossil plants actually grew, as distinct from the places where we find their remains at the present day. For example, some fossil plants found today in Yorkshire might well have grown near the equator when that piece of land was thousands of miles south of where it is now.
Clearly it would require enormous amounts of energy to move continents around. Strangely enough these quantities of energy are actually available. Other, much more familiar, effects of this energy are seen when volcanoes erupt and when earthquakes take place. We are all unfortunately aware of the frightening amounts of energy that are released by atomic bombs. Internally in the Earth this kind of energy from the radioactive decay of certain elements is continuously being released. Fortunately, the rate of release does not cause atomic explosions, but it certainly gives rise to a lot of heat energy (e.g. as in volcanoes).
Another effect of this heat is to cause convection currents below the continents so that they are moved around (again, a simplification of a very complicated process). You can see something like this if you watch the surface of milk that is boiling in a saucepan. Were you to drop small pieces of bread on to the milk they would be pushed around, because you are providing the energy to do this by applying heat to the bottom of the saucepan.
You may be wondering why we are going to a great length at this stage to explain what is known as Continental Drift. The reason is that at about 350 million years ago there was no Atlantic Ocean and North America was actually joined on to Western Europe. A vast deltaic swamp extended from the area now known as Kansas right across to the Don Basin in the USSR just north of the Black Sea. Also, because this area at the time lay across the Equator there existed ideal conditions for plant growth on a colossal scale. Hot tropical conditions with an abundant water supply made possible forest growth similar to the present day tropical rain forest. Whereas under normal circumstances dead plants would be consumed by fungi, in this deltaic region the marshy ground was so waterlogged that it was deficient in oxygen so that fungi could not live. Great depths of unrotted plant material accumulated as peat. Subsequent deposition of rocks on this peat converted it to coal. Because the sea level was continuously going up and down, alternating layers of peat and rock gradually built up to form the familiar coal seams of the present day.
An explanation is now available as to why it was stated at the end of Chapter 3 that this period in the Earth’s geological history was to be so important to mankind. Along that line (as it was then) from Kansas to the Don Basin lie the locations that in the 19th Century were the centres of the Industrial Revolution fuelled by the underlying coal. It has been said, rather cynically, that whatever else was manufactured by the Industrial Revolution, it certainly manufactured people. The great expansion of the population at this time in human history provided the cheap labour to dig out the coal that had been formed hundreds of millions of years previously.
The subject of coal now impinges on the human race again because burning it in large quantities releases carbon dioxide and exacerbates the greenhouse effect. It is a sobering thought that the released carbon in the carbon dioxide was taken in by plants a very long time back in geological time.
What kinds of plants grew in such profusion as to produce quantities of coal that in the UK still represent a 300-year supply at the present rate of mining? It so happens that we really know rather a lot about these so-called Coal Measure plants; more, probably, than we know about the plants in any other geological period. There are a number of reasons for this:
- Conditions for preserving plants were at their best;
- Mining brings up huge quantities of them to the surface;
- A lot get dumped on coal tips so that they can be collected over long periods at leisure;
- They have been readily available for study in just those industrialized areas of western civilization where there is the greatest number of educated people with the knowledge to study them.
The most outstanding ones were clubmosses and horsetails. There were other types of plants, but they were either much smaller or less numerous. The clubmosses were enormous [Figs. 12 and 13]. Portions of trunks 100 feet long have been found, with basal cross-sections greater than 3 feet. It is estimated that the tallest were about 150 feet high.
All that growth by a type of plant which today is hard to find! How was it achieved? Well, in the first place they did not have the solid trunks like the oaks and pines of today.
Secondly, it would appear that they were not very long-lived, only a few years or less, maybe. They reached maturity, produced a crop of spore-bearing organs (sporangia in cone-like structures) and then died.
In describing their structure we find ourselves back again to the concept of the strength of the cylinder. These trees were similar in their structure to a long, hollow roll of vinyl floor covering. There was little in them of true woody tissue. When they died and fell to the ground they became compressed almost flat by the weight of sediment that gradually accumulated upon them. It is apparent that in the competition between these plants success went to those that attained the greatest stature with the use of the least material. The hollow, cylindrical form provided a good structural solution to the problem.
Another way in which these giant clubmosses differed from trees in other plant groups was in their root systems. Although the basal structures seen in [Figs. 12 and 13] look like roots, in fact they are not. They are the root-bearers which might attain a length of 12 yards and were developed in a very symmetrical fashion such that each divided into two. True roots were produced on these organs, the site of each root can be seen as a small dot. A further curious feature of these trees is that their trunks were covered with a very attractive pattern produced by the marks left where leaves had dropped off. The form of the pattern is what is known as a tessellation, with large numbers of the leaf-scars arranged like a mosaic with an underlying spiral trend up the trunk.
In the Coal Measures Flora the horsetails [Fig. 14] were also very large. Curiously, they too had hollow cylindrical trunks, but were not quite of the same calibre as the clubmosses. Indeed it is thought that many of them may have acquired extra support by leaning on the neighbouring clubmosses. Their trunks are not so interestingly marked as those of the clubmosses; they are merely striated with parallel lines along the length of each segment. Again, as was the case with the fossil clubmosses, their trunks became flattened when lying horizontally after death.
Among the numerous, large fossilized remains of clubmosses and horsetails there is found a considerable quantity of fossil fern-like foliage. In fact, so much that at one time this period in geological history was known as the Age of Ferns. However, as more of the material was studied it was frequently noticed that undoubted seeds were attached to some of the foliage. These discoveries led to the realization that they were a whole new group of seed plants – the seed-ferns. These are now extinct at the present day but at the time we are considering, about 330 million years ago, they formed an important part of the land flora. Some were certainly very handsome plants [Fig. 15]. That there were some ferns as well as the seed-ferns is now certain [Fig. 16], but they were nothing like so abundant as was at first thought.
One other major plant group that flourished with those already mentioned, were some considered to be the ancestors of the conifers [Fig. 17]. With their long, strap-shaped leaves and seeds borne singly on separate stalks, not in cones, they did not look much like any conifer existing now. However, expert botanists can discern features that are significant in determining their relationship to conifers.