<P> Ogives (or Forbes bands) are alternating wave crests and valleys that appear as dark and light bands of ice on glacier surfaces . They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals the annual movement of the glacier . Ogives are formed when ice from an icefall is severely broken up, increasing ablation surface area during summer . This creates a swale and space for snow accumulation in the winter, which in turn creates a ridge . Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives . </P> <P> Glaciers are present on every continent and approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant subantarctic island territories . Extensive glaciers are found in Antarctica, Chile, Canada, Alaska, Greenland and Iceland . Mountain glaciers are widespread, especially in the Andes, the Himalayas, the Rocky Mountains, the Caucasus, Scandinavian mountains and the Alps . Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period . In New Guinea, small, rapidly diminishing, glaciers are located on its highest summit massif of Puncak Jaya . Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya and in the Rwenzori Mountains . Oceanic islands with glaciers include Iceland, several of the islands off the coast of Norway including Svalbard and Jan Mayen to the far North, New Zealand and the subantarctic islands of Marion, Heard, Grande Terre (Kerguelen) and Bouvet . During glacial periods of the Quaternary, Taiwan, Hawaii on Mauna Kea and Tenerife also had large alpine glaciers, while the Faroe and Crozet Islands were completely glaciated . </P> <P> The permanent snow cover necessary for glacier formation is affected by factors such as the degree of slope on the land, amount of snowfall and the winds . Glaciers can be found in all latitudes except from 20 ° to 27 ° north and south of the equator where the presence of the descending limb of the Hadley circulation lowers precipitation so much that with high insolation snow lines reach above 6,500 m (21,330 ft). Between 19 _̊ N and 19 _̊ S, however, precipitation is higher and the mountains above 5,000 m (16,400 ft) usually have permanent snow . </P> <P> Even at high latitudes, glacier formation is not inevitable . Areas of the Arctic, such as Banks Island, and the McMurdo Dry Valleys in Antarctica are considered polar deserts where glaciers cannot form because they receive little snowfall despite the bitter cold . Cold air, unlike warm air, is unable to transport much water vapor . Even during glacial periods of the Quaternary, Manchuria, lowland Siberia, and central and northern Alaska, though extraordinarily cold, had such light snowfall that glaciers could not form . </P>

How does a glacier act on the land in a valley