Sometimes, when trying to describe the particular travails of Texas horticulture, it’s necessary to go to great efforts to explain why basic techniques that produce great results elsewhere are nearly worthless here. At other times, those demonstrations literally pop out of the ground. In my case, I nearly ran into one while bicycling home from the Day Job one afternoon this week.
Travelers to Alberta might be familiar with the famed hoodoos around Drumheller. Those rock formations are completely natural, having formed when weather-resistant caprocks protect the softer stone beneath them from erosion. Dallas isn’t amenable to real hoodoos, what with our horrendous storms and thick clay soils, so all of the boulders one sees on the sides of strip mall driveways and in suburban front yards came from elsewhere. Most of the ones seen around here today are legacies for the SUV craze of the late Nineties, where the only way to get inattentive or incompetent drivers to make their turns in the correct places was to delineate the driveway with rocks slightly smaller than the vehicles in question. The piles here were a lot smaller, and composed of local chalk, but they also had a very human origin.
Based on the filled-in hole in the center among the four piles, someone needed to do some digging for practical reasons, such as to fix a fiber optic cable or possibly a water line. Judging by the piles themselves, whatever went into the hole had as much volume as the displaced rock, and it apparently made more sense to leave the piles behind than to haul them away. It certainly offered more amusement to the drivers stopping at the intersection by the piles to gawk at them. We Texans are a very easily entertained people.
The whys and wherefores of the hoodoos make sense, and now it’s time to look at what else they can teach. When you hear North Texas gardeners complain about “black gumbo,” they’re talking about the Blackland Prairie soil that covers most of the area. Most experts agree that this soil is potentially some of the most fertile on the planet, or would be as soon as worms, grubs, and the occasional armadillo mixes it up with lots and lots of organic matter. Right now, it’s thick and sticky when wet, and its high water content means that it shrinks and produces gigantic cracks when dry. During full-scale drought, that shrinkage means that the subsidence can damage house foundations, break water mains, and threaten life and limb. I mean that literally: during the drought of 2011, I nearly broke my ankle while mowing the front yard when I stepped in a monstrous crack in the yard that wasn’t there a few days before. Given a few more weeks of severe drought, that crack might have caught small animals: one here in Garland nearly ate a puppy. Further north and west, at the edges of the Woodbine Formation, the clay changes color from black to deep red and orange, and goes down 10 to 20 feet, interspersed with flat boulders of blue quartzite strong enough to break the bucket on a steam shovel. (Yes, I witnessed this firsthand when I was a kid.)
Here, though, the Black Prairie soil is relatively shallow, and it’s usually frosting on the thick and deep sponge cake of the Austin Chalk. Back about the time dinosaurs were stomping around near Glen Rose, the Dallas area was underneath the warm and shallow waters of the North American Seaway, and it would have been possible to paddle or raft your way from the Gulf of Mexico to Hudson Bay. Not that you’d want to, considering the number of aggressive and very hungry sea-going reptiles that considered that waterway home.
Now, for palaeontology buffs, the Austin Chalk is a treasure. It’s a little coarser in grain than the fine sediments of the Eagle Ford Shale: unlike the Eagle Ford, you generally won’t find ammonite shells in the Austin Chalk. However, shells are occasionally found, along with a lot of trace fossils from digging organisms. If you have patience, a couple of tons of spare chalk, and a lot of dilute acid, you can treat the rock with acid and find teeth from the many species of shark that also called the North American Seaway their home. You won’t find anything on a par with Carcharocles megalodon teeth, but the Seaway had a lot of specialist sharks and rays that scattered discarded and worn teeth all through the Austin Chalk sediments.
Fast-forward 90 million years or so, and contemplate the chalk formed by sediment pressure times time. People who don’t live here always ask why houses in Dallas generally don’t have basements, and the chalk is a big reason why. It’s why we don’t have subways as well: it’s structurally weak, so tunnels made in it have to be shored up to the point where it isn’t economically viable to continue. (Not that this stopped the Dallas Area Rapid Transit system from putting in a train tunnel on the edge of Highland Park in the mid-Nineties, but that was due less to improved digging techniques and more with politics.) Between the shifting clay on top and the porous chalk below, any building foundation either has to be dug deep into the chalk or put in as a slab that “floats” atop the clay, and basements on residences are generally the worst of both worlds.
The same geologic factors that affect construction also affect plant growth. Take a look at our indigenous trees, and they’re also shaped by the battle between clay and chalk. Either they produce shallow but wide roots through the clay, such as the various oaks in the area, or they produce extremely long and strong roots that punch through cracks in the chalk, such as with mesquite. Everything else does its best to hang on in whatever clay it can find, thus helping to explain our fleeting but tremendous displays of spring wildflowers. In areas such as this one, which might have a centimeter of soil or less atop chalk, even lupines like the Texas bluebonnet can’t get established. Those areas stand out by their dearth of ground cover, as only a mist of grass can survive there in spring and fall, and the summer months are notable for the feel of barely-concealed rock underneath your boots.
As for these hoodoos, they’re sadly not long for this world, one way or another. If some human effort doesn’t knock them down or otherwise disperse them, the combination of gullywasher storms in spring and brutal sun in summer will crumble them all in about a year. Five years from now, their only trace will be random piles of chalk chips, along with whatever bits of fossil shell and tooth may have been concealed within. And so it goes.