Geography of the Christmas Eve Blizzard

After the Christmas Eve Blizzard of the southern Plains states (photos from Norman), a NASA MODIS satellite shot this fine image of the swath of snow left behind.

Click the image above to open a much larger and more finely detailed version that I’ve annotated with interesting geographic features. You then can move about it while following along with the descriptions below.

Starting our journey around the photo in central and eastern Oklahoma, Tulsa and Oklahoma City stand out as gray splotches amidst the whiter snow of surrounding rural areas. To the west of OKC, grasslands and farms prevail, so the snow cover is brighter (higher albedo), despite being somewhat less thick than over OKC proper. Snow-covered buildings reflect less sunlight (lower albedo) than snow-covered flat plains, so they appear grayer. Still darker are deciduous forests without leaves on the trees, as is the case in the Western Crosstimbers area that extends from from OKC down into north Texas west of Ft. Worth. Just because the color is darker doesn’t mean the snow is less deep, necessarily. It depends strongly on the land cover and usage, as we’ll continue to see.

Over eastern Oklahoma, less snow fell, but still enough to show up very nicely atop hidden layers of sleet and frozen rain. Fort Gibson Reservoir, impounding the Grand (Neosho) River, also nicely demarcates agricultural areas east and southeast of Tulsa from the mode heavily forested Ozark Plateau extending into Arkansas. [Go here to see ground photos of the the Ft. Gibson area without snow cover.] Lake Eufaula, an enormous reservoir that dams the Canadian River and several major tributaries, stands out well with its size and faint brown-orange tint. It drains the Permian redbed regions of western and central Oklahoma (more below), so at times of high winds and/or flooding upstream, it turns deep rust-orange with suspended red clay particles.

Nearby, to the southeast, the ridges of the Ouachita Mountain fold belt stand out brilliantly, with their shadows and differential snow cover. Green areas denote pine forests that extend southward across east Texas as the Piney Woods. These very old mountains — a cut-off western extension of the Appalachians that formed at the same time and in the same continental-collision process — extend southwest under Dallas and Austin as a buried range, wind generally westward across southwest Texas underground, and barely crop out again for the last time in the Big Bend region.

The snow cover tapered over the Dallas area, southwestward across the Hill Country. The Hill country is much like a drier version of the Ozarks in that it is mostly a dissected limestone plateau, cut into sometimes very rugged relief by streams, and underlain by caverns. This limestone is much younger, though — Cretaceous (~100-130 million years old), as opposed to the Mississippian and Ordovician rocks of the Ozarks (roughly 300-500 million years). Also like the Ozarks, the Hill Country includes a small core area of very old (Precambrian, over a billion years) rocks that reach the surface.

The Balcones Escarpment, parallel to and overlying the buried Ouachita Mountains, separates the Hill Country from the flatter Blackland Prairie region to its east. In the blacklands, which extend from San Antonio through Dallas and east-northeastward, younger, softer Cretaceous limestone, chalk and marl weather to a rich variety of thick, deep, black soils used for a wide variety of heat-tolerant crops. Crossing the Blacklands east of Austin, a contrail (condensation trail from an airplane) appears, with its shadow well to the north. As of the snow wasn’t evidence enough, the steep sun angle shows that the satellite photo was taken on a winter’s day! Between the norther parts of Dallas and Ft. Worth, look closely to see two neatly packed sets of dots; those are the terminal areas of DFW Airport. The Western Crosstimbers country farther west overlies some of the same Pennsylvanian (~280-320 million years back) rocks as appear again with different names from eastern Oklahoma to northwestern Missouri and Iowa.

Jumping back up to northern Oklahoma, the Great Salt Plains area surrounds the southwestern parts of muddy Great Salt Plains Reservoir, but doesn’t stand out since the salt flats and snow are the same color. We’ve gone digging for selenite (gypsum) crystals there before, the best time being in April right after the northward bird migrations end and digging areas are opened on the salt flats. Some of those crystals are buried in mud, have sharp edges and can cut skin, so gloves come in handy.

The ancient Wichita Mountains of southwest Oklahoma, near Lawton, persist as but tiny nubs of the towering peaks they were hundreds of millions of years ago. [Go here to see ground photos of that area without snow cover.] Worn down to remnants of their Cambrian granite cores, now the Wichitas just barely stand above the surrounding plains as knobby hills, but still look like mountains when approached from the distance on I-44 or US-62.

Traveling south, the darker areas around the Pease, Brazos and Wichita Rivers in northwest Texas are Permian redbeds deeply eroded into ravines, gullies and small canyons in dry, nutrient-poor, somewhat salty, mainly clay soil that is highly erosion-prone and unsuitable for agriculture. The choppy terrain reflects less light when snow-covered than the more level fields to the north.

Out in west Texas, the Caprock Escarpment stands out brilliantly as a darker outline to the east edge of the tabletop-flat High Plains (a.k.a. Llano Estacado, or Staked Plains). There, in person, a relatively hard ledge of caliche and caliche-cemented Ogallala sandstone that holds up the High Plains breaks and tumbles down eastward under slow erosional attack from water runoff and gravity, revealing layers of soft, multicolored Triassic sediments that bottom out on Permian red sale and sandstone. Although the slopes of the Caprock look gray here, given a mix of snow, dark scrub brush and rocks, the reddening colors of the Permian layers do appear in the Palo Duro Canyon. While not approaching the scope of the Grand Canyon, Palo Duro is very spectacular and photogenic, well worth a couple of days’ exploration during springtime or the milder parts of winter for any avid hiker, mountain biker, jeeper, or outdoor photographer.

On the much flatter terrain inside the Caprock edges, irrigated farming for cotton, wheat and other dry-land crops dominates, whereas ranching and rangeland make up most of the land of poorer soils below the Caprock. On those High Plains, cities and towns such as Lubbock and Plainview stand out strongly. Look closely at Lubbock, where the mile-by-mile checkerboard of primary thoroughfares stands out even at this scale.

Farther southward, the snow peters out across a grid of roads and farms that long ago was laid out west-southwest/east-northeast, including those in the Midland-Odessa area. One north-northwest/south-southeast trending strip of snow-covered ground stands out nicely northwest of Midland. Southwest of Abilene, a line of low hills experienced slightly greater snow accumulation than areas nearer to the southern Permian redbeds.

Over southern New Mexico, the White Sands region looks snow-covered, but of course, the high-albedo dunes of gypsum sand stay visible from space year-round. Much higher terrain west of the Pecos River valley, such as the Sacramento and Guadalupe Mountains, caught more snow. Areas in the higher parts of the Sacramentos that are forested by conifers show up well as dark green, cut by small canyons. The Guadalupes, being not quite so high, and farther south into the fringes of the Chihuahuan Desert, don’t bear such a dense veneer of evergreens to darken what actually is thinner snow cover. The process that formed them is different too: an erosionally resistant, Permian reef of oceanic limestone, as opposed to the fault-block range of the Sacramentos that formed in much the same way as the rest of the southern Rockies.

The High Plains caprock also has western edges in eastern New Mexico that drop into the Pecos and Canadian River valleys, though not as high above their bases as the escarpment in Texas. The western Caprock shows up well as a break in the snow albedo.

Heading northward toward Colorado, large fields of extinct volcanoes dot both the plains east of the Sangre De Cristos, and the San Luis Valley to the west, two of numerous volcanic areas in the Land of Enchantment. Where snow cover was thicker, the volcanoes look like dark blisters on the landscape. Raton Mesa stands high above the surrounding plains along the Colorado-New Mexico line, its hard basalt layers resistant to erosion, its loftier elevation catching more snow. Raton, along with the other mesas trending eastward along the border toward the northwest part of the Oklahoma Panhandle, are hardened lava flows that filled former river valleys. The softer ground around them eroded away, leaving behind nice tracers of old rivers now standing above adjacent areas! Much like with Palo Duro in the Texas Panhandle, the canyons of the Purgatoire River stand out for their lack of snow cover compared to surrounding plains.

Look closely at the parts of western Kansas not covered by visible snow, and hundreds of tiny does and circles snow up. These are the fields moistened by center-pivot irrigators, tapping an ever-lowering water level of the Ogallala Aquifer. This aquifer extends northward to southwestern South Dakota and southward across the Panhandles. While replenished by snowmelt, and under the southern High Plains caprock by seepage from intermittent playa lakes, irrigation draws water out faster than it trickles in from above.

An enormous swath of Permian red sedimentary rock, expanding laterally at the surface from south of Abilene, Texas, extends northward across the Panhandles below the Caprock, most of central and western Oklahoma, and parts of southwestern Kansas. The Kansas part locally carries the name Red Hills, visible here as a slightly reddish hue thanks to areas of exposed rock and soil, dissected and eroded much like the Pease, Brazos and Wichita River valley in Texas, but not snow-covered.

And with that circuit, we have completed our grand flyover tour of the geography of the Christmas Eve 2009 blizzard. I could have covered a lot more…


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