Thursday, May 29, 2008
Them Ogallala Blues
I have been thinking a great deal lately, what with the rising price of food, about the body of water that has made a lot of American crop production possible in the last 50 years or so. Not a body you can see, swim in, or boat on. This is the High Plains Aquifer, that underlies much of Nebraska, Western Kansas, eastern Colorado, and that general area. (It is popularly known as the Ogallala Aquifer, but technically the Ogallala is only part of it. "Ogallala" is much more fun to say, though.) At its peak - which was a while ago - it held as much water as Lake Huron.
It's a fossil formation, with fossil water. Back in the late Cretaceous, most of the Great Plains were covered by a shallow sea, but as the so-called “Laramide revolution” pushed up the southern Rocky Mountains, the inland sea dried up, leaving a flat thick bed of sediments. But not for long. Rivers and streams rushing off the rising mountains to the west cut numerous channels and valleys in the relatively soft limestone and sandstone of the former sea floor. Eventually – and we’re now in the late Miocene, 5 to 10 million years ago – the valleys and river beds started accumulating vast quantities of sediments still being washed off the mountains in the form of boulders, cobbles, gravels, and coarse and fine sands. (The same process is going on today: look at any broad stream at the foot of large mountains, which in dry summer months may look like a gravel field with a trickle of water in it. Come back every August for 500,000 years and you’ll likely see a significant build-up of washed-down mountain.)
The sediments eventually overflowed the banks of their river channels and met to form a largely continuous sheet of sand and gravel. Around 3 or 4 million years ago, the climate started to get a lot drier and the Great Plains area warped upwards, drastically slowing the process by which Colorado mountains were being transformed into Kansas subsurface. The sediments were gradually covered over by other layers, some wind-blo
wn, some water-borne, until the landscape is largely as it is today, a thick layer of very fertile soil overlying a bed of sand and gravel that is some 900 feet thick at its thickest. Because the sand and gravel are what geologists call “unconsolidated” – not cemented together as a rock – the bed is full of water. The water has accumulated over millions of years: the old Cretaceous seabed formed an impermeable barrier underneath, and drainage from the Rockies filled it with Pliocene water. Changes in the land contour over the ages have cut the hydraulic connection to the Rockies, but precipitation, sparse though it is, still percolates through the soil to the aquifer. Just not fast enough to make up for what we're taking out of it.
It's a fossil formation, with fossil water. Back in the late Cretaceous, most of the Great Plains were covered by a shallow sea, but as the so-called “Laramide revolution” pushed up the southern Rocky Mountains, the inland sea dried up, leaving a flat thick bed of sediments. But not for long. Rivers and streams rushing off the rising mountains to the west cut numerous channels and valleys in the relatively soft limestone and sandstone of the former sea floor. Eventually – and we’re now in the late Miocene, 5 to 10 million years ago – the valleys and river beds started accumulating vast quantities of sediments still being washed off the mountains in the form of boulders, cobbles, gravels, and coarse and fine sands. (The same process is going on today: look at any broad stream at the foot of large mountains, which in dry summer months may look like a gravel field with a trickle of water in it. Come back every August for 500,000 years and you’ll likely see a significant build-up of washed-down mountain.)
The sediments eventually overflowed the banks of their river channels and met to form a largely continuous sheet of sand and gravel. Around 3 or 4 million years ago, the climate started to get a lot drier and the Great Plains area warped upwards, drastically slowing the process by which Colorado mountains were being transformed into Kansas subsurface. The sediments were gradually covered over by other layers, some wind-blo
wn, some water-borne, until the landscape is largely as it is today, a thick layer of very fertile soil overlying a bed of sand and gravel that is some 900 feet thick at its thickest. Because the sand and gravel are what geologists call “unconsolidated” – not cemented together as a rock – the bed is full of water. The water has accumulated over millions of years: the old Cretaceous seabed formed an impermeable barrier underneath, and drainage from the Rockies filled it with Pliocene water. Changes in the land contour over the ages have cut the hydraulic connection to the Rockies, but precipitation, sparse though it is, still percolates through the soil to the aquifer. Just not fast enough to make up for what we're taking out of it.
