Great Works: Map of Britain (c.1250), Matthrew ParisChronicle of England, entitled 'Cronica facta sub compendio'

British Library, London

Very like an island...? All maps before a certain date are free. There are no uniform or standard plans. They are simply possibilities. True, some of them are trying to make documentary surveys. Some are fixed to myths and formulae. But they are limited. And beyond that, there is another realm of likeness. A piece of ground can't help become a shape. An island in particular is a piece of drawing. Its coastline makes a defining contour, which can naturally create beasts. Islands breed fantasies.

Take this pair of British islands. One is a map of Great Britain, about 1250, and drawn by Matthew Paris. He was a Benedictine monk, an artist in illuminated manuscripts, based in St Albans' Abbey. The other island is also Great Britain. It comes from a Chronicle of England, at the coronation of Henri IV in 1399, made for a map of Totius Britanniae Tabula Chorographica. The cartographer is anonymous.

What connects these two maps? Both of them are to us recognisably British. Their likenesses may seem a little remote, but they have clearly the anatomies that we still know and name, and they share identities between themselves and between many more modern maps. On the other hand, their different imageries are equally impressive.

Look at these two forms. Ancient or modern maps, it is normally possible to see Great Britain as a body with a head. But here there is a visible contrast. One of them shows the head up; the other one shows the head down. More than that, the Matthew Paris map suggests Scotland as a head with an open mouth. And seeing that, its whole body is like a creature. Its contour has jagged edges like vertebrae all round its coastline. It could be a primitive fish, a whale.

Meanwhile, the coronation map is clearly upside down, and the head itself is much less conspicuous. In fact, it doesn't register as a body at all. This map has coastline with a soft outline – curvaceous, looping, billowy. There is no hint of creature now. It is a puffy cloud.

Or at least, you can see this, if you wish. This whale, this cloud, are only fictions, and fictions seen only by ourselves, not by their original makers, who would not even acknowledge these figures. But for us these acts of shaping can generate inspirations. After all, we can look for images in thrown ink patterns. We can read pictures in nature's forms. Leonardo is famous for suggesting scenes in chance stains on walls. Blobs and silhouettes and bingo!

The only difference is that maps can appear both as designs and as accidental formations. They pull us towards and away, tempting us to geography and to association. Or perhaps we are strongly bound to found likenesses. These two islands propose wild and opposing inventions. One swims in the sea. One floats in the sky.

Follow further lines of fiction. There are books that have images, which echo those likenesses. There is an island that is a whale. There is an island that floats. There is a cloud that is a whale. Milton's Paradise Lost shows a vision of Satan. His monstrous body is compared to the gigantic whale, so big that fishermen believe that it is a land...

"that sea-beast

Leviathan, which God of all his works

Created hugest that swim th' .........  ocean-stream.

Him, haply slumbering on the Norway oam,

The pilot of some small night-foundered ......... skiff,

Deeming some island, oft, as seamen tell,

With fixed anchor in his scaly rind,

Moors by his side under the lee, while night

Invests the sea, and wished morn delays."

An island is a whale. And Milton's comparison between Satan and whale is also inspired by medieval images. The whale's mouth is often pictured as the mouth of hell.

Or take the political fantasy in Swift's Gulliver's Travels. The Kingdom of Laputa is "The Floating or Flying Island". Now, it is not itself a floating cloud. On the contrary, its description and its map show it as exactly circular. It hovers in the air though a magnetic lodestone: "it is in the power of the monarch to raise the island about the region of the clouds and vapours..." And it is principally a war machine, designed to bring down power upon the subject cities beneath it. The Kingdom of Laputa, in fact, is an anti-cloud-island. It rains down destruction.

Or you can see these two islands, one cloud, one whale – and what connects them is in Hamlet, in the teasing-maddening dialogue between Hamlet and Polonius.

Hamlet: Do you see yonder cloud that's almost in shape of a camel? Polonius: By th' Mass, and 'tis like a camel, indeed. Hamlet: Methinks it is like a weasel. Polonius: It is backed like a weasel. Hamlet: Or like a whale. Polonius: Very like a whale.

These similarities can become a relay of echoes. Whale is island. Island is cloud. Cloud is whale. Of course, there is no authority or genealogy in these parallels. There are no intended likenesses in this map or that map. There are no links betweens maps and these later books, or among these three books themselves. They come together entirely by luck. But these early maps are treasure trove. Imagined coastlines become new lands, new worlds. Old Great Britains are discoveries. Very like an island...?