We spent a couple of days in Moab hiking between 5-6 miles each day in Arches National Park. As a little background as stated on their brochure, the park lies atop a underground salt bed that is basically responsible for the arches, spires, balanced rocks, sandstone fins and eroded monoliths. Thousands of feet thick in places, this salt bed was deposited across the Colorado Plateau 300 million years ago which a sea flowed into the region and eventually evaporated. Over millions of years, residue from floods, winds, and the oceans that came and went blanketed the salt bed. The debris was compressed as rock, at one time possibly a mile thick.
Salt under pressure is unstable, and the salt bed lying below Arches was no match for the weight of this thick cover of rock. The salt layer shifted, buckled, liquefied, and repositioned itself, thrusting the rock layers upward as domes, and whole sections fell into the cavities.
Faults deep in the Earth made the surface even more unstable. Fault-caused vertical cracks later contributed to the development of arches. As the salt's subsurface shifting shaped the Earth, surface erosion stripped off younger rocks layers. Except for isolated remnants, today's major formations are salmon-colored Entrada Sandstone, in which most arches form, and buff-colored Navajo Sandstone.
Over time warter seeped into superficial cracks, joints, and folds. Ice formed in the fissures, expanding and pressuring the rock, breaking off bits and pieces. Wind later cleaned out the loose particles, leaving a series of free-standing fins. Wind and water then attacked these finds until the cementing material in some gave way and chunks of rock tumbled out. Many of these damaged fins collapsed. Others, hard enough and balanced, survived despite missing sections. These became the famous arches. Porthole arches are formed by chemical weathering as water collects in natural depressions and then eventually cuts through to the layer below. This is the geologic story of Arches.
We spent a couple of days in Moab hiking between 5-6 miles each day in Arches National Park. As a little background as stated on their brochure, the park lies atop a underground salt bed that is basically responsible for the arches, spires, balanced rocks, sandstone fins and eroded monoliths. Thousands of feet thick in places, this salt bed was deposited across the Colorado Plateau 300 million years ago which a sea flowed into the region and eventually evaporated. Over millions of years, residue from floods, winds, and the oceans that came and went blanketed the salt bed. The debris was compressed as rock, at one time possibly a mile thick.
Salt under pressure is unstable, and the salt bed lying below Arches was no match for the weight of this thick cover of rock. The salt layer shifted, buckled, liquefied, and repositioned itself, thrusting the rock layers upward as domes, and whole sections fell into the cavities.
Faults deep in the Earth made the surface even more unstable. Fault-caused vertical cracks later contributed to the development of arches. As the salt's subsurface shifting shaped the Earth, surface erosion stripped off younger rocks layers. Except for isolated remnants, today's major formations are salmon-colored Entrada Sandstone, in which most arches form, and buff-colored Navajo Sandstone.
Over time warter seeped into superficial cracks, joints, and folds. Ice formed in the fissures, expanding and pressuring the rock, breaking off bits and pieces. Wind later cleaned out the loose particles, leaving a series of free-standing fins. Wind and water then attacked these finds until the cementing material in some gave way and chunks of rock tumbled out. Many of these damaged fins collapsed. Others, hard enough and balanced, survived despite missing sections. These became the famous arches. Porthole arches are formed by chemical weathering as water collects in natural depressions and then eventually cuts through to the layer below. This is the geologic story of Arches.
