Such continental ice sheets advanced at least four times as climates cooled, and as many times they retreated during warmer intervals. With each advance and the consequent cooler, wetter climate, there was undoubtedly a southward shift of vegetation belts. In the mountains there would also have been a downward shift of forest types, particularly those of the higher elevations. That is, the higher elevation species would begin to grow down the slope. In sheltered coves temperatures probably did not drop as much as they did higher up or out in the open lowlands, and soils in coves were deeper and more fertile. The coves of the Southern Appalachians thus may have formed a refuge for many temperate species of plants, including some forced southward by the spreading ice. This is a factor in today’s biotic richness or abundance in the Smokies. On top of the Smokies and other high mountains of the Southern Appalachians, tundra (treeless areas) may have developed as winter climates became too cold and windy even for spruce and fir, which is the situation today on high peaks of the Adirondacks and White Mountains in New England. Accumulations of blocky boulders in higher parts of the Smokies resemble block fields in the northern Appalachians that probably were formed above timberline in late stages of glaciation. From the location of block fields, geologists postulate a treeline in the Smokies somewhere between 900 and 1,500 meters (3,000 and 5,000 feet) elevation during the last glacial period, some 15,000 to 25,000 years ago. If islands of tundra did exist in the Southern Appalachians, it is not likely that tundra mammals would have migrated from the tundra bordering the ice front through the intervening forest to reach such Arctic pastures in the sky. But some birds might have. Water pipits, which today nest in the Arctic and above timberline on our Western mountains, might have bred on these patches of southern tundra. And the few snow buntings which have been seen wintering on Southern Appalachian balds may have been returning to ancestral nesting grounds of the species. Although Pleistocene tundra in the Smokies is a rather speculative notion, it seems certain that spruce-fir forest existed below today’s 1,400-meter (4,600-foot) limit. This supposition is supported by the fact that fossil pollen and other fragments of spruce and fir have been found in several lowland bog deposits of the South.