Sunday, May 9, 2010
The photo above may not look like much, but it took hundreds of years to create. It is a pothole in the bedrock of Mather Gorge, which I saw on my recent trip to the Great Falls of the Potomac.
The red lines I drew show the general shape of the "pothole". The blue chicken scratch was my attempt to show water swirling inside, carving out the pothole. About 35,000 years ago, during the Wisconsin Glaciation, huge amounts of melt water from the glaciers poured through the Mather Gorge. The spot where I was standing to take this picture was under water.
A turbulent vortex of water, created by some obstacle such as a rock, started spinning above the future location of this pothole. The swiftly flowing water carried sand and stones, which acted like sandpaper against the bedrock. Over hundreds of years, a circular pothole was formed.
In the thousands of years since, weather and water have caused the large crack at the back of the pothole, and completely ripped off the front half. What you see is half a pothole.
The above photo is the largest pothole I came across. It was about 5 or 6 feet deep. There are many potholes along both banks and on the islands in the river. Here is a top view of a small one, only about a foot across. It became filled with silt over the years.
Where the rock is softer, it gets eroded much quicker, and can form very large potholes. The pothole below is in Moab, Utah. The photo is from the Utah Geological Survey.
As you can see, if the pothole has not cracked or been filled with silt like the ones I saw, it will hold water. This pothole in Utah is in a dry area, and the water at the bottom is rain.
Amazingly, organisms have adapted to live in these transient pools of rainwater. They include mites, nematodes, tardigrades, and many others. The conditions in the pools are harsh, with wide temperature variations including freezing of the water during the winter. Oxygen and pH levels also swing widely. The smaller pools periodically dry up completely.
Some residents of these potholes, or at least their eggs, have been known to live dormant for many years without water. Some of these organisms are so well adapted to their harsh environments that they have not changed in millions of years. Their ability to withstand these conditions makes them of great interest to scientists looking for life on other planets. Click here to read more about this in NASA's Astrobiology Magazine.
at 6:36 PM