While driving along I-15 through the Wasatch Front Megalopolis (see previous post), there are some interesting features. The most obvious is the steep front of the Wasatch Range to the east. This mountain range was uplifted along a large, active fault at the base of the mountains termed the Wasatch Fault (which has some interesting geologic hazard aspects for this rapidly growing area). There are two other interesting features along the front the mountains. One is that prime housing developments (and some fancy golf courses) are very common on a series of flat surfaces (terraces) along the mountain front (see figure below). These have great views of both the mountains and the valley below and so are prime real estate. The terraces are also mined for gravel to fuel development. Terraces are extensive extending along the entire Wasatch Front and beyond around the edges of the Great Salt Lake Valley. The largest and most complex terraces are associated with large canyons or prominent ridges extending out from the range front. What are these features and what can they tell us about the paleoclimate of the Wasatch Front and the role humans play in modifying the Earth’s landscapes?
To answer that question we need to go back to the mid-late 1800s and examine the work of one of the West’s most famous Geologists, Grove Karl Gilbert. In 1890, G.K. Gilbert (as he is mostly known) published a report on a large prehistoric lake that filled the Great Salt Lake basin thousands of years ago. This work was based on extensive field work he and explorers before him had done in the Great Salt Lake basin. He named the prehistoric lake they discovered, Lake Bonneville, after explorer Benjamin Louis Eulalie de Bonneville (1796–1878), and identified a number of features formed by the lake. At it greatest extent it was over 500 km long, 200 km wide, and 300 m deep filling the Salt Lake Valley. Gilbert found evidence for Lake Bonneville in the terraces seen along the mountain fronts (figure below). He recognized that the basin was filled with a much larger lake during the last part of the geologic epoch called the Pleistocene.
These terraces and other shoreline features (deltas, spits, tombolos and barrier bars) established that the lake existed for a long time at several different elevation or stands. It remained at some particular elevations for long periods of time (hundreds to thousands or years–first figure below) forming large deposits of sand and gravel. Research since Gilbert’s pioneering work, has dated these stands and established the geologic history of Lake Bonneville and contemporaneous lakes throughout the Great Basin to the east of the Wasatch Range (second figure below). Lake Bonneville and contemporaneous paleo-lakes in the Great Basin are called Late Pleistocene lakes because they were at their high stands during the last part of that epoch. The Late Pleistocene encompasses the last major continental ice sheet advance from about 126,000 year ago to 11,700 year ago. The North American ice sheet reached its maximum southward extent (termed last glacial maximum or LGM) during the Late Pleistocene. The timing of the LGM, about 30,000-17,500 years ago encompassing the time of the high stands of Lake Bonneville. By 15,000-11,500 the Earth was warming and moving into the present interglacial, the Holocene (starting about 11,700 years ago and extending to the present).
The vast extent of Late Pleistocene lakes in the Great Basin and the huge size of Lake Bonneville in particular (and Lake Lahonton on the west side of the Great Basin), show that the climate was very different about 20,000 years ago compared to now. Some researchers think that rainfall in the region needed to be from 140-280% of present values to get high lake stands, while evaporation was likely about 30% lower due to decreased temperatures during glacial times (maybe as much as 5-10ºF). Annual mean precipitation for Salt Lake City is now 16 inches. Late Pleistocene precipitation would therefore be on the order of 23-46 inches, for a average of about 34 inches. The low end is about like that of present-day San Francisco, CA (21 inches), the high about like Tampa, FL (46 inches), and the mean pretty much like Seattle (38 inches). So, Salt Lake City in the late Pleistocene was a fairly wet place compared to now! Let’s now return to the Lake Bonneville shoreline deposits and look at how humans utilize those deposits and what that says about our ability to modify the landscape.
The higher precipitation in along the Wasatch Front lead to higher runoff. More flow in the streams eroded the surrounding mountains transporting sediment into Lake Bonnevile forming extensive shoreline deposits. Strong and persistent winds transported sediments along the shore forming the terraces, spits and beach ridges. Where rivers and streams entered the lake, deltas were formed. As the lake dropped sediment was spread out, forming the present-day deposits. These sediments are unconsolidated so easy to excavate. They are also very close to where all the building is taking place so easy to transport to building sites. So, they make building roads and structures along the Wasatch Front relatively cheap–there is nearly always a gravel bar close at hand to new development. These terrace deposits are the foundation of the Wasatch Front Megalopolis. From 1994-2007 (last date data is available), 287 million metric tons of sand and gravel were extracted in the region along the Wasatch Front (data from U.S. Minerals Information Service, see figure below). The reconstruction of Interstate-15 and associated federal highways in the run up to the 2002 Salt Lake City Olympics used over 12 million metric tons of sand and gravel. Construction of high-rise buildings, Olympic Villages and other associated structures used more, forming the large spike in the plot of sand and gravel mined from 1997-2000 (figure below).
Now, another boom in building is happening that is not yet completely captured by data provided by the U.S. Minerals Information Service because their last reported data is for 2007 production. In some places the excavation of sand and gravel has removed a substantial proportion of the deposits that formed over several thousand years along the shores of Lake Bonneville. Let’s look at one of those deposits, the Point of Mountain Spit.
Point of Mountain is a spectacular spot just south of Salt Lake City. A high, traverse ridge extends westward several miles from the Wasatch Range separating the Salt Lake Valley from Utah Valley at the Jordan Narrows. Strong shoreline currents carried sediment southward toward this spot, forming a gigantic spit of sand and gravel that extends (extended) from the Salt Lake Valley into Utah Valley. When Gilbert described this spit, it was an intact feature. In 1993 much of the spit was still enact and the shoreline terrace to the north was relatively undeveloped (figure below), but some gravel mining had destroyed the end of the spit (where I-15 curves around the ridge in the figure below), likely to build the first stages of I-15.
Twenty years later in 2013, the spit was completely transformed (figure below).
The entire end of the spit and much of the underlying gravels have been excavated in a huge gravel mine. Even the ends of the mountain ridge has been mined for rock. The shoreline terrace is nearly completely covered with housing developments (the undeveloped end is a paraglider park, so was saved). Much of the farm land in the valley has been transformed to housing developments and roads. This shows the accessibility of Lake Bonneville deposits to the demand for construction materials. Gravel from the spit goes right into adjacent roads, houses, business parks and shopping malls. The figure below is a closer and vertical view that better illustrates the magnitude of these excavations–all of the lighter areas are the sand/gravel/rock mines and roughly outline the previous extent of the spit.
Extensive Lake Bonneville deposits like the Point of Mountain Spit probably took from 500-1500 years to form (length of a stand in the lake and following drawdown). Humans have nearly completely excavated it in about 20 years. Or a rate of destruction 25-75 times faster than construction. This shows what a tremendous power direct human actions are in the modern world. Research looking at human actions at the global scale show a similar rate of “human erosion”, about 40 times geologic rates (see “Moving Dirt” in the post archives). Humans now have become the premier mover of material on the Earth’s surface, more efficient than all other geologic processes. The Wasatch Front Megalopolis is a prime example of that ability. In a little over a century we have come a long way in completely transforming a natural landscape that took many thousands of years to develop into a human construct. That is incredible power. Is the next stage Trantor?