Sometimes an extraordinary landscape seems to defy explanation. Large parts of western Utah in the USA have bizarre landforms including vast fields of boulders, strange swirling gorges, coves surrounded by steep cliffs and pronounced formations that resemble beaches and shorelines yet are hundreds of miles from the sea.
There are great linear scars where the soil has been scoured away and it is impossible to cultivate crops.
Scientists, particularly geologist GK Gilbert in the 1870s, have had to use a lot of detective work to understand what happened to create these extraordinary landscape features. And the culprit is a single catastrophic event towards the end of the Pleistocene period, deep in prehistory some 17,400 years ago. It is known as the Bonneville Flood.
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This was a sudden draining of the great Lake Bonneville that extended over much of modern Utah and elements of neighbouring Nevada and Idaho that within a few short weeks reworked the landscape to dramatic and contorted effect.
Lake Bonneville formed about 30,000 years ago as water was slowly captured in the Great Basin depression of western North America. The water came largely from rain and melting glaciers though this was accelerated when the Bear River was diverted by volcanic activity away from the Snake River and directly into the Basin.
Lake Bonneville grew and filled the Basin and the inland ridges that can be seen today are evidence of periods of stability when wind-driven waves created 'beaches' on the lake's shores. At its greatest extent, it reached some 52000 square km, and nearly 350m deep. As Lake Bonneville reached grew to its natural high point, water began to seep out at the weakest point. Then, suddenly, it broke through a fault in the lake's shore at what is now Red Rock Pass.
Quickly the fault became a huge hole and the water gushed out at what has been estimated at 1000000 cubic metres a second, some five times the flow rate of the modern Amazon.
Initially the lake water surged into the existing Snake River but there was too great a volume and pressure and a second, wider watercourse formed to the north, known as the Eden Channel.
The water roaring along the more constricted path of the Snake River carved a canyon through pure erosive power, and the eddies and back currents hollowed out the coves and strange circular cliff formations that baffle visitors today. Many of these were eroded down below the water table and now contain pools of pure spring water.
Meanwhile, the Eden Channel conveyed a wider sweep of immensely powerful water racing at 30-40 miles an hour. Carrying basalt boulders and sediment, it stripped the land back to the bedrock, from which it has never recovered. Satellite photos show the course of the Eden Channel even today, essentially bare rock where almost nothing grows. These are known as the scablands.
As the force of the Flood slowed, silt and boulders were deposited in great, bizarre fields of rocks across the Snake River Plain, particularly at Massacre Rocks State Park. Many of these are as big as a car and some even the size of a small house though most are smaller. In many areas, the boulders are naturally aligned to show the immense flow of the water that carried them there and have been smoothed by their violent journey through the Flood.
The boulder fields are known as 'melon gravels' – and the name is attributed to a sign put up in the 1950s that advertised “Petrified Watermelons – take one home to your mother-in-law!” It seems the power of advertising is almost as strong as the flow of the prehistoric flood!
Native American people have long viewed the melon gravels as sacred and there are many beautiful and extraordinary drawings, called petroglyphs, etched in the stones. These feature sun shapes, abstract designs and geometric shapes.
After a few weeks or months, Lake Bonneville was mostly drained by the flood, although remnants of it can be found today in Utah's Great Salt Lake, Utah Lake and the Bonneville Salt Flats. The Great Salt Lake is so named because, unlike Bonneville, it has no outlet and water only leaves through evaporation, concentrating the minerals and salts within the water body. And it is little more than a 13th of its prehistoric ancestor's size.
