It seems from the readings that ice ages are caused by many things, including Carbon dioxide and methane concentrations, changes in the Earth's orbit around the sun (Milankovitch Cycyles), and the motion of tectonic plates resulting in the relative location and the amount of continental and oceanic crust on the Earth's surface. Most of the issues begin with the idea that the Milankovitch cycle creates a period, in the North, that causes snow to not melt in the summer time from previous seasons. This is caused from lower than needed temperatures and as this cooling happens snow accumulates and keeps continuing up until it reaches areas that are warm enough for it to melt. As the snow is unable to melt it causes Carbon dioxide levels and temperature to decrease causing a period in which the cold only exist, from not enough heat getting trapped and instead reflected by the snow and becomes huge ice sheets that cover a mast amount of the Northern continents. There are of course other influences, such as winds taking water vapor to lower latitudes and ocean currents taking warmer water lower as well, but the main ideas behind the cause of an ice age are the period listed above.
Figure 1: This shows how ice ages can occur,more specifically the Milankovitch cycle. The Earth axis tilt over time and at points when the North or south is as far away as possible there comes the point when temperatures fail to reach a critical value to melt the ice and snow.
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