A reader, in response to WC’s point that High Island is the result of a subterranean salt dome, asked what a “salt dome” was. It proved to be an irresistible opportunity for WC to geek out on geology.
Imagine a salt pan, an area where salt is a major constituent of surface deposits. The Great Salt Lake and its surrounding salt flats would be a good example. A desert ocean shore where the ocean advances and recedes over millennia might be another. Now imagine that erosion of the surrounding mountains buries that salt pan under thick layers of sediment. When that overburden is thick enough it starts to lithify – turn to rock – like sandstone or mudstone. With more and more pressure from those overtopping layers of rock, something strange start to happen to that buried salt: it starts to behave as a plastic.
If later stresses in the layers of rock immediately above the salt layer cause that overlying rock to fracture, the pressure will cause that salt to ooze up through the cracks, to widen the cracks and to be squeezed up like toothpaste from a tube. As the salt dome rises, it pushes up on the overlying layers, lifting them up. The amount of uplift will depend on the progress of the salt channel and the depth and weight of both the layers above the original salt strata and the weight of the remaining layers above the rising bulb of the salt bubble.
Salt domes can force their way out onto the surface and create a salt glacier. Or, as in Texas, the weight of the overlying rock layers hasn’t been quite enough to push the rising salt to the surface. In many places around the world, the salt isn’t exposed by breaking through the surface, but rather by erosion of the overlying rock.
Salt domes were very important to early oil exploration. After the discovery of crude oil at Texas’s Spindletop Salt Dome, oil wells were drilled into and around salt domes across the basin. The salt itself is largely impervious to oil and gas, and many of the mudstones are as well. So the rising salt domes create “oil traps” around their margins, and less often at the top of the salt strata. Today, there are dozens of abandoned, rusting oil pumps surrounding High Island.1 A very few are still pumping crude oil.
The surface expression of the High Island’s salt dome today is a nearly circular rise of about 38 vertical feet above mean sea level at its highest. The dome is a little less than two miles in diameter. That’s not much of a hill, but it is the highest point on the Texas Gulf Coast between the Yucatan and Mobile, Alabama by a considerable margin. More importantly for birds and birders, the rise is just enough to allow trees to grow; everywhere else the ground water is too saline for trees. High Island is just high enough that it has a lens of fresh water. And trees.
Birds migrating from South America to North America, across the 600 mile wide Gulf of Mexico, see the High Island trees. Migrants that feed on the bugs that infest trees target the trees. So you get a remarkable, sometimes astonishing, concentration of birds. Better still, while small birds migrate at night when there are fewer predators and the birds can use stars for navigation, the flight across the Gulf takes about 18 hours, so the birds arrive, hungry, thirsty and exhausted in mid-afternoon. Perfect for birders.
All of which is why WC believes geology and birding aren’t really all that unrelated.
1 Texas is notably much better at drilling for crude oil and gas than at cleaning up the debris and detritus from earlier hydrocarbon extraction.