The summit of Nevado del Ruiz is covered by glaciers (nevado means snow-covered in Spanish), which formed over many thousands of years, and have generally retreated since the last glacial maximum. From 28,000 to 21,000 years ago, glaciers occupied about of the Ruiz-Tolima massif. As late as 12,000 years ago, when the ice sheets from the last glacial period were retreating, they still covered . During the Little Ice Age, which lasted from about 1600 to 1900 CE, the ice cap covered approximately .