Twenty thousand years ago Earth was in the grip of an Ice Age (technically we are still in an Ice Age, in one of the interglacial warming periods). Massive sheets of ice covered much of North America, northern Europe, and parts of north Asia and locked up so much water that the contours of the continents were different because the sea levels were lower. As temperatures began to rise, the ice began to melt and the oceans to rise.
Back in the day, the island we call Britain was not an island but part of Europe. The English Channel did not exist, the Rhine River flowed a lot further south before it reached the Atlantic Ocean, and people lived and probably often thrived on what was then an expanse of land that now lies beneath the North Sea.
Coincidentally, the December 2012 National Geographic includes an article about this region, called Doggerland after the Dogger Banks, a shallow area in the North Sea well known to fishermen.
There is a fabulous map at the NatGeo site which I can’t post here, but you totally should go there and look at it (scroll up, for some reason the link deposits you at the end of the page). The graphic clearly displays how the expanse of land changes across time as the ice shrinks and the oceans rise. The shoreline in Spiritwalker falls somewhat close to what is shown here as the year 8000 B.C.E (Before the Common Era), although of course this mapping is educated guesswork.
When I “built” the landscape of Spiritwalker, I wanted enough ice that Britain would be attached to the continent but not so much that most of Europe would be too cold for extensive human habitation.
Europe’s Lost World: The rediscovery of Doggerland by V Gaffney, S Fitch and D Smith (CBA Research Report 160, 2009) provided a great deal of information by some of the scientists at the forefront of this research.
The book also provided a crucial set of figures depicting “Isopollen maps showing changes in vegetation over the postglacial” (in Europe). I needed to know how a late glacial landscape would differ from today’s European landscape in terms of climate zones and vegetation, not just shorelines. For instance, how far north could people farm? Would there be other geographical variations in vegetation zones? What could farmers grow? Some grains can grow in harsher conditions with shorter growing seasons; others need warmer, longer seasons. I never go into detail about issues like this although they are alluded to, and specifically if briefly mentioned in Cold Steel.
The city of Adurnam is actually in what is now the English Channel, south of Portsmouth, on the old paleolithic watercourse of the Solent River, more or less (despite being named after Portus Adurni, the Roman fort at what is now Portchester, a suburb of Portsmouth). The land controlled by Four Moons House lies east of London and Canturbury, in what is now the southern part of the North Sea but which in Spiritwalker is all land. For these landscapes I consulted references like the Journal of Quaternary Science, which has an entire journal volume dedicated to the Quaternary history of the English Channel.
I also wanted to know how melting would occur, how quickly vegetation could “migrate” north, and by what “mechanism” the trolls (the feathered people, that is, the intelligent descendents of troodons) might have survived into the “present day” of the novel while at the same time allowing for human migration into the Americas.
The land bridge between Asia and the Americas was generally ice free during the Ice Age due to climate variables. Called Berengia, this land bridge had a significant population of mammoths and other now extinct mammals. Meanwhile, however, the massive North American ice sheet for a long time cut off Berengia from the ice free southern part of the North American continent.
E.C. Pielou’s excellent After the Ice Age: The Return of Life to Glaciated North America (The University of Chicago Press, 1991) is a superb resource for a fantasy writer. It taught me about Berengia’s ice free corridor, conditions in newly deglaciated landscapes, and how plants return to those zones, which they can do remarkably quickly under the right circumstances.
It also taught me about refugia, which are ice free areas, large ones like Berengia and then also small ones: nunataks are ice free zones at high elevations like mountaintops and coastal refugia are small ice free sections of “coastal plain in the lee of high mountains” (Pielou). Some plant and animal species survived in refugia, surrounded by ice and thus cut off from other populations for long periods.
Refugia and nunataks gave me a rationale for the survival of my intelligent descendents of troodons. Also, the existence of coastal refugia made it possible to suggest that humans, after crossing ice-free Berengia either on foot or by boat along the coast, had coast hopped down a string of coastal refugia to the ice-free lower portion of North America (as they may have done in our world). Meanwhile, the ice would have kept the two populations, the small but expanding feathered people population in the north and east and the small but expanding human population in the west and south, from meeting until rather late in this prehistory at which point contact between outlying groups would have brought caution, conflict, cooperation, trade, and eventually yet more complex interaction.
It was easy to find information on Europe and North America–the above referenced books and articles are not the only resources I used–and far more difficult to find information on how the Ice Age affected, for instance, the climate of the Caribbean, something I needed to know for Cold Fire. I did what I could with maps of the sea floor in the Caribbean to consider how the ocean currents would work since the islands of the Caribbean Basin are larger in this alternate landscape, I posited that the hurricane season would be shorter due to water temperature changes, and I winged it a bit.
I dredged around to find what I could about world regional climate variation–for instance, although my map of the Eastern Hemisphere doesn’t extend that far south, I posit that Lake Chad in the West and Central Sahel is huge because of the climate making west and central Africa wetter–but mostly I focused on areas I knew I was going to visit in the story.