Glacial periods occur when the Earth's orbit is elongate, when the Earth's axis has a low tilt, and when northern hemisphere summer occurs at a position on the orbit farther away from the sun so that it is cool.Roughly 10,000 years ago, for example, Earth woke up from a deep freeze and the massive ice sheets sitting atop what are now Canada melted. As ice mass fled, and the depressed crust rebounded, the distribution of the planets mass changed and the North Pole started to drift west.