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Chapala Basin Geologic History

For millions of years the area stretching from around Puerta Vallarta through Guadalajara, Mexico City, Puebla and eastward to the Gulf Coast around Veracruz has been a band of active volcanoes. North of that region is on a tectonic plate moving eastward, while to the south is a plate moving to the west. The land has built up to a high plateau, with even higher volcanic mountain ranges dominating the landscape.

The fertile valleys between these mountain ranges were occupied by a rich diversity of prehistoric fauna, including mammoths, mastodons, horses and camels. The major river systems, cutting through deep layers of volcanic debris, were much as they are today, with the Santiago-Verde and their tributary systems providing the main drainage for southern Jalisco.

The remains of at least two mastodons have been found in the lakebed near Chapala, but it is not clear if these fossils pre-date the existence of the lake, or are from the very end of the mega-fauna period. Likewise, I have seen suggestions that they were 'driven' into the marshy edge of the lake by human hunters, but nowhere have I seen mention of any artifacts having been found with the skeletons.

About 40,000 years ago a series of volcanic eruptions conspired to block all of the large drainage systems, and gradually a large lake formed. Modern Lake Chapala is just a remnant of this great lake, called Lake Jalisco, which stretched from Tuxpan in the south to Ciudad Aguascalientes in the north. Encompassing some 8500 square miles (a little larger than modern Lake Ontario ... compared to about 825 square miles for Lake Chapala), the giant lake averaged more than 800 feet deep. Modern day Guadalajara would have been under almost 700 feet of water.

The climate in this area was much cooler and wetter than today, as the far north was locked in a series of ice ages that covered most of Canada and many of the northern U.S. states with glaciers. The mega-fauna (giant animals like mastodons) were probably not as numerous on the steep and rocky slopes surrounding Lake Jalisco as they had been in the preceding valley systems.

Eventually, the Santiago River found its way through the volcanic plug that blocked its path, carving the amazing Barranca de Oblatos gorge in the process. This great canyon is more than 2000 feet deep, and was cut over the relative short period (geologically speaking) of about 5000 years. Lake Chapala is the largest of several remnants left when the great lake drained out to the sea.