Major ice ages

There have been at least four main ice ages in the Earth's past.

The most basic hypothesized ice age is believed to have occurred around 2.7 to 2.3 billion years ago during the early Proterozoic Age.

Main article: Snowball Earth.
The most basic well-documented ice age, and probably the most severe of the last 1 billion years, occurred from 800 to 600 million years ago and it has been recommended that it produced a Snowball Earth in which permanent sea ice extended to or very near the equator. It has been recommended that the end of this ice age was responsible for the subsequent Cambrian Explosion, though this theory is current and contentious.
A negligible ice age occurred from 460 to 430 million years ago, through the Late Ordovician Period.

Sediment records viewing the changeable sequences of glacials and interglacials during the last several million years. The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciations with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period finished about 10,000 years ago.

The timing of ice ages throughout geologic history is in part restricted by the position of the continental plates on the surface of the Earth. Small changes in solar energy can tip the balance between summers in which the winter snow mass totally melts and summers in which the winter snow persists until the following winter. Due to the positions of Greenland, Antarctica, and the northern portions of Europe, Asia, and North America in Polar Regions, the Earth today is considered prone to ice age glaciations.

Proof for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the statement of till or tallies and glacial erratic. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. It took some time for the current theory to be worked out. Analyses of ice cores and ocean sediment cores unambiguously show the record of glacial and interglacial over the past few million years.