The continental masses that stand proud of their surrounding oceanic crust have never occupied fixed positions on the Earth's surface. They are constantly carried around on the tectonic plates rather like logs embedded in the ice-floes of a partly frozen river.

The movement is going on at the present day, with North America moving away from Europe at a rate of about 2 to 3 centimeters per year, and the movement of Africa against Europe made evident by the intensity of earthquake activity and the presence of active vol­canoes in the Mediterranean area.

The proof that this has been happening throughout geological time takes a number of forms.

The first line of evidence - in fact the first obser­vation that suggested that the continents are in motion - is the apparent fit of one continental coastline with another. The eastern coast of South America and the western coast of Africa are so similar in shape that it seems quite obvious that the two once fitted together like the pieces of a jig­saw.

The other continents can also be pieced together in a similar way but usually the fit is not so obvious; for example, Africa, India, Antarctica and Australia would also mate together. It is the edges of the continental shelves, rather than the actual coastlines that provide the neat fit.

If the continents were placed together certain physical features could be seen to be continuous from one to another across the joins. Mountains that formed 400 million years ago and now found in south-eastern Canada and eastern Greenland would be continuous with those of the same age now found in Scotland and Norway, if North America and Europe were placed together. Mountain ranges in Brazil would be continuous with those in Nigeria if South America and Africa were brought together.

Evidence for ancient climates is also a good indicator of continental drift. Northern Europe went through a phase of desert conditions about 400 million years ago, followed by a phase of tropical forest 300 million years ago, and then another desert phase 200 million years ago. This is consistent with the movement of that area from the southern desert climate zone of the Earth, through the equatorial forest zone and into the northern desert zone.

About 280 million years ago an ice age gripped the southern hemisphere. The evidence for this includes ice-formed deposits and glacier marks from that period found in South America, south­ern Africa, Australia and, significantly, India -which is now in the Northern Hemisphere. If the continents were reassembled and the directions of ice movement analyzed, they would point to an ice cap with its center in Antarctica.

The evidence from fossils is just as spectacular. Fossils of the same land animals and plants have been found on all the southern continents in rocks dating from about 250 million years ago. These are creatures that could not have evolved independently on separate continents.

Mesosaurus fossilMesosaurus was a freshwater reptile, resembling a small crocodile, and its remains have been found both in South America and South Africa. Lystrosaurus was like a reptilian hippopotamus and its remains have been found in India, Africa and Antarctica. The fern­like plant Glossopteris is typical of the plants that lived at the same time as these creatures and its remains have been found in South America, Africa, India and Australia.

Similar biological evidence is found in the Northern Hemisphere where the dinosaurs of Europe 150 million years ago were similar to those of North America.

The mammals that developed in various parts of the world during the last 65 million years also reveal evidence of the movements of the conti­nents. Up to about 10 million years ago the domi­nant mammals of South America were the pouched marsupials, similar to those of Australia today. This suggests that their origin lies in a single southern continent.

Later most of the South American marsupials became extinct after a sud­den influx of more advanced placental mammals from North America, suggesting that South and North America became attached to one another about 10 million years ago.

India was a similar iso­lated continent, broken away from the southern landmass, until it collided with Asia about 50 mil­lion years ago. It would be interesting to see if the mammals of India before this date were marsupi­als or not, but no Indian mammal fossils have been found for the relevant period. In 1980 a fos­sil marsupial was found in Antarctica, helping to substantiate the theories.

The positions of the Earth's magnetic poles change over a long period of time. Clues as to their location in any particular geological period lie in the way in which particles in the rocks that formed in that period have been magnetized. As rocks are formed, the magnetic particles in them line up with the magnetic field of the Earth at that time, and are then locked in position when the rock solidifies. This phenomenon is sometimes known as Remanent Magnetism and it has been actively studied since the 19 60s. It has been found that the remanent magnetism for different periods in each of the continents point to a single north pole only if the continents are "moved" in relation to each other.