The strata were laid down in several cycles of glaciations and warmer interglacial interludes. Low sea levels, interglacial and glacial periods facilitated these migrations. Consequently, any climatic perturbations produce slow responses, occurring over glacial and interglacial periods. These consisting of glacial periods where conditions are colder than normal, separated by interglacial periods. The oscillation between glacial and interglacial periods is due to the Milankovitch cycles. During glacial periods they expanded, and during interglacial periods they contracted. Much of these shelves has been exposed during glacial periods and interglacial periods. The Holocene epoch is the current interglacial. The EPICA core covers eight glacial/interglacial cycles. Such a period between glacial maxima is known as an "interglacial". This has proven crucial in studies on glacial/interglacial temperature. They date to an Upper Pleistocene interglacial. It indicates a glacial (below zero) or an interglacial (above zero). This was the start of our present interglacial, the Holocene. During an icehouse, only 20% of the time is spent in interglacial, or warmer times.