An esker is an ancient river bed that has been formed inside or on top of a glacier. Eskers vary in length from a fraction of a mile to scores of miles, and in height from a few feet up to several hundred and more feet. More than a thousand eskers have been found in Michigan. Holt is home to the most famous esker in Michigan, the Mason Esker. An ancient glacial esker, the Mason Esker is the longest in Michigan at roughly twenty miles, and can be traced from south Lansing through Holt to just beyond Mason, Michigan.