Question

What caused virtually all the Canada And some of the northern US to be once covered by thick glaciers ice Imo’s 15,000 years ago

Answer 1

According to most scientists it was several factors involved. But the main cause was b) changes in the earth's orbit which occurs every 10,000 to 15,000 years. Other contributing factors are changes in the ocean's currents and major shifts in platonic plates of the planet.