High country Have you ever wondered how the mountains we see every day were formed? Click on the graphic above to find out. Really mammoth Thousands of years ago, the Sonoran Desert had lots of large animals that scientists call megafauna. Plant-eaters included mammoths, mastodons, horses, camels, ground sloths, giant peccaries, oxen, bison, antelopes, giant beavers, tapirs and giant armadillos. There were also carnivorous animals that often made meals out of some of those plant-eaters. These included giant bears, dire wolves, two types of saber-tooth cats, cheetahs and American lions, which looked a lot like today's African lions. They disappeared about the same time that humans showed up in North America. Some scientists say they were hunted to extinction. Others think the megafauna disappeared because the climate became hotter and drier, or because diseases spread, or all of the above.

'Book' of Sonoran history begins 1.7 billion years ago

Geology is the study of the Earth and its history. Geologists study everything from the rocks of the Earth and how they got here to all kinds of fossils, the remains or traces of ancient forms of life.

Think of the geology of the Sonoran Desert as a book with four chapters.

* Chapter One - Precambrian Era: From about 1.7 billion to 550 million years ago, a huge mountain range formed in this region. Then it eroded little by little all the way back down to sea level. These eroded rocks like schist and granite form the foundation for most of the Sonoran Desert.

* Chapter Two - Paleozoic Era: From about 550 million to 250 million years ago, this area was dry land sometimes and under water the rest of the time. The seas came and went many times, and the sediments left behind dried and hardened into rocks like limestone, sandstone and shale.

* Chapter Three - Mesozoic Era: From about 250 million to 65 million years ago, pressure beneath the surface of the Earth caused molten rock, or magma, to shoot up as volcanos and crystallize as big blobs called plutons that eventually floated up. These processes formed what geologists call igneous rock. Volcanism formed rocks like basalt, while plutonism formed large bodies of granite.

* Chapter Four - Cenozoic Era: Somewhere around the middle of this era, which stretched from about 65 million years ago to the present, stress and pressure got molten rock under the surface stirred up again. This time, it pulled and pushed the surface into basins and ranges running north and south.

The book ended with land that has basins about 2,500 feet above sea level and mountains, or sky islands, of almost 10,000 feet.

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