Making sense of the math of climate change
I was able to dig up this (although without being able to find full sources):



It shows a pattern where the temperatures are dramatically lower where ice sheets have built up relative to today. Interestingly, a lower overall temperature means weaker mixing across latitudes, so some areas are actually warmer.  Boston gets chilled by -20 according to the map.

Also note the altitude effect. If the surface is raised by 1 kilometer, then due to the lapse rate, the temperature at the new surface will drop by 6.5 degrees. This is a very important factor in stabilizing large ice sheets.