Geology of the Great Basin

16. Selected GB Parks

16 selected great basin parks national parks Great Basin National Park, Lehman Caves Wheeler Peak, one of the highest mountains in the Great Basin, is the thirteen-thousand-foot summit of the Snake Range along the border of Nevada and Utah. The National Park straddles the highlands, and high on the eastern flank of Wheeler peak is Lehman Cave. The cave was carved by acidic groundwater following fractures in lightly metamorphosed carbonate rocks. The large rooms are filled with impressive cave formations. Large shields or thin round disks of the mineral calcite are common in this cave system and rare elsewhere. We can only speculate on their origin. What had precipitated out of the ocean was now returning to solution again, only in groundwater this time. The subsurface water armed itself with carbonic acid, derived from the carbon dioxide of the atmosphere and the soils. Limestone is especially vulnerable to acid water, and its chemical defenses were quickly breached. Originally, the solution followed fractures and enlarged them. Like an uncorrected cavity in a tooth, they slowly enlarged with time. Eventually, they coalesced into an anastomosing pattern of rooms, passages, and collapsed chambers. This activity occurred below the water table, where the groundwater could slowly seep through the rocks. Sometime within the past few million years, the land surface rose and lifted the caverns above the water table. Now high and dry, the dissolving effect of groundwater has mostly ended. The roofs and walls of some of the rooms, freed from the supporting pressure of internal water, collapsed under their own weight. Selected Great Basin Parks 211 The austere water-smoothed ceilings and walls of the caverns began to acquire decorations. Water dripping along fractures and through the pores in the limestone was saturated with carbonate. As the water emerged into the air of the rooms, it evaporated, leaving the lime minerals behind. These travertine deposits form ribbons over the walls, hang as stalactites from the ceiling, rise as stalagmites from the floors below the drippings of the overhanging stalactites, and occasionally join as great columns that stretch from floor to ceiling. Where stalactites coalesce, draperies or curtains form. A most unusual speleothem is the helictite, a stalactite that twists and turns like a drunken sailor careening off the walls of the cave. A helictite might result from the inner tube of the stalactite being so small that the water moves by capillary action, even though it might move against the flow of gravity. The helictite behavior may also result from the fact that the carbonate, or calcite , crystals are not perfectly square but are squashed cubes. As the minerals form, the slightly off-square sides of the crystals might act as preferential crystallizing directions. The long crystal axis, the C-axis, is often the direction of growth. Mineral-laden waters flow over the floors of the rooms and deposit horizontal layers of flowstone. The rate of growth of these speleothems is exceedingly slow, but time ticks with infinite patience in a cave. Death Valley National Park The deep sun-blistered trench sandwiched between the oven walls of mountain heights was named Death Valley by the forty-niners. It was called Tomesha by the Panamint Indians, who lived in the valley for millennia before the arrival of the miners. Tomesha means “ground afire.” Death Valley, produced by the titanic struggle and collision of gigantic plates of the earth’s crust, is an eyeful. The deep trough, overpowered by great mountains, seems to lie lifeless under the blazing desert sun. Names attribute a netherworld source for its features—Devil’s Cornfield, Dante’s View, Furnace Creek, and the Devil’s Golfcourse—names that penetrate the mind with frightful images. Names, however, reflect human perceptions. The plants, animals, and desert-adapted humans find this a delightful place to live or visit. Some are so content here that they would be unable or unwilling to live anywhere else. Here, too, the parched but gentle hand of Aeolus, the wind god, has added an artistic finishing touch to the landscape. Fine sands and muds, washed down from surrounding mountains, lie unconsolidated and vulnerable on the valley floor. Desert winds have gently scooped out the loose detritus and shaped deflation hollows several feet deep. Roots of widely spaced arrow weed are exposed as the land surface is Great Basin state and national parks.° National Park—administered areas. 1. Death Valley National Park 2. Great...