Are these watermelons?
Petrified watermelons, melon rocks, melon gravel – these are all names for basalt boulders scattered across the Snake River corridor. While the process of petrification can turn organic material like watermelons into stone, watermelons did not even exist when these rock monoliths were formed. The colloquial name “petrified watermelons” comes from a gas station sign near a large melon gravel field that pronounced “Petrified watermelons. Take one home to your mother in law”. Like these rocks, the hatred of mother-in-laws endures for millennia.
Where did melon gravel come from?
The catastrophic Lake Bonneville megaflood deposited these melon boulders approximately 17,400 years ago. Lake Bonneville no longer exists, but was previously the size of Lake Michigan. This giant freshwater bathtub covered much of western Utah and parts of Idaho and Nevada. Today. the Great Salt Lake in Utah remains in the footprint of its ancient predecessor.
Lake Bonneville was fed by surrounding rivers and streams, but had no outlet of its own. The cool, wet climate of the Pleistocene limited water evaporation. Over thousands of years, this combination of factor led the water levels of Lake Bonneville to rise.
One fateful day, the water reached the rim of the basin at Red Rock Pass in southern Idaho and began spilling over. Like a dam bursting, the trickle quickly became a violent torrent, decimating everything in its path.
The discharged continued for weeks, raging with the force of 400 Olympic-sized swimming pools per second. The force far exceeded the flow of any river existing today, carving through hundreds of feet of loose material in the process. The flood waters traversed southwestern Idaho until joining with the Snake River, then the Columbia River, and eventually the Pacific Ocean.
Fragments of rock dislodged in the burst began tumbling along the newly chiseled river floor. The the tumbling shaped the pieces into rounded boulders. As the force of the flow subsided and the larger boulders settled in place, continued pelting by smaller rocks and sand helped to shape and polish them further.
What do melon boulders look like now?
Melon boulders today still tell the story of their ancient past. Sharply split, jagged specimens rest nestled amongst their smooth and rounded counter parts. These jagged, angular rocks may have occurred during the flood as the result of violent collisions with other rocks, or later as part of natural freeze-thaw cycles. In these cycles, water can enter small cracks in the rock surface, which then expands upon freezing. Repeated cycles of this process turn small fissures into massive cracks, which can eventually cleave the boulders completely.
Keen eyed observers may also note petroglyphs of animals, humans, and other symbols caved into the melon gravel. In this regions, petroglyphs exclusively decorate the melon boulders and not the canyon walls. It is speculated this may be because the melon boulders were viewed as sacred objects by ancient peoples. These are believed to be from the first humans entering Idaho between 11,000 – 14,000 years ago.
Melon boulders display a variety of colors, thanks to a combination of desert varnish, lichens, and other weathering effects. Desert varnish is a thin layer of clay oxides ranging from yellow-orange to black that slowly accumulates on the rocks over thousands of years. Colorful lichens are well adapted to the arid desert environment, able to withstand long periods of drought. Lichens are a type of epiphyte (like air plants) that do not damage the structure they attach to. These epiphytes are able to derive all their necessary nutrients from dust particles and water molecules in the air.
Where can I find melon boulders?
Melon boulders are distributed across the Snake River plain. The rate of boulder deposition depended on the speed of the flood at any given time. The waters rushed at 70 miles per hour through narrow canyons, but slowed considerably when they encounter broader plateaus. You’ll find the largest boulders in regions where narrow canyons dramatically widen. Numerous examples of these curious geological remnants are visible at Celebration Park, about an hour south of Boise, Idaho. Visitors can meander on their own hike through the boulders and search for petroglyphs.
Learn more
The first chapter of Geology Underfoot in Southern Idaho by Shawn Willsey has a more detailed explanation of the flood and where in Celebration Park you can see specific geological remnants.
Awesome unique content. Always love learning new things, and I can’t wait to read more! 😁