Southgate Road - Danville Entrance
Northgate Road - Walnut Creek Entrance
Summit Road - Junction Ranger Station to Summit

Southgate Road - Danville Entrance

0.00 (Odometer reading)
Set odometer to "zero" at entrance to Southgate Road as you turn north off of Diablo Road. For the first half mile you will be crossing non-marine strata of Pliocene and upper-Miocene age. The beds are almost vertical and you will be traveling 90 degrees to the strike of the beds, progressing over older strata as you proceed north past the Athenian School.

These beds are the same age as those containing the Blackhawk Ranch Quarry mammal fossils. They have been mapped as "Blackhawk Ranch Formation" (Crane), "Green Valley" (Savage), "Tassajara including Green Valley" (Jones, Brabb), and Sycamore Fm (Anderson). They have also been placed in the "Contra Costa Group" by some and treated as "Orinda equivalent" by others.

Up ahead you can see the light tan stratified rocks of the Eocene "Domengine" at the top of the ridge.

0.6
The rounded hills to the right with scattered oaks (in the housing development) is mapped as Diablo Formation by Crane. Most authors continue to treat as "Green Valley" or "Tassajara". The Diablo represents the transition from non-marine to marine in this area.

You will continue in the Diablo Formation for about four-tenths of a mile, past the first sharp turn to the left until you reach the second sharp turn to the left.

0.95
Here you take a second sharp turn to the left. Almost all of the maps show this as the upper boundary of the Neroly Formation. According to Crane, a basal tuff (in the Diablo Formation) overlies the Neroly at this location. The Neroly sandstone and shale is brownish in color along the roadcuts and contain rich fossil clam beds, well exposed farther east in Sycamore Canyon. Although non-marine in some areas, the Neroly has been recognized as marine in this area.

1.15
Boundary Gate to park. You will traverse eastward (and occasionally northward) and begin your climb up the south flank of Mt. Diablo. You will continue to travel over increasingly older Neroly for the next one-third mile. The Neroly topographic expression is as rounded hills -- grass covered with oak trees prominent.

2.2
Final turn north in the Neroly brings into view the steep (almost vertical) chaparral covered slope of the Briones Formation just ahead.

2.45
As you make a sharp turn to the right, you will begin the serious climb up the south side of Mt. Diablo, driving along the strike of the Miocene Briones Formation. The shallow marine sandstone beds are not only almost vertical here, they are in fact slightly overturned, which means the "underside of the beds" (facing south) is younger than the "topsides" (facing north). The beds frequently contain marine fossil shells. The Briones Formation as used here includes beds mapped as Cierbo Formation by some geologists.

As you climb up the Briones "wall", the difference in the character of the topography and plant communities between the Neroly and Briones formations is very apparent. The Neroly is represented by the adjacent grass and oak covered rounded hills to the south, which stand in sharp contrast to the steep chaparral covered slope of the Briones to the north.

3.65 This is a rather spectacular overlook across Sycamore Canyon to the "hogback ridges"plunging down the western edge of Blackhawk Ridge into the canyon. Immediately to the left beside the road are corresponding beds forming Fossil Ridge. There are several beds with fossils exposed here, but the park does not allow "pull outs" except in an emergency. I suggest you proceed to the South Gate Entrance Station just ahead and get permission to park there briefly and walk back down to the overlook. These "hogbacks" represent the basal (oldest) beds of the Briones Formation. Hogback ridges plunging down Blackhawk Ridge into Sycamore Canyon. These rocks are Upper Miocene in age, about 15 million years old.

As you walk (drive) back to the entrance station you will be in an underlying siltstone unit, with pebbly siltstones and conglomerates. Less resistant to erosion, this interval of Miocene beds forms a characteristic grassy "saddle" between the overlying Briones, and the underlying Mid-Eocene "Domengine". This lower mid-Miocene unit is often mapped as "Monterey" - representing deposition near a shore line not far to the east of Mt. Diablo (James Hill, 1983).

3.80
South Entrance Station. After passing the entrance station, you will be entering the mid-Eocene "Domengine Formation". These massive tan colored coarse sandstone beds are interpreted as Sierra derived sands deposited as flow sediments (turbidites) on a westward deeping paleo-slope in fairly deep ocean. Graded bedding and "flame structures" argue for this type of deposition and the presence of radiolaria indicates depth. The rocks exhibit a wide variety of erosion features. As we now travel northwest away from the entrance station, the deep valley to the north (right) is formed by the West Fork of Sycamore Creek. The rocky slope across the valley is Domengine as well.

It is interesting to note that after passing the chaparral covered Briones, we now find that the gray pine has become a prominent tree. They seem to thrive in the sandy soils of the Domengine.

4.4
This rocks on the right are part of the "Boy Scout Rocks" group popular with climbers from the valley below. Several hundred feet ahead on the left is the massive monolith called "Gibraltar Rock".

4.65 Rock City. Restrooms. Picnic areas. Many short trails for exploring. If you use your imagination, you will be able to tell how this rock got its name - Elephant Rock. There are many interesting erosional features in the sandstone rock around Rock City.

4.85
After passing the rangers house on the right, you will be driving over rocks that are less sandy and more shaly then the massive sandstone of Rock City. During the next 1/2 mile, you will notice the trees give way to more open grassland. There is an interesting Turritella marker bed in this part of the Eocene section. Ivan Colburn has identified the fossils that make up this marker bed as "Turritella uvasana aedificata Merriam" (Colburn -1961). Although Colburn maps this section as "Domengine - member 2", some geologist use the name "Meganos formation" for this early Eocene unit.

5.45
Curry Point. There is a pull out here at Curry Point that provides a view of the distant Sierra on a clear day and the summit of Mt. Diablo to the north. The mountain is capped by rocks of the Franciscan Assemblage that have been thrust southeastward over the Cretaceous Great Valley shale exposed in the east-west aligned valley immediately north of Curry Point. The large boulder lying just above the valley floor on the lower slope of the mountain probably slid down from the in-place Franciscan higher up the mountain. Looking north from bottom to top - the valley contains outcrops of Great Valley shale of upper Cretaceous age. The Cretaceous beds are bounded on the north by the Mt. Diablo Thrust Fault. The fault trace on the surface is mostly covered by landslide deposits. Above the thrust fault is Franciscan melange material and farther up the hard resistant greenstone and chert forming the rocky crest of Mt.Diablo Peak and North Peak. The protruding rock to the east of Mt. Diablo summit is Devils Pulpit, a resistant chert outcrop.

After leaving Curry Point, you will be driving west, parallel and just above the contact between Upper Cretaceous Great Valley rocks (in this case a brown colored shale) to the north and the younger Eocene "Meganos" (lighter colored sandy shale) on which you are driving.

6.1
Near this point, you will be crossing over the Cretaceous/Eocene contact. This contact is a major unconformity with a hiatus (interval of time represented by the missing rocks) of about 25 million years where 15 million years of Upper Cretaceous rocks and the entire record of Paleocene history is missing. That unconformity marks the boundary between the age the dinosaurs and the age of mammals

6.2
Here you turn to cross the open valley to the north.

6.4
Mount Diablo Thrust Fault. After passing a sign that says "2000 Feet", you will notice on your right a slight dip in the surface of the road bank with a sign that says "Authorized Vehicles Only" and "Heliport" (this is the second "Heliport" sign). If you examine the road bank (looking east), you will notice that to the left of the "Authorized Vehicles Only" sign you will find blocks of Franciscan chert and greenstone. To the right of the "Heliport" sign, down the road about 20 to 30 feet, you will notice that these rock types are missing, finding instead an exposure of brown shale in the road cut . These two rock types, Franciscan to the north and Upper Cretaceous Great Valley to the south are separated by the Mt. Diablo Thrust Fault.

From this point to the Junction Office, you will be driving in Franciscan melange, where large blocks of typical Franciscan rocks are exposed along the side of the road. The red-brown soils are characteristic of the iron-rich greenstones that make up much of the Franciscan on the mountain. Red radiolarian cherts, along with lesser amounts of shale and graywacke are also exposed.

7.0 Junction Office - End

Northgate Road - Walnut Creek Entrance

0.00
Set odometer to "zero" at the North Gate entrance kiosk. For the next 1.6 miles you will be traveling over Upper Cretaceous Great Valley sedimentary rocks made up mostly of shale with some sandstone beds. The topography consists of low rounded hills cut by the Little Pine Creek running parallel to the road. Beds dip steeply to the southwest.

1.6
Enter park at this point. Continue traveling on Upper Cretaceous rocks. The top of the hills to the right (west) is Eocene in age.

2.0
As you turn left and then right crossing a stream gully, you will notice gray-green rocks along the left side of the road cut just before you cross over the gully. The gray-green rock outcrop itself is a badly weathered exposure of serpentinite, forming the western most edge of the long serpentinite band that runs prominently east-west across the mountain. The fault (Coast Range Fault) that separates the serpentinite from the Franciscan is probably expressed topographically by the east trending gully at this point. The fault plane dips down and to the north.

The serpentinite is separated from the upper Cretaceous Great Valley beds (on which you have been traveling) by another major fault, the Mt. Diablo Thrust Fault that cuts off further westward extension of the Coast Range Fault at his point almost at right angles as it trends roughly north and south (the Coast Range Fault trends east-west).

As you cross the gully and round the next bend (again driving briefly on the Upper Cretaceous shale), you will notice a large rock outcrop ahead on the left side of the road. It is at this point you once again cross the Mt. Diablo Thrust Fault (Cretaceous Great Valley to the west, Franciscan to the east) and will continue in the Franciscan from this point on. The large imposing rock outcrop is a greenstone outcrop (not serpentine).

A prominent outcrop of red chert.

As you continue to drive south, keep an eye out for the typical assemblage of rocks you find in the Franciscan Complex - greenstone (basalt), red radiolarian chert, graywacke, and shale along with a few blocks of dark blue schist. In this area, these varied rock types are enclosed in a melange. Franciscan soils are typically reddish brown in color. The Mt. Diablo Thrust Fault continues to run parallel to and west of the road, separating Franciscan rocks on the east from Cretaceous Great Valley rocks on the west.

3.2
Camel Rock. As you approach 3.2 miles, watch for Camel Rock - a rock outcrop that (with the help of a little imagination) resembles the outline of a single hump camel. You will first see it ahead and up a short slope.

3.35
Just past Camel Rock, you go over a rise of a hill and you will notice ranch building ahead. The prominent ridge across the valley to the west is made up of lower Eocene age sandstone and shales while the valley below on the west side of the Mt. Diablo Thrust Fault is Upper Cretaceous in age and part of the Great Valley Group. The Mt. Diablo Thrust Fault itself runs along the slope about half way between the road and the creekbed. The hills are relatively treeless, typical of an unstable soil such as a melange.

4.4
As you round a bend at this point you will notice a pile of rocks ahead. This rock formation is called Turtle Rock. It is made of radiolarian chert and from this perspective doesn't look much like a turtle. Better views later.

4.8
Turtle Rock (behind the gate to "Diablo Ranch" on the right). We are now traveling on material formed by a large landslide. This material also encloses blocks of Franciscan rock types and looks a lot like melange topography and it is often difficult to distinguish the two land forms from each other.

You will be traveling on landslide material containing large blocks of Franciscan rocks up several major switch backs for about 1 1/2 miles. Look back as you go and you will see that Turtle Rock is aptly named (shell to the south and head to the north).

6.4
By the time you reach the ranger's house, you will be out of the landslide and back into Franciscan melange topography.

6.65
Notice the small blue black boulder in the gully on the left side of the road about 100 yards before you reach the Junction Ranger Station. This is a "blue schist". This sodic amphibole schist is common in the Franciscan melange and is named for the noticeable blue color of the mineral glaucophane. Blue schists are largely altered basalt and reflect a history of hi-pressure/low- temperature metamorphism and on Mt. Diablo are about 165 million years old (younger than the greenstone, but older than the graywacke).

Junction Office - End

Summit Road - Junction Ranger Station to Summit

0.00
Set your odometer to "zero" when you turn up Summit Road at the Junction Ranger Station. The entire drive to the summit will be in the Franciscan Assemblage. Watch for the characteristic exposures of Franciscan type rocks including red cherts, greenstone (basalt), and a few outcrops of graywacke and blueschist. The soils are typically red-brown in color due to the high iron content of the parent rocks. Between here and near Toyon Picnic area, the drive will be through Franciscan melange.

1.2
To the left you will pass a prominent exposure of red radiolarian chert. If you pull off at the turnout, you will also be able to look out over the ridges of younger rocks to the south. As you look south and southwest, the valley below contains Cretaceous rocks bordered on the north by the Mt. Diablo Thrust Fault and on the south by younger Eocene age rocks that include the yellow-tan sandstone exposed around Rock City and Castle Rocks. The more distant row of ridges (Fossil Ridge, Blackhawk Ridge, and Oyster Point) are Miocene in age and are quite fossiliferous.

If you look to the north, you will see the rugged "chert/greenstone" capped summit of Mt. Diablo.

1.5
At about 1.5 miles, you will leave the "melange" part of the Franciscan and the chert and greenstone outcrops become more frequent.

2.0
Around 2.0 miles, on the right side of the road and continuing for another 0.2 miles, is a major (and typical) outcrop of sheared and broken greenstone (basalt).

2.35
Pull out on left at Juniper Camp Ground. Good views to the west. You will be looking out over the San Ramon Valley and the towns of Danville, Alamo and Walnut Creek (left to right). The Calaveras Fault runs down the valley. The ridge on the far side of the valley is Las Trampas Ridge and is made up of Miocene age rocks similar to the Miocene age rocks on Mt. Diablo including highly fossiliferous sandstone beds.

3.3
Another good exposure of greenstone, highly sheared with slickensides visible.

3.6
As you make a sharp turn to the left, you will see more chert and greenstone rocks toward the north. These rocks are resistant to erosion and are responsible for the rugged topography. The red rock on the right as you pull out of your turn is thin bedded radiolarian chert with interbeds of reddish shale. Continue to the summit.

4.5
Summit parking lot. I suggest you visit the summit museum open Wednesday through Sunday from 11 am until 5 pm. Excellent geology displays as well as animal and plant dioramas. Gift shop.

If time, I also suggest a short walk along the Summit Fire Interpretive Trail. It is a level path around the summit beginning at a trailhead near the picnic table where the loop part of the summit roads comes together near the lower parking lot. A quarter mile walk along the northside to the overlook platform will provide an opportunity to inspect greenstone, graywacke, red chert, and a small amount of shale up close. The views to the north are spectacular on a clear day. The quarry you see to the north is quarrying diabase (part of the Mt. Diablo Ophiolite) for crushed rock used in the construction of road beds, etc.

Reproduced from Geology of Mt. Diablo - A Training Manual

MDIA is part of the TRAIL THROUGH TIME Project that plans to construct an interpretive trail from the base of Mt. Diablo near Blackhawk to the summit of Mt. Diablo. There will be panels to provide information on the geology and landscapes along the way.

If you would like to be involved in the Trail Through Time Project in the fundraising, trail construction, or interpretive aspects of the project, please contact me. I also invite your comments and suggestions concerning the Road Log article above. I want to assure accuracy on the geological presentation. Please send your comments to Roi Peers.