At 6,862 feet, Wrightwood has the distinction of being the highest point on the San Andreas Fault, California’s largest rift zone which runs from the Gulf of California toward San Francisco.

 

Responsible for two of California’s largest tremors — the 1857 Southern California quake and the famous 1906 San Francisco earthquake — the San Andreas Fault is about 650 miles long. From Wrightwood, it continues in a southeasterly direction through Lone Pine Canyon toward San Bernardino.

 

Following is an updated two-hour driving tour of some of the fault’s most noticeable geological features. It was prepared by the Big Pines Visitors Center.

 

1. Big Pines Divide

(Tour starting point; near the intersection of the Angeles Crest Highway 2 and Highway N4: adequate parking.)

 

Big Pines Divide, situated directly on the San Andreas Fault, is a dip in the mountainous terrain created by the grinding action of the fault. This grinding action accelerated the rate of erosion and caused a saddle to be formed. The Divide, which is the location of the Big Pines Visitor Center, is the highest point (6,862 feet) along the entire length of the fault. The large rock tower, just to the west of the Visitor Center, marks the exact location of the San Andreas’ highest elevation. The U.S. Geological Survey marker is located next to the tower.

 

2. Road Cuts

(Highway N4; 0.7 miles northwest of the Visitor Center; parking very limited).

 

As you drive along Highway N4, you are on the edge of the San Andreas Fault zone. You may notice a white powdery substance visible inthe road cuts. This substance is a rock material that has been crushed by the shearing action of the fault. Because of its very fine consistency, some people have called this substance “fault flour.” It is a very unstable material, and during the winter it often becomes saturated with water and slumps down onto the road.

 

3. Appletree Campground

(Highway N4; 2 miles northwest of the Visitor Center; excellent parking).

 

A short hike to the east of Appletree Campground will bring you to an excellent view of the San An-dreas Fault zone. To reach this overview, park at Appletree Campground and walk about 1⁄2 mile up the old dirt road at the eastern end of the parking area. Along the south side of this dirt road you can see evidence of vertical uplift in the form of several rock outcroppings.

 

By examining these outcroppings you will find some that are covered with a red varnish, commonly called “slickenside.” This coating is the result of chemical changes caused by heat and pressure from fault movement. When you reach a large water tank, you can view examples of fault flour slumping and sliding into the canyon on the far side. Beyond the water tank, at the end of the dirt road, is the location of the most recent fault activity in the area.

 

Here a scarp, or steep embankment, probably a result of the 1857 earthquake, breaks the mountain slope. This scarp has been eroded by the creek and exposes a rock material known as “fault gouge.” Fault gouge is severely crushed granite reduced to a clay-like substance. This exposure of crushed rock is probably representative of the rock conditions existing below the surface in this part of the fault.

 

4. Jackson Lake

(Highway N4; 2.7 miles northwest of the Visitor Center; excellent parking in the picnic area).

 

Jackson Lake is actually a fault sag, or a small area of land which sank below the surrounding terrain as a result of fault movement alongthe San Andreas zone. The lake itself was created in 1916 when the basin was flooded by the building of a dam across Mescal Creek. Water  is collected in the lake from rainfall, spring snow melt and underground springs.

 

Across the highway on the north side, just opposite the lake, is a small lateral fault to the San Andreas. It is visible from the road and provides a good exposure of crushed rock. From Jackson Lake, the San Andreas Fault continues past the Devil’s Punchbowl through the community of Littlerock and toward San Francisco.

 

To continue the fault tour you will need to retrace your route back to the highway intersection of Big  Pines Divide. At the divide, check your odometer and turn west (right) on the Angeles Crest (Highway 2);  Inspiration Point will be two miles  ahead on the left.

 

5. Inspiration Point  

(Highway 2; 1.9 miles west of theVisitor Center; excellent parking).

 

As you approach the top of the ridge, watch for the Inspiration Point parking area on the left side of the highway. From this magnificent vista you can see strong evidence of the Punchbowl Fault. The east fork of the San Gabriel River is visible as it comes north and then makes an unusual split into opposite directions, up Vincent Gulch on one side and the Prairie Fork on the other. The branching of the river is caused by the Punchbowl Fault which cuts through the mountains at this point. Surface water, flowing along the more easily eroded  course of the fault, has created the  Vincent Gulch and Prairie Fork  canyons.

 

6. Vincent Gap

(Highway 2; 5.0 miles west of the Visitor Center, excellent parking).

 

Vincent Gulch Divide, or Vincent Gap, provides a good view directly into the Punchbowl Fault zone. The gap is located precisely on the fault and was formed by the grinding action of the fault. The fault line here is marked by brilliant red sedimentary rock which contrasts sharply with the dull gray schists of the area. These red rocks have been shifted many miles within the fault zone to their present location.

 

From this viewpoint, the steep narrow gorges of the San Gabriel Mountains can be seen, indicating recent uplifting in the area. The Vincent Thrust Fault, the oldest known fault in the San Gabriel range, is also visible as it dips south and west in an irregular pattern toward the East Fork of the San Gabriel River.

 

To continue the tour, you will now need to return to Big Pines Divide. From there you will continue east on Highway 2 toward the village of Wrightwood. Again, check your odometer at the Visitor Center to determine the appropriate distance.

 

7. Swarthout Valley

(Highway 2; 1.3 miles east of the Visitor Center; limited parking)

 

As you leave Big Pines Divide heading toward Wrightwood, you will quickly find yourself in the long, narrow Swarthout Valley. About 0.4 miles beyond the Mountain High East Ski Resort, you will find adequate parking on the left side of the road. From this vantage point the straight linear nature of Swarthout Valley can be seen.

 

The San Andreas Fault runs through this valley and separates two entirely different types of rock on each side. The south slope is made up of a series of shales and

siltstones changed by heat and pressure. The north slope consists of a group of weathered and decomposed granites. Both types of rock are about 500 million years old and were brought in contact with each other through the horizontal movement of the fault.

 

As you look across the road toward Mountain High, you will notice a swampy, sagging area covered with willows and other water-loving plants. At this location ground water is forced to remain close to the surface because a zone of crushed rock within the fault prevents percolation into the earth.

 

8. Wrightwood Village

 

(Highway 2; 3.5 miles east of the Visitor Center.)

 

The village of Wrightwood is located on the fault zone at the eastern end of Swarthout Valley. As you travel through the center of Wrightwood, you can see a large landslide scar high on the southern ridge above town. This is the Heath Canyon mud flow which was indirectly caused by long term activity of the San Andreas Fault.

 

Over a long period of time, the rock on the ridge was repeatedly fractured by fault action and became very unstable. During the spring of 1941, a rapid snow melt saturated the rocks until the surface layer became too heavy to adhere to the steep slope. A large mud slide washed down the slope and buried several houses and roads. Mud flow activity in Heath Canyon has continued to occur each year but on a much more modest scale.

 

From this point in Swarthout Valley, the San Andreas Fault continues in a southeasterly direction through Lone Pine Canyon and toward San Bernardino.

 

Source: Big Pines Visitors Center