Stop # 2 Bonneville Shoreline Overlook

From this point turn and look directly to the south across the valley toward the flat terrace just above the houses. Also look further down across the canyon towards the west, where you can see another flat terraced area. These flat sections are ancient terraces carved by Lake Bonneville. The lowest flat portion is actually a portion of the highest which has been displaced by movement along the Wasatch fault. This terrace was developed between 16,000 and 14,500 when Lake Bonneville was at its maximum size covering nearly 20,000 square miles and was almost 1000 feet deep. This terrace is called the Bonneville terrace or Bonneville level. Above the LDS temple to the south, along the base of Y mountain, the Bonneville terrace can be clearly seen. It is the high level ground upon which many homes have now been constructed. If you had been standing here 15,000 years ago, you would have been wading on the Lake Bonneville shoreline.

About 14,500 years ago a natural dam close to the Utah-Idaho border failed catastrophically. Because of the failure of this natural dam, the lake level was lowered rapidly as water drained into the Snake River Plain. The lake level dropped by about 350 feet before it stabilized again and began to form a new lakeshore and a new terrace. The main BYU campus is built on this lower lake terrace, called the Provo terrace or Provo level. It is quite flat and drops off steeply to the south and west into the Provo River valley. The Provo terrace was developed from 14,500-12,500 years ago.

Goodbye Lake Bonneville

Beginning about 12,500 years ago the earth’s climate began to change and enter the present interglacial period. This climatic change brought an increase in temperature and a decrease in precipitation that caused Lake Bonneville to gradually decrease in size. Today the remnants of this giant lake are Utah Lake and the Great Salt Lake. Utah Lake is drained by the Jordan River and thus does accumulate salt like the stagnant Great Salt Lake has.

Recent Faulting

Now turn and face directly towards the west, looking at the road you drove up on, 2300 North. Look on the north side of 2300 North for the access road which leads to the U.S. Forest Service Fire & Heliport station. Just past this entryway to the east you can see a sharp break in the slope of the land. Turn to the geologic map, you can see this break represented on the map by a black line running north-south, with short teeth pointing to the west. The helicopter port is on the east side of this line. This break in slope can be quite easily seen by looking at the change in slope of the fence surrounding the U.S. Forest Service Facility, which can be seen as you are driving out of the canyon.

This break in slope is called a fault scarp, and was formed by earthquakes on the Wasatch fault zone. Most major breaks in the Earth’s surface, like the Wasatch fault, do not occur as simple, single fault planes; rather, these major breaks occur as zones of multiple planes of breakage. Look back again at the geologic map. Included on the map is a cross section of the Rock Canyon area. A cross section is a view of the interior of the Earth along a line; it is like the view you would see if you sliced down through the ground along that line and then looked at the side of the slice. The Wasatch fault zone can be clearly seen as a zone of several faults near the mouth of the canyon.

Wasatch Fault Facts and Figures

The Wasatch fault zone is part of the deformation that has occurred due to the extension of the western U.S. during the last 25 million years (Phase III in the history of Rock Canyon). During this time period, the rocks on the east side of the Wasatch fault have been uplifted an average of about 0.5 to 1.0 mm/year relative to the valley on the west. The valley has been filled with sediment shed from the mountains. This valley fill is between 10,000-15,000 feet deep near the mountain front. The Wasatch fault is composed of about 10 segments; each one somewhat disconnected from the others, so that an earthquake occurring on one segment of the fault does not trigger earthquakes on other segments. Major earthquakes (magnitude 7.0 or greater) occur about every 2,000-3,000 years on each fault segment or about every 200-300 years somewhere on the fault zone. The last major earthquake centered on the Wasatch fault zone occurred nearly 500 years ago.

Shattered Quartzite

Directly behind you is further evidence of the immense forces created by movement along the Wasatch fault. This is the plane in the fault zone along which most of the movement has occurred. This main fault plane is located approximately where the bedrock of the mountain rises up from the unconsolidated valley sediment. You are looking at white, bleached outcrops of Tintic Quartzite (refer to geologic map). The quartzite has been crushed and fractured into pieces by movement along the fault.

Rock Canyon Creek

Now turn your attention directly west again. Follow the creek to the grassy park region. This is Rock Canyon Park. It was built after the flooding occurred in 1983. It was built as an emergency catchment basin, in the event that Rock Canyon Creek reached unusually high levels. Rock Canyon Creek has its source in the form of run-off and multiple springs. In 1965, the city of Provo re-routed Rock Canyon Creek halfway down the mountain into pipes which emptied into large storage tanks. This water was used for culinary purposes throughout the city. The remnants of these old pipes can still be seen in many places while walking up the canyon. Since then new underground perforated collection pipes have been installed. These pipes collect the water from 15 naturally occurring springs throughout the canyon. The springs are monitored each year to determine the cleanliness of the water. About 5% of Provo’s drinking water comes from these springs. Further up the canyon sits a small building, this is used to chlorinate the water before it is channeled into the large water storage tanks located near the mouth of the canyon.

Any water not diverted through the perforated pipes is allowed to flow down the creek bed. Eventually this stream reaches the park area and is diverted into the storm system which flows down 2620 North. This water continues to flow through the storm system until it is diverted to the Provo River, and eventually flows into Utah Lake.

Alluvial Fans

Turn back to the geologic map at the end of this guide. Notice the large, yellow, fan shaped area near the mouth of the canyon. This is called an alluvial fan. This occurs as the creek cascades down the mountain and then suddenly flattens out. This change of slope is accompanied by a loss of energy in the creek. Once a flowing creek loses energy, it loses its ability to transport coarse sediment. Sand and gravel are deposited by the creek, eventually blocking its flow. The creek diverts its path around these deposits and continues this cycle further downstream. This cycle of diversions and blockage creates the fan shape. This is very similar to the formation of a delta, the difference being, a delta is deposited in a body of water rather than on land.

Follow the Bonneville Shoreline Pathway back to the main path, and continue up the canyon. At this point take time to notice the angle of the bedding planes in the bedrock. The formations are lying virtually horizontal on top of each other. Shortly, this will all change.

Up ahead on the north side of the canyon you will be able to see a large water tank which is fenced in, follow the small trail which leads up to the tank. This trail crosses the creek again, so if you are unable to cross continue to Stop #4. Hike to the north side of the tank and make some observations on the olive brown rock unit directly behind the tank.