Sunday, October 3, 2004

Plant for the Day: Quaking Aspen, Populus tremuloides

In the wetter region in the east of its range, quaking aspen often reproduces from seed; but in the semiarid west, where conditions are much harsher, large clones from vegetative reproduction are more common. A hillslope of aspen in southwestern Colorado usually contains a mosaic of different clones. Some clones can be quite large: One giant aspen clone studied in Utah extends over several acres and includes more than 47,000 individual stems! Scientists estimate that many clones date back to the end of the last glacial period, about 10,000 years ago, when conditions in parts of the West were more favorable for their sexual reproduction. Aspen is an important food source for a variety of animals, including deer, elk, rabbits, and grouse.

Darkness prevented our visual appreciation of the rainbow of hues so evident in the buttes and mesas of Momument Valley during daylight. We could only see the dark forms of massive rock silhouetted by the dim light from stars and the waning moon. These rock giants are erosional remnants, capped with a resistant layer of rock that protects the weaker layers below while differential erosion removes the surrounding rock. With time, however, these holdouts against erosion will succumb as well, as has already happened in the intervening areas where the lower rock layers are all that remain.

Shiprock was visible for miles as we approached the Four Corners, even in the dark of night. The Navajo name for Shiprock is Tse Bitai, or ‘winged rock’. The mountain is a spectacular example of a volcanic neck with dikes radiating away from the central chamber. It originally formed many hundreds of feet below the surface, as the lava in the central chamber of an ancient volcano slowly cooled and solidified. Over millions of years after the eruption, overlying weaker sandstone and shale were gradually eroded away, exposing the more resistant solidified lava. Today the main part of Shiprock stands nearly 2000’ above the surrounding desert, with prominent feeder dikes radiating away in several directions. Shiprock and its dikes are part of the Navajo volcanic field, which covers thousands of square miles in the Four Corners area. It includes many intrusive volcanic features that are now evident at the earth’s surface, thanks to erosion of the overlying country rock that has exposed these deeper, extinct volcanoes.

The southwestern corner of Colorado is high desert, but as we approached the foothills of the San Juan Mountains near Cortez, elevations increased. Just south of Cortez lies Mesa Verde National Park (MVNP), which protects over 4000 separate pre-Columbian ruins. The north rim of the park is a ridge trending east-west, with many long thin mesas fingering southward. The narrow protected canyons between these mesas were inhabited from about A.D. 600 to A.D. 1300, with dwellings built in alcoves of the canyon walls near the end of this period. Like the settlements in Navajo National Monument, this area was abandoned around A.D. 1300, for reasons that are not fully understood.

As we headed from the area around MVNP towards Durango, we were treated to occasional glimpses of Sleeping Ute Mountain and La Plata Mountains, which formed from magma that was intruded over 65 million years ago. Sleeping Ute Mountain is a laccolith, with two resistant volcanic necks, much like Shiprock, forming Ute’s toes. Laccoliths are bodies of intrusive igneous rock that are squeezed between layers of existing rock, causing the surrounding rock to dome upward. They become exposed at the surface when the overlying rocks are eroded away. These mountains lie on the southwestern fringes of the San Juan Mountains, an area with spectacular, rugged topography that was carved by glaciers during the last glacial period. The San Juans form the southern part of the Rocky Mountains and extend eastward through the Continental Divide. Volcanic activity played a major role in their formation, and volcanic features are more evident here than they are in some of the other parts of the Rockies.

East of Pagosa Springs, we dropped into New Mexico to cross the Continental Divide. The divide runs along the spine of the Rocky Mountains, which separates streams that flow westward to the Pacific from those that flow to the east. In northern New Mexico, the divide is in an area of relatively low relief; but we then climbed up over Cumbres Pass, back across the state line in Colorado, which tops out at 10,022’. As we ascended the pass, we climbed through a typical gradient of forest types, with changes occurring in response to the increased stress associated with climates found at higher elevations: Douglas-fir forests, through Engelmann spruce and subalpine fir, first growing as stately trees, then as stunted shrubby plants, or krummholz.

Dropping out of the mountains, we encountered the broad expanse of the San Luis Valley. I’ve been watching for sandhill cranes, which migrate through the San Luis Valley in the fall, with numbers peaking in this area in October. They are an impressive sight. They stand over 3’ tall and have an elaborate courtship dance that they perform in the spring. Monte Vista National Wildlife, just north of our route, is a major stopover location for cranes on their journey from their breeding grounds near Yellowstone to their wintering grounds in the Rio Grande Valley in New Mexico and northern Mexico. Cranes migrate as a small family unit; the parents and their one offspring stay together through fall, winter, and the northward migration again in the spring, before the young bird disperses. Sandhill cranes live from 20-30 years. An unsuccessful attempt to establish a population of the endangered whooping crane in this area, with sandhill cranes as foster parents to whooper juveniles, was made in the 1980’s.

The San Luis Valley lies between the San Juan Mountains to the west and the Sange de Cristo Mountains to the east. The valley is part of the Rio Grande Rift. The uplift of the San Juan and Sangre de Cristo Mountains stretched the earth’s crust in this area and widened the intervening basin, causing the San Luis Valley to sink. Sediment delivered to the valley from ash flows, wind, and streams draining the adjacent uplands began to accumulate in the valley. The unique geography of the San Luis Valley has led to the formation of the largest sand dunes in North America. Just north of our route across the valley lies Great Sand Dunes National Park (formerly a national monument, but just granted national park status last month), where the massive dunes reach over 700’! The dune field is nestled up against the western flank of the Sange de Cristo Mountains. Prevailing winds from the west blow sediment accumulated in the valley toward the mountains. The mountains, combined with frequent, more localized down-valley winds, trap the sediment west of the mountains. Sand is also transported westward away from the mountains by streams surrounding the dune field that drain the Sangre de Cristo Mountains. This “recycles” some of the sand back to the western part of the valley, where it begins a slow migration eastward again under the prevailing winds. Thanks to the interplay of these factors, instead of migrating downwind, as dunes often do, the shifting sands accumulate just west of the mountains, reaching greater heights than most dunes, and form a greater variety of wind-blown landforms than is evident in most dune fields.

A little farther along, to the south of our route, we could see the Spanish Peaks rising in the distance. They are prominent features in the landscape, and from the east are visible from 100 miles away! They served as important navigational landmarks for native people inhabiting this area, and later, for European settlers pushing westward. During the upfaulting of the Sangre de Cristo Mountain block over 20 million years ago, magma pushed up into the cracks formed in the lower rock layers, where it cooled slowly and solidified, forming large batholiths of granite. At the time, these batholiths were hidden below thousands of feet of overlying sedimentary rock. Gradually over time, erosional processes stripped away the weaker rock above, to exume the igneous batholiths below. These now form the Spanish Peaks. Equally spectacular are the numerous granite dikes radiating out from several of the peaks, like the spokes of a wheel. When the batholiths formed, molten magma also rose into vertical fractures spreading out from the main area of magma intrusion. These, too, solidified and have gradually been exposed over time as weaker rock has been removed. These dikes stand as impressive linear ridges in the landscape. I once had the treat of flying over this area; and the Spanish Peaks, the associated dikes, and the Great Sand Dunes to the north are clearly visible from the air. With a bird’s eye view of the terrain, it’s easy to visualize how these distinctive features of this landscape formed.