The last glacial maximum (LGM) in Europe is the time period when the ice sheets descended to their furthest extent southwards. The LGM occured between approx. 22,000 and 17,000 BP. Places like Scotland and Denmark were buried under the glaciers and regions like northern France were turned into tundras. The humans who lived in Europe during this time were hunter-gatherers who normally liked to be spread out in small groups across the landscape, but the changing environment of the LGM forced them all to move southwards and settle in sheltered river valleys in places like southern France and northern Spain. This led to much higher population densities than these people would have been used to, and also happens to coincide with an explosion in the number of rock art sites being decorated. Researchers have speculated that these two things are related (population density and increase in art production) and while varying theories have been proposed, no one knows exactly what the relationship is. What reasons can you think of for this happening?