In the Company of Plants and Rocks

Thursday, March 31, 2016

Bad Water, Sweet Water, and Greasewood

Healthy greasewood—dig here! (photo by Cory Maylett)

While traveling up the Missouri River through today’s northeast Montana, the great American explorer Meriwether Lewis came upon a shrub he didn't recognize, growing in large stands. "Hereafter I shall call it the fleshey leafed thorn” he wrote in his journal, on May 11, 1805. Lewis didn’t much like it, noting it was “extremely troublesome” and that animals avoided it (source).

“Fleshey leafed thorn” with succulent leaves, sharp-tipped twigs, and red winged fruit. It's now called greasewood (photo by Jim Morefield).

For the most part, folks agree there’s little to like about greasewood, including its eponymous habitat—wet greasy mud, where vehicles slide around before becoming firmly stuck. The scientific name, Sarcobatus vermiculatus, means fleshy bramble of small worms. Indeed, greasewood branchlets develop into stiff sharp painful spines, and the succulent leaves look like little green worms. And they’re toxic, fatal to any livestock that eat them. For greasewood grows where water is bad—salty, alkali, poison.

Many desert basins in southern Wyoming are closed—water runs in but not out, ponding at low points. Some soaks into the heavy soil, but much evaporates, leaving behind whatever chemicals were carried in—sodium, potassium, magnesium, calcium, and sometimes weird nasty things like boron and mind-altering selenium. Salty crusts encircle wetlands. When lakes dry up, brilliant white playas remain.

Amazingly, greasewood appears to thrive on these harsh sites! On basin margins it grows mixed with other salt-tolerant species, but on the chemical-rich heavy soils of the lowlands, it forms pure stands where few other plants can survive.

Greasewood flat in southern Wyoming, by Dan Lewis, The Wyoming Naturalist. Used with permission.

Alkaline and saline soils present insurmountable challenges to most plants, because their roots can’t absorb water with high concentrations of solutes (dissolved chemicals). But greasewood is a halophyte—a “salt plant.” The root cells contain high concentrations of solutes, and take up water even in these difficult situations. Greasewood stores toxic salts (oxalates) in its succulent leaves, and being deciduous, disposes of them at the end of the growing season, making the soil below especially salty.

Greasewood leaves, to 4 cm long (NPS).

From Meinzer 1927.

But greasewood can’t flourish on the paltry amount of water available at the surface. Fortunately it has another trick up its sleeve … or rather down its root. And this is the reason why greasewood has a fan club, albeit a small one.

• • •

Let’s walk down into a closed desert basin to a healthy stand of pure greasewood in the very bottom, and start digging.

First we have to get through heavy fine soil laced with small roots—there to absorb any water that might soak in. The networks can be dense. Donovan and colleagues (1996) found 140 km of roots per cubic meter under greasewood canopies!

Next, we dig through fine roots for several feet while navigating around substantial lateral roots 3 to 12 feet long. These are equipped with adventitious buds that send up shoots (clones) when a plant is damaged. Burned or cut plants can crown-sprout as well. No wonder the US Department of Agriculture warns land managers to leave greasewood stands alone:

“… treatment of the site will most likely fail or be a very poor investment of capital. … Areas of black greasewood that are burned, crowned, brush beat, or shallow plowed and/or shallow disked will often result in a much higher density of black greasewood. … Thus extreme caution should be exercised when selecting which sites have the best potential for improvement.” (“treatment” and “improvement” mean eradication; more details here)

By the time we’re six feet below the surface, we’ve left behind the fine roots, lateral roots and developed soil. But the tap root continues on. And it’s large—several inches in diameter:

“Near Moab, Utah, along a creek where the water had cut away the bank, exposing the roots, a greasewood 6 feet tall had roots down 18 feet, a taproot 3 inches in diameter down 6 feet, and abundant feeding roots, some 10 feet long, at a depth of 10 to 12 feet.” (Shantz 1940)

How far do we have to dig to find the tap root’s end? Usually at least 10 to 15 feet, often 20 or 30 feet, and sometimes more:

“Near Grandview, Idaho, H.T. Sterns observed roots of greasewood penetrating the roof of a tunnel 57 feet below the surface.” (Meinzer 1929; italics added)

Finally the tap root reaches its destination—the blessed capillary fringe! Here root hairs absorb sweet water that has seeped up from the water table. It's sucked up the tap root 10, 20, maybe even 50 feet—whatever it takes to reach the thirsty greasewood plant, standing in hot sun on an alkali mudflat.

“These plants have been called phreatophytes. The term is obtained from two Greek roots and means a ‘well plant.’” (Meinzer 1927; arrow added).

Old timers knew that a healthy stand of greasewood meant sweet water wasn't all that far away. They knew greasewood could help them site wells. But it wasn’t until the early 1900s that ecologists and hydrologists were convinced:

“Greasewood was not at first regarded as an indicator of ground water, because to a large extent it grows on land that lies some distance above the water table. Information now at hand, however, makes it practically certain that greasewood habitually sends its well-developed taproot to considerable depths … It is, thus, one of the most trustworthy of all ground-water indicators.” (Meinzer 1929; italics added)

Prickly, toxic and hardly photogenic, greasewood is helpful too—a most trustworthy groundwater indicator (photo courtesy BLM).

Sources (in addition to links in post)

Donovan, LA, and colleagues. 1996. Water Relations and leaf chemistry of Chrysothamnus nauseosus ssp. consimilis (Asteraceae) and Sarcobatus vermiculatus (Chenopodiaceae). Amer. J. Bot. 83: 1637-1646.

Groeneveld, DP. 1990. Shrub rooting and water acquisition on threatened shallow groundwater habitats in the Owens Valley, California in Proceedings: symposium on cheatgrass invasion, shrub die-off, and other aspects of shrub biology and management. Available here.

Knight, DK, and colleagues. 2014. Mountains and plains; the ecology of Wyoming landscapes, 2nd ed. Yale University Press.

Meinzer, OE. 1929. Plants as indicators of groundwater. USGS Water Supply Paper 577. Available here.

Shantz, HL, and Piemeisel, RL. 1940. Types of vegetation in Escalante Valley, Utah, as indicators of soil conditions. Tech. Bull. 713. Washington, DC: US Department of Agriculture. 46 p. Available here.

USDA NRCS Plant Guide: Black Greasewood.

Thursday, March 17, 2016

Of Woolsacks, Witches, Cheesewrings & Tors

Baa baa black sheep,

Have you any wool?

Yes sir, yes sir,

Three bags full.

One for my master,

One for the king,

One for the Geologist

Who likes such things!

Between Laramie and Cheyenne, Interstate 80 crosses the crest of the Laramie Mountains. But there are no rugged high peaks, no sparkling alpine lakes, no steep narrow canyons. Instead, the highway traverses a broad plain populated with peculiar rocks. They rise abruptly—like simple alters, or ancient castle walls, or stone creatures frozen on their way to stone temples. Travelers curious enough to stop will find they’re made of blocks: block walls, stacks of blocks, blocks scattered across the ground. The blocks themselves are distinctive. The rounded edges indicate they were born deep underground, where they were shaped by woolsack weathering. Only much later did they emerge into this world.

“granites rise in thick picturesque ridges, 50 to 100 feet high, like ruined walls, lending a peculiar as well as picturesque appearance to the landscape” wrote geologist FV Hayden of his visit to the Laramie Mountains in 1870. Photo by WH Jackson (USGS).

Why "woolsacks"? … these rocks look more like wool bales. In fact, woolsack means bale in the case of rocks—specifically the bale of wool on which King Edward III (1327-1377) commanded his Lord Chancellor to sit while in council, in recognition of the importance of the wool trade. It became known as The Woolsack. Six centuries later it’s still in use, by the Lord Speaker in the House of Lords.

Wool bales, 1900 (source).

The Woolsack, 1897 (source).

Woolsack weathering isn’t unusual, but the resulting forms are so fanciful that they grab our attention and spark our imaginations. Often they have evocative names and other-worldly explanations. We designate parks around them, for protection and public access.

The Cheesewring of Bodmin Moor, southwest England; John MacCulloch (1814).

The Cheesewring was named for its resemblance to slabs of cheese on a press. But legend says otherwise—it's a stack of rocks created in a contest between a Giant and a Saint. In spite of the great weight of the stones, the diminutive Saint built the taller stack (about 15 feet high), thereby avoiding death. The Giant was so impressed that he immediately converted to Christianity.

Vedauwoo Glen, in the Laramie Mountains, is home to Earthborn Spirits. It's managed by the US Forest Service.

The Great Staple Tor in Dartmoor National Park, a textbook case of woolsack weathering.

A stack or pile of woolsack rocks is often called a tor, possibly derived from the Old English torr—related to Scottish Gaelic tórr for a bulging hill—or possibly from the Celtic word twr meaning tower. Dartmoor National Park in southern England is famous for its legendary tors. There are at least 160, most with colorful names and stories.

Vixen Tor; John MacCulloch (1814).

Vixen Tor was home to the wicked witch Vixana. Whenever a traveler foolishly passed nearby, she called up a thick mist—so thick that the traveler lost his way, stumbled into a bog, and met an excruciating end, sucked screaming to his death. Vixana would dispel the mist just in time to enjoy the final moments of his desperate and hideous struggle. Finally Vixana herself was killed—by a handsome young moorman, of course!

The tors of Dartmoor are legendary not just for tales and spirits, but also for pioneering studies by early geologists. In 1754, antiquarian, geologist and naturalist William Borlase concluded that Druids carved the tors, based on the prevalence of blocks. Druids were said to worship cubes, symbolic of the god Mercury—even though almost nothing was known about Druid culture then, as now.

In 1814, Scottish geologist John MacCulloch read a paper before the Geological Society of London titled On the granite tors of Cornwall, in which he discounted Borlase’s theory. He didn’t mince words:

“… learned antiquarians have tortured their inventions and have constructed religious systems for the purpose of explaining these very simple and intelligible natural appearances, by the rites of a mysterious and Druidical worship. … It is unnecessary to suppose that the chisel of Druidism has been employed to reduce it [the Cheesewring] to an image of Saturn. Natural causes are sufficient to account for its appearance.”

The tors’ rounded blocks convinced MacCulloch that they had been shaped by air and water, not by human hands:

“The changes which they undergo in their places of rest, by their more rapid disintegration at the angles than at the sides, are sufficient to prove that this spheroidal shape may be produced by chemical action of air and water, without the necessity of any mechanical violence. However difficult it may be to give a very satisfactory account of this peculiarity, the fact is undoubted.”

Woolsack weathering—“more rapid disintegration at the angles than at the sides”

MacCulloch was only partly right. The rounded shapes were produced by chemical action of water but not air. That would be impossible, for woolsacks and tors form underground.

Woolsacks at Vedauwoo, Laramie Mountains. For origins, see diagram below.

Our local woolsacks are made of Sherman granite, which began as magma intruded into the crust about 1.4 billion years ago. It never reached the surface, but cooled underground into a huge mass of granite, shrinking and cracking in the process. Fractures often formed 90º angles—the beginnings of blocks. Groundwater percolated through the cracks, and chemicals broke down the rock, rounding the edges (spheroidal weathering).

Then roughly 70-40 million years ago, during an episode of mountain-building (Laramide Orogeny), the Laramie Mountains rose, erosion set in, and the Sherman granite was gradually exposed. Weathered debris washed out of the fractures, and a multitude of wondrous rock forms emerged.

Birth and emergence of the woolsacks in the previous photo (click on image for details).

This land of science and the supernatural, where knowledge coexists with legend and whimsy, is just 15 miles from town. We're lucky to be able to wander among the real and the otherworldly as our mood sees fit.