FIELD TRIP LEADERS
BOB HOWARD, FRANK HANNA, AND GEORGE DUNNE
Photographed and compiled by GENE FRITSCHE
After leaving Texas Springs Campground in the morning, we headed south toward Badwater. Enroute to Badwater, we came to this ridge that is oriented east-west, perpendicular to the axis of Death Valley and the common wind direction. The cobbles and boulders on the ridge are ventifacts (rocks whose shape has been modified by the sandblasting effect of the wind). Here Julie Laity describes the origin and usefulness of ventifacts. The side of the rocks facing the camera is the side that has been grooved and polished by wind erosion.
In this picture our group is standing on the salt playa surface at Badwater (280 feet below sea level) and looking up toward the sign on the side of the mountain where sea level is marked. Reflections behind the group are from the surface of a small pond that marks the level of the water table in the area. Rocks in the background are Proterozoic crystalline rocks. The rounded surface of the mountain is the locus of a folded detachment fault referred to as the Badwater Turtleback. The rocks above the detachment fault were younger and softer than the underlying crystalline rocks and they eroded away, leaving the detachment fault surface exposed.
Farther out on the playa at Badwater we came to this area of salt polygons. Frank Hanna described for us two possible origins for these salt polygons, and the actual origin may be a combination of both processes. Both processes begin with the drying and polygonal cracking of the playa surface as water evaporates. One process proposes that the polygonal sheets expand laterally as salt crystallizes within them and the edges of the sheets override each other. The other process suggests that salt crystallizes due to capillary action only along the polygonal cracks and that the salt grows upward from the cracks in the form of inverted salt stalactites. In the middle ground on the right is the Badwater Turtleback and behind that are additional parts of the Black Mountains.
South of Badwater we stopped in a canyon along the front of the Black Mountains to see one of the detachment faults (arrow). Here the Proterozoic crystalline rocks are overlain in sharp fault contact by Pliocene-lower Pleistocene, well cemented alluvial fan deposits. The contact has a low dip and the age of faulting is obviously very young. Frank Hanna (far left) is wondering why that arrow is hanging up in the sky.
A little east of the point where Highway 178 turns east and heads out of Death Valley, we stopped to look at an exposure of the Amargosa Chaos. This rock unit is a megabreccia that contains up to house-size blocks of almost exclusively Kingston Range Granite, a late Miocene intrusive unit. The formation originated during the latest Miocene as a rock-avalanche deposit that fell off a steep, west-facing mountain front composed of Kingston Range Granite. Observations on the character of the deposit indicate that it could not have slid more than 10 km from it's source in the proto-Kingston Range, which today is far to the southeast. To bring the deposit to it's present location requires latest Miocene-Pliocene extension of about 30 km along the Amargosa detachment system.
East of Shoshone, at a roadcut in the Resting Spring Range, we came across this dark band of rock. It is an amygdaloidal obsidian (volcanic glass), and all the amygdules (gas bubbles with mineral deposits inside) are flattened. It occurs in the middle of a thick, volcanic ash deposit (tuff) of rhyolitic composition. The explanation is that the ashes were still very hot when they fell and that heat was retained longest in the center of the deposit where it was hot enough to remelt the rhyolitic ashes and convert them to obsidian. Gas bubbles created in the ash fall collapsed as the weight of the overlying ash increased.
As the sun dipped behind the mountains, we drove out on Silver Dry Lake, north of Baker, to examine the margin of a playa. Here the sediment grades rather abruptly from coarse pebbles to silt and clay. Whenever you see an auto commercial that shows the car on a dry lake, this is most likely the place where the commercial was made. The clay surface was very hard and smooth. The group seems to be searching for the best route to take to Baker.
The Pleistocene lake that formed along the Mojave River behind a dam at Afton Canyon (between Barstow and Baker) is called Lake Manix. Here our group is once again standing on an old beach ridge that formed at the east end of Lake Manix, where the Interstate 10 offramp to Afton Canyon exists today. Note the abundant, flat, rounded, skipping-type pebbles in the foreground. Frank Hanna is describing the history of Lake Manix.
Here at the head of Afton Canyon we are looking at the yellow-green clay deposits of Lake Manix (arrow), that have since their origin been mostly eroded away after the dam was breached and eroded. Deposits below the ledge at the level of Frank Hanna's hat and George Dunne's nose are river and alluvial fan deposits that crept out across the lake basin when there was no water in the lake. As the lake filled during one of the glacial stages, the surrounding rivers were forced to deposit their sediment in deltas around the edges of the lake and the center of the lake received only clay deposits. Above the lake deposits, not seen in the picture, is another set of river deposits marking the draining or evaporation of the lake and the advancement of the surrounding rivers into the center of the playa. After four days in the field, we had all learned a great deal and owe a big thank you to George Dunne, Bob Howard, and Frank Hanna, the field trip leaders.