Bridge of The Gods (land Bridge) - Geologic History

Geologic History

Interpretations of the age of the Bonneville landslide have evolved as more investigators have studied it and as more modern dating techniques have become available. Early work based on dendrochronology and radiocarbon dating suggested the landslide occurred between 1060 and 1180 or between 1250 and 1280. The year 1100 A.D. has often been cited as the date of the Bonneville landslide. More recent work using radiocarbon dating and lichenometry has suggested dates between 1500 and 1760 or between 1670 and 1760. These younger radiocarbon ages permitted a possible a link to the 1700 Cascadia earthquake. However, more recent investigations using radiocarbon dating and dendrochronology show the landslide occurred around 1450, more than two centuries before the great earthquake

The Bonneville landslide sent a large amount of debris south from Table Mountain and Greenleaf Peak, covering more than 5.5 square miles (14 km2). The debris slid into the Columbia Gorge close to modern-day Cascade Locks, Oregon, blocking the Columbia River with a natural dam approximately 200 feet (61 m) high and 3.5 miles (5.6 km) long. The impounded river formed a lake and drowned a forest of trees for about 35 miles (56 km). Native Americans may have been able to cross the river on the dam or, as their oral histories say, a bridge. Although no one knows how long it took, the Columbia River eventually broke through the dam and washed away most of the debris, forming the Cascade Rapids. Geologists have determined that debris from several distinct landslides in the same area overlap, forming what is called the Cascades landslide complex. The Bonneville landslide was the most recent, and perhaps the largest landslide of the complex. Understanding the nature of the landslide complex and mapping the lobes of individual landslide events was done during the 1960s and 1970s. The composite nature of the landslide complex may explain the early discrepancies between date estimates of the Bonneville Landslide.