Smaller lakes known as tarns occupy glacial cirques at higher elevations. Glacial erosion produced numerous granite outcroppings, tarns, cirques and knife-edged ridges. Numerous smaller lakes, known as tarns, are located in cirques formed by glacial erosion. Many glacial cirques contain tarns dammed by either till (debris) or a bedrock threshold. Alpine Gardens drops off precipitously into two prominent glacial cirques. Several small glaciers and permanent snowfields are found in the high mountain cirques. Glacier-related features like cirques, talus, alpine lakes and moraine are common. At the upper ends of Denali's glaciers are steep-walled semicircular basins called cirques. In the high mountains, the major rivers have their headwaters in alpine glacially carved cirques. Several further glaciations followed, forming the current landscape, with glacial valleys, cirques, nunataks etc. The bottom of cirques are often occupied by glacial lakes. The body of this batholith has been deeply cut by glacial cirques and canyons. They all originate from glacial cirques below the actual summit. Cols are saddle-shaped depressions in the ridge between cirques. Glaciers are now restricted to the highest cirques of the Brooks Range.