Monday, October 4, 2004

Plant for the Day: Plains cottonwood, Populus deltoides var. occidentalis

Plains cottonwood occurs throughout the Great Plains from the southern prairie provinces of Canada to the high plains of northern Texas. It is a fast-growing tree that likes moist sites. Consequently it is common in bottomlands and along streams, where it occurs in pure stands on sandbars in permanent streams and on the bed of the channel in intermittent streams. In a landscape dominated by grasses and other nonwoody plants, cottonwoods act as prominent markers of a reliable water source, visible for many miles.

Once we passed the Sangre de Cristo Mountains and the Spanish Peaks, we flushed out onto the more open landscape of the Colorado Piedmont, east of the Rockies. While major upheavals of the crust were occurring just to the west during construction of the Rockies, this area experienced relatively stable geologic conditions. We rode out beyond Walsenburg, our route taking us just north of Comanche National Grasslands, and turned to the north near La Junta. Comanche National Grasslands was designated in 1960. It preserves a diverse physical landscape, including deep canyons with vertical rock walls and more gently rolling terrain, and a rich history of occupation by humans and other creatures. It has the largest continuous set of preserved dinosaur tracks in North America—over 1300 footprints preserved in the rock! The tracks were made by both allosaurus and brontosaurus inhabiting this area 150 million years ago. The brontosaurus tracks include footprints made by a large animal, as well as many made by smaller brontosaurs, all traveling as a group along a pre-historic shoreline lined with clams, crustaceans, and fish. Much later this area was inhabited by pre-historic hunters and gatherers, who left rock art on the rock walls of the canyons. Historically this area was inhabited first by the Comanche in the late 1700s, then by ranchers in the late 1800s, and later yet by homesteaders in the early 1900s. Twenty-two miles of the Santa Fe Trail cut through the grassland reserve. Today the predominantly short-grass prairie is home to a diversity of animals, including golden and bald eagles, and the rare lesser prairie chicken.

As we rode north, we paralleled the Front Range on our way towards Denver. The Front Range is just one part of the Rocky Mountains, one of the most impressive mountain systems in North America. The history of the Rockies is long and complex, and geologists still debate several aspects of their formation. About 65 to 75 million years ago, the basement rocks that form the continental crust in Colorado and neighboring states rose during a series of uplifts. They were pushed many thousands of feet higher than the relatively undisturbed rock layers beneath the High Plains just east of the mountains. The uplift facilitated erosion, which removed overlying younger sedimentary rocks, exposed the metamorphic and igneous basement rocks below, and lowered the terrain in general. From about 20 to 40 million years ago, during a period of volcanic activity, many of the rocks that make up the San Juan Mountains, the Spanish Peaks, and other volcanic peaks in the southern Rockies were formed. The uplift that has occurred in the last 20 million years elevated the land many thousands of feet above the High Plains. The rugged terrain in this area results from erosional processes, like running water and glacial ice, stripping away weaker rock to sculpt the spectacular landscape we see in the Rocky Mountains today.

As we made our way northward, we paralleled the Front Range and the hogbacks that are visible in several places just east of the Front Range, like Colorado Springs, where we trained during the summer. Earlier uplifts that first elevated sedimentary rock units thousands of feet, then facilitated removal of the weaker rocks through erosion, left crystalline basement rocks of the Rockies next to upturned rocks layers at the western edge of the High Plains. These hogbacks, or sharp-crested ridges, are the remaining limbs of the anticlinal arch, or upwarped fold, that formed the Front Range. The other rocks having long since been eroded away. The hogbacks are formed in Dakota sandstone, a strong rock unit that resists weathering and erosion. The Dakota sandstone forms the Flatirons west of Boulder and the Garden of the Gods west of Colorado Springs. These are some of the most spectacular hogbacks visible anywhere.

Once we got as far north as Denver we turned west, back towards the mountains and into Denver. As we made our way towards Denver, we crossed a sequence of progressively older rocks, although it wasn’t conspicuously expressed in the surrounding topography. Denver lies in a structural basin, whose underlying rock layers are bowed downward in the middle, like a giant bowl. The basin contains layer upon layer of sediments worn down from the modern and ancestral Rocky Mountains and transported to the east, where they accumulated in the basin.

What a spectacular view to see all the peaks along the Front Range of the Rockies as we dropped off the High Plains east of the mountains into Denver. We could see at least three of Colorado’s peaks that top out over an elevation of 14,000’—Pikes Peak to the south, Mount Evans west of Denver, and Long’s Peak to the north, in Rocky Mountain National Park. The same orogeny (i.e., period of mountain building) that uplifted the mountains of Colorado was responsible for mountain building in Wyoming and New Mexico, yet those states lack the concentration of peaks over 14,000’ that characterizes the Colorado Rockies. Geologists have observed that the “fourteeners”, as they are called, are almost all located in two areas of Colorado—either adjacent to the Rio Grande rift, in the southern part of the state near the Great Sand Dunes and the Spanish Peaks, or in the Colorado mineral belt. The latter is a swath stretching from northeast to southwest that contains many igneous intrusions. Most of the major mining resources of the state are located within the belt. Colorado’s fourteeners that are outside of these two areas are Long’s Peak and Pike’s Peak; these were likely remnants from erosion of terrain uplifted 65 to 75 million years ago, only to be elevated again during more recent tectonic activity.

After a Hope Rally at the University of Colorado Cancer Center, we pedaled out of town to resume our eastward journey. Most of the way to the Colorado-Kansas state line we cycled through the wide open spaces typical of short-grass prairie. Today, most of this land is in ranches and farms, and little of the native prairie remains. Historically, though, this area was dominated by grasses that are low in stature, with most of their biomass underground. Most of the plants are perennials; their above-ground biomass dies back annually in the winter, but the root systems and basal shoots survive from year to year. Overall plant biomass in the short-grass prairie is sparse enough that fires weren’t as common as they were in the mixed and tall grass prairie farther east—thanks to insufficient fuel buildup. Prairie dogs and pronghorns are some of the more conspicuous mammals of the short grass prairie. The pronghorn relies on good visibility and speed to outrun predators. Prairie dogs dwell in elaborate towns; sentinels are stationed at underground burrow entrances to watch for danger and sound an alarm to warn other prairie dogs of a dangerous intruder, such as a badger. Historically bison found prairie dog towns to be particularly favorable habitats, because the light grazing around prairie dog town entrances stimulated growth of the herbaceous species considered especially tasty by the bison.