Rocks
The most prominent rock types within Joshua Tree are metamorphic rocks and igneous rocks.
The metamorphic rocks started forming as early as the middle Proterozoic era. originally called the Pinto Gneiss, the rocks became widespread along the Pacific margin of North America due to the Mesozoic tectonic arc. today, the four sub-units of the Pinto Gneiss within Joshua tree are known as the Joshua Tree Augen Gneiss, the meta-sedimentary suit of Placer Canyon, the augen gneiss of Monument Mountain, and the meta-sedimentary suite of Pinkham Canyon. |
Landformssome of the most common landscapes found within Joshua Tree include Arroyos, Playas, Alluvial fans, Bajadas, and Pediments.
The Arroyos in Joshua Tree are flat floored stream channels (usually dry throughout the year) surrounded by high vertical walls. these streams can sometimes cut as deep as 50 meters into the ground floor. Arroyos are usually form during unusual climate changes; the most current landforms being from around 1865 to 1915 (Vogt) The Playas in Joshua Tree are dry lakes that only contain water a few times a year. it is formed over thousands of years due to the rocks being washed away eroding into dust. as a result, the dusty landform can only grow creosote bush. |
The Alluvial fans in Joshua tree are sediment deposits formed around the bases of mountains and canyons, and are common in arid regions. the fans are created from streams that deposit alluvium along the valley floor, which allows it to spread around a broad area, giving it the fan shape. One well known fan is the Lucy Gray fan, which spreads from the base of the Lucy Gray Mountains into the Ivanpah Valley north of the Mojave. Many believe that Alluvial fan development started at the end of the Ice Age.
Bajadas in Joshua Tree are groups of Alluvial fans that formed together to create a broad slope of sediments at the base of a mountain. |
Pediments
Pediments are a common type of landform found in the southwestern parts of the United States. They take on the same appearence as Bajadas, but they are bedrock surfaces (homogeneous crystalline rocks like granite), created from erosion. they generally have a very light slope, ranging from around a-half-a degree to 6 degrees. Pediments also can become covered with a small layer of gravel, however if the layer of gravel is over 10 feet, the Pediment is classified as a bajada. Observers have to pay close attention to these gently sloped landforms in order to classify which they are viewing.
Pediments are formed by the retreat or erosion of a mountain front. When the mountain front retreat, it leaves behind a monumental area of planed bedrock surface (the surface shows the direction the mountain retreated to). the excess debris , minus alluvium and gravel, are washed/eroded away by various agents such as rill wash, sheet-floods, winds, and lateral planation by streams.
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InselBergs & Boulder -Mantled slopes |
Many believe that the origin of pediments within the southwest are related to Inselbergs. Inselbergs are steep residual hills and mountains rising from erosional planes (Trent), characteristic to the desert and formed during the Tertiary and Quaternary periods.
Another type of landform common in Joshua Tree are boulder-mantled slopes. Boulder-mantled slopes. Boulder mantled slopes are the pre-eroded form of inselbergs. The whole process started with varied spacing within the joints of rocks about 20 million years ago during the Triassic period of the Mesozoic era. As time passed on into the Pliocene Epoch, water had percolated through the space between the joints and decomposed large portions of the rock . The slopes themselves developed during the later part of the Pleistocene Epoch(Trent), not because of water, but air. If we fast forward to the present, most of the boulder mantles' rock in higher elevations were decomposed. Many of the slopes have crumpled into Grus and turned into inselbergs.
Another type of landform common in Joshua Tree are boulder-mantled slopes. Boulder-mantled slopes. Boulder mantled slopes are the pre-eroded form of inselbergs. The whole process started with varied spacing within the joints of rocks about 20 million years ago during the Triassic period of the Mesozoic era. As time passed on into the Pliocene Epoch, water had percolated through the space between the joints and decomposed large portions of the rock . The slopes themselves developed during the later part of the Pleistocene Epoch(Trent), not because of water, but air. If we fast forward to the present, most of the boulder mantles' rock in higher elevations were decomposed. Many of the slopes have crumpled into Grus and turned into inselbergs.