Of course, such a scene is imaginary, but it could have taken place about 10,000 years ago, almost anywhere among the valley and canyon bottoms of what is now known as the Four Corners country of the Southwest. The great continental ice sheets never got this far south, and 10,000 years ago they were already retreating northward from their farthest expansion. Yet smaller glaciers in some of the surrounding mountains were also shrinking, and the general climate of the area must have been far different than it is today. No doubt it was colder and damper, with more rain and many swamps, lagoons, and lakes abounding in game animals and birds of all varieties. With a colder climate, the scenery too did not resemble that of today. High grasslands, extensive hardwood forests, and full-flowing streams and rivers characterized the region. The general land formations, however—the mountains, canyons, mesas, and plateaus—had been formed long millenia before, and as short a time as 10,000 years ago they would have been very much as they are today. Geologists believe that after the last of the four great continental glaciations (i.e. the Wisconsin) there were three broad climatic periods over most of the western United States. These are called the Anathermal, which was cool and moist, becoming gradually warmer; the Altithermal, which was exceptionally dry; and the Medithermal, a relatively cool, moist period which is still in progress. Our imaginary tale about the hunters, if it had taken place, would have occurred during the Anathermal period. These variations in climate have been determined by studying old stream terraces (streams cut deeper when there is more water, and they can carry a greater load of abrasive sands and gravels); old beach levels around ancient lakes (such as are prominent today around the Great Salt Lake in Utah); and ancient annual lake deposits of fine silts which form thin bands, or varves. Studies of animal life, both vertebrate and invertebrate, and plant life are important, for some life forms can exist only under certain limited climatic conditions. Bones and shells found in various deposits may help to indicate the type of climate existing when they were laid down. Plant fragments, pollen grains, and diatoms (microscopic plants with siliceous skeletons), are clues in telling the story of prehistoric times.