Geology Field Trip to Death Valley, California, Spring 1995
Stop 1 Walker Lake
Part of the history of the Great Basin are the Ice Age lakes. The most recent ice age began one million years ago and lasted until about 20,000 years ago. From here we can see Walker Lake, a remnant of one of the largest. During this time the terrain was much the same as now (the normal faulting had already occurred 30 million years ago) but the climate was different. There was more moisture, less evaporation and the valleys contained a hundred lakes. One of the largest was Lake Walker, the predecessor to the current lake. Against the base of the hills we can make out the “bathtub rings” of the old lake levels. Water levels went up and down but when lake levels were stable, the water eroded little notches in the surrounding hills. There is also a sediment history of these lakes so there was lots of sediment dumped into these lakes. Up the road today is a gravel plant extracting the old lake gravels which were deposited at the edge of the lake. At this same time period Death Valley was Lake Manly. There is evidence in caves at the south end of this lake of human habitation as well as bones of giant ground sloths, camels and sabertooths. The fossil pollen of spruce and pine indicate that there was forest down to the edge of the lake with many animal inhabitants. Starting about 10,000 to 11,000 years ago the area dried out and some plants adapted and survived and others, such as trees, disappeared.
Stop 2 The Sump
The rocks in this basin are known as the Esmerelda Formation, composed of mudstone, siltstone, sandstone and volcanic ash. These record the earliest stages of the extension of the Great Basin 30 million years ago. This was the start of the stretching and thinning of the crust, resulting in the land dropping and basins forming. Rocks were washed into them from the high rocks on either side of the basin. Volcanic activity was part of this extension so each basin filled with mud, sand, ash and lava flows. The Sump is a typical example of this. This formation is 8 to 10 million years old (Miocene) and contains some fossils. Before the Sierra Nevadas rose this was a subtropical grassland. The orange spots we see scattered about are the remains of tree stumps on the basin floor. The white layers are volcanic ash. This area was like the Serengeti Plain in Africa today with lots of water and animals, such as camels, horses, pigs and beardogs. The volcanic ash shows different degrees of welding, which is temperature related. The biotite that occurs in ash allows the dating of the sediments. Death Valley was also like this area at that time.
From the top of the hill we can see a good view of the Esmerelda Fm. This Miocene formation stretches all the way to Benton CA. Geology is a discipline which benefits from “viewing deeply”—seeing what is represented by what we can see. There were no volcanoes here, but they erupted elsewhere and the ash settled here. There were two eruptions at least since there are mudstones in between ash layers. The mudstones show evidence of herds of animals and predators. There were rodents and rabbits. This was a huge open basin with the animals' lives occasionally interrupted by eruptions of ash. The small pillows of brown we see are old tree stumps which show patches of exhumed forests. The landscape has always been changing and the old basin isn't here anymore and the mountains are.