The black mat formed on top of the layer of proxies,' Allen replies. 'Down here it has a lot of charcoal in it. But it also has algal remains so it's not just fire. The Younger Dryas changed the climate and made the area much wetter. Algae began to grow along the edges of the lakes.' He puts a hand on the black stripe along the arroyo wall: 'So the remains of about 1,000 years of dead algae, charcoal, and a lot of other stuff are all embedded in here, and at the bottom of it, where the impact happened, we find iridium, platinum, and a layer of melted spherules where the temperatures must have been so high that they would have melted a modern car into a pool of metal.