Dating archaeological artifacts
About 13,000 years ago climate apparently cooled again, and spruce became more abundant and black ash less common.
During this time birch (Betula) and alder (Alnus) were also important components of the vegetation.[At that time, the summers here were one month shorter than today and rivers had dropped to their near present levels.
A more recent theory propose prehistoric Solutreans of Ice Age France also sailed west to America across the Atlantic Ocean along the south ridge of the polar ice cap more than 18,000 years ago.
It is thought they brought Clovis point technology (earlier, similar points were found in France) and genetic diversity (such as red hair and large noses) to Native Americans.
What happens next is like something out of a Dooms-Day sci-fi movie: The exploding comet creates a giant white-hot tornado and sets forests ablaze killing off just about everything and everybody in North America.By the end of this Mini-Ice Age 11,700 years ago, the climate warmed. After the floods, gravity continued its work on the riverstones, again using water as a tool, albeit in less dramatic fashion: Since glaciers follow the course of rivers, the exposed rocks were washed down and further polished by flowing water.After it all settled, the ice and floods had cobbled many sections of the clear riverbeds with smooth pebbles and stones - some rare and semiprecious.The remaining vegetation would have been charred, forcing starvation upon surviving mega fauna. This comet melted a good portion the Laurentide Ice Sheet and the resultant flood waters changed the Atlantic currents.This combined with ash and soot in the atmosphere, plunged the Northern Hemisphere into a Mini-Ice Age for another 1,200 years.
Balsam fir (Abies balsamea) also was present, as was spruce, although not as abundantly as before.