Both the northern and the southern hemispheres have had their heavy, slow-going floods of ice that appear to have swept from the polar world far toward the equator. During the great glacial period, which may have lasted for ages, a mountainous flood of ice overspread America from the north and extended far down the Mississippi Valley. This ice may have been a mile or more in depth. It utterly changed the topography and made a new earth. Lakes were filled and new ones made. New landscapes were formed: mountains were rubbed down to plains, morainal hills were built upon plains, and streams were moved bodily. It is probable that during the last ice age the location and course of both the Ohio and the Missouri Rivers were changed. Originally the Missouri flowed east and north, probably emptying into a lake that had possession of the Lake Superior territory. The Ice King deliberately shoved this river hundreds of miles toward the south. The Ohio probably had a similar experience. These rivers appear to mark the "Farthest South" of the ice; their position probably was determined by the ice. Had a line been traced on the map along the ragged edge and front of the glacier at its maximum extension, this line would almost answer for the present position of the Missouri and Ohio Rivers. The most suggestive and revealing words concerning glaciers that I have ever read are these of John Muir in "The Mountains of California": "When we bear in mind that all the Sierra forests are young, growing upon moraine soil recently deposited, and that the flank of the range itself, with all its landscapes, is new-born, recently sculptured, and brought to light of day from beneath the ice mantle of the glacial winter, then a thousand lawless mysteries disappear and broad harmonies take their places." "A glacier," says Judge Junius Henderson, in the best definition that I have heard, "is a body of ice originating in an area where the annual accumulation of snow exceeds the dissipation, and moving downward and outward to an area where dissipation exceeds accumulation." [Illustration: THE HALLETT GLACIER] A glacier may move forward only a few feet in a year or it may move several feet in a day. It may be only a few hundred feet in length, or, as during the Ice Age, have an area of thousands of square miles. The Arapahoe Glacier moves slowly, as do all small glaciers and some large ones. One year's measured movement was 27.7 feet near the centre and 11.15 near the edge. This, too, is about the average for one year, and also an approximate movement for most small mountain glaciers. The centre of the glacier, meeting less resistance than the edges, commonly flows much more rapidly. The enormous Alaskan glaciers have a much more rapid flow, many moving forward five or more feet a day.