Any theory that attempts to explain the causes of glacial ages must successfully answer two basic questions: (1) What causes the onset of glacial conditions? For continental ice sheets to have formed, average temperatures must have been somewhat lower than at present and perhaps substantially lower than throughout much of geologic time. Thus, a successful theory would have to account for the cooling that finally leads to glacial conditions. (2) What caused the alternation of glacial and interglacial stages that have been documented for the Pleistocene epoch ? The first question Deals with long—term trends in temperature on a scale of millions of years, but this second question relates to much shorter-term changes.