It is the cool summers which are thought to allow snow and ice to last from year to year in high latitudes, eventually building up into massive ice sheets.
The closest approach of the earth to the sun is called perihelion, and it now occurs in January, making northern hemisphere winters slightly milder.
This change in timing of perihelion is known as the precession of the equinoxes, and occurs on a period of 22,000 years.
11,000 years ago, perihelion occurred in July, making the seasons more severe than today.
The "roundness", or eccentricity, of the earth's orbit varies on cycles of 100,000 and 400,000 years, and this affects how important the timing of perihelion is to the strength of the seasons.
The combination of the 41,000 year tilt cycle and the 22,000 year precession cycles, plus the smaller eccentricity signal, affect the relative severity of summer and winter, and are thought to control the growth and retreat of ice sheets.
Cool summers in the northern hemisphere, where most of the earth's land mass is located, appear to allow snow and ice to persist to the next winter, allowing the development of large ice sheets over hundreds to thousands of years.
Conversely, warmer summers shrink ice sheets by melting more ice than the amount accumulating during the winter.
Anatomy of a Glacier (glacial zones)