California Geological Survey California Division of Mines and Geology
Bulletin 182
Geologic Guide to the Merced Canyon and Yosemite Valley, California
ROAD LOG 3
EL PORTAL TO WAWONA TUNNEL AND A CIRCUIT OF YOSEMITE VALLEY, CALIFORNIA*
By DALLAS L. PECK
U.S. Geological Survey, Menlo Park, California
CLYDE WAHRHAFTIG
University of California, Berkeley, California, and
U.S. Geological Survey, Menlo Park, California
and FRANK C. CALKINS
U.S. Geological Survey, Menlo Park, California
*Publication authorized by the Director, U.S. Geological Survey.

EL PORTAL TO THE EAST PORTAL OF THE WAWONA TUNNEL

FIGURE 1. Some of the features seen from the east portal of Wawona Tunnel.

FIGURE 2. Sketch map of the Yosemite Valley area, California. (click on image for a PDF version)
Mileage
0El Portal (Standard Oil Company Service Station). The contact between the Calaveras formation (late Paleozic) and the granitic rocks of the Yosemite area trends due north a few hundred feet east of here. The marginal intrusive rocks include coarse diorite and some norite. Farther east is the V-shaped gorge of the Merced River, which, although glaciated during two pre-Wisconsin stages when the glaciers extended about one mile below here, was not glaciated during the Wisconsin. El Portal was formerly the terminus of the Yosemite Valley Railroad. For reference see the sketch map of the Yosemite Valley area (fig. 2), the generalized geologic map (Calkins and Peck herein, fig. 1), and the glacial map (Wahrhaftig, herein, fig. 5).
3.5Arch Rock Entrance Station, National Park Service.
3.6Arch Rock. Two large fallen blocks are in contact at the top but are separated at the bottom by enough space for passage of the old road. Talus of Arch Rock granite† is exposed in a quarry on the north side of the road. The granite contains sparse inclusions of an unidentified darker-gray rock similar in appearance to the granodiorite at the Gateway.

†For descriptions of this and other rock units in this section see paper by Calkins and Peck in this guidebook.
4.7Elephant Rock is straight ahead.
6.0Junction with the Coulterville Road, the first road into Yosemite Valley (completed as a toll road on June 17, 1874).
6.2Wildcat Creek. El Capitan granite is exposed at the falls just west of here.
6.5Cascade Creek. El Capitan granite (probably Cretaceous) is exposed in a nearby cliff, and some large fallen blocks of it can be seen from the road.
8.4Junction with the Big Oak Flat Road. The original road, which lies farther up the slope, was completed one month after the Coulterville Road. At the road junction are exposures and talus of the older diorite (described as "diorite of the Rockslides" by Calkins and Peck). Here the diorite contains light-colored aggregates consisting mainly of plagioclase that probably formed as the result of metamorphism by the nearby El Capitan granite.
9.4Turn right across Pohono Bridge.
10.4Turn right on Wawona Road.
12.2Stop in parking lot at the east portal of Wawona Tunnel. View to the east of El Capitan, Sentinel Rock, Cathedral Rocks, the hanging valley of Bridalveil Creek, and Bridalveil Fall (fig. 1). Nearby exposures of various granitic rocks and of diorite.
Photo 1. North wall of the Yosemite Valley above the Church Bowl. Light colored, nearly flat-lying dikes of coarse pegmatite and Half Dame quartz manzonite intrude Sentinel granodiorite.

Photo 2. Sentinel Cascade and its alcoves. Spalling of sheets of granodiorite around the cascade has enlarged the alcove and steepened the cliff over which the cascade plunges. Photo by U.S. National Park Service.

Photo 3. Cathedral Rocks. Light-colored dikes of Bridalveil granite intrude a maze of older granitic and diaritic rocks. Photo by U.S. National Park Service.

The abundance of joints in the diorite in the opposite valley wall (directly north of here) contrasts strongly with their scarcity in the massive cliffs of El Capitan and the Cathedral Rocks (composed mostly of El Capitan, Bridalveil, and Taft granites, all probably of Cretaceous age). The construction in the valley between El Capitan and the Cathedral Rocks may be due to the massive nature of the granitic rocks at this point. The great abundance of talus in the cliffs directly north of here, in contrast to the paucity of talus farther up the valley, is due to the close jointing of the diorite of the cliffs.

The U-shape of Yosemite Valley, in contrast to the V-shape of the gorge of the Merced below El Portal, is well displayed here. The bottom of the U, however, is much flatter than in typical glaciated valleys. According to Gutenberg, Buwalda, and Sharp (1956, pl. 5), the bedrock surface lies almost 1,000 feet beneath the floor of the valley between El Capitan and Cathedral Rocks, and what we see is essentially a plain floored by lake sediments.

The top of the highest glacier in Yosemite Valley, according to Matthes (1930), reached about to the brow of El Capitan, and was about 300 feet above the top of the Cathedral Rocks. The glacier swept around the flank of Sentinel Dome, but did not cover the dome. The upper 700 feet of Half Dome, likewise, was unglaciated. These domes owe their form to concentric spalling of massive unjointed rock, not to glacial erosion.

The steep lower course of Bridalveil Creek above Bridalveil Fall is graded to the level established by the Merced River during the most recent of three distinct stages of preglacial erosion (from oldest to youngest, the Broad Valley, Mountain Valley, and Canyon stages of Matthes, 1930, p. 45-50); hence it helps to define the amount of glacial erosion in Yosemite Valley.

The V-shaped form of the gorge of the creek, although typical of stream erosion, is preserved because the sloping walls of the gorge coincide with throughgoing joints in the otherwise nearly unjointed rock. The upper part of Fireplace Creek, a little downstream from us on the opposite wall, is graded to the Mountain Valley stage of the Merced Canyon. Ribbon Creek, above the head of Ribbon Fall (which cannot be seen from here, but can be seen on the north wall of the canyon from places a mile or two down the road), is graded to the Broad Valley stage of the Merced (see Wahrhaftig, herein, fig. 4).

The blasted rock face at the west end of the parking lot exposes a complicated mixture of diorite and El Capitan granite. The porphyritic phase of the Taft granite is well exposed on the slope just to the west. El Capitan granite along the south side of the road contains blocks of partially assimilated diorite, and has a steeply dipping foliation.

Turn around and drive east along the south side of the Valley, across Sentinel Bridge and to the Ahwahnee Hotel.


CIRCUIT OF YOSEMITE VALLEY

0Ahwahnee Hotel. The route is plotted on the sketch map of the Yosemite Valley area (fig. 2).
0.3Entrance to Ahwahnee Hotel, turn right.
0.5STOP 1. Church Bowl. View across valley of Glacier Point. In the cliff face west of Glacier Point, note the contrast between unjointed Half Dome quartz monzonite (probably Cretaceous) below, and jointed granodiorite above. In the center of the valley south of here as much as 2,000 feet of glacio-lacustrine debris overlies the bedrock (Gutenberg, Buwalda, and Sharp, 1956; see Wahrhaftig, herein, fig. 5). In talus and in cliff faces on the north side of the valley at this stop dark-colored Sentinel granodiorite is cut by gently dipping light-colored dikes of coarse pegmatite and Half Dome quartz monzonite at the margin of the large body of Half Dome quartz monzonite. In some of the dikes of quartz monzonite unequal concentration of dark minerals produces a nearly horizontal layering.
0.7STOP 2. Parking lot at Yosemite National Park Headquarters. Visit to Museum.
1.7View of Sentinel Rock across valley. Along the cascade of Sentinel Creek to the right (west) of the rock, can be seen recesses caused by spalling of the granitic rocks around the cascade. Enlargement of the recesses appears to be developing vertical waterfalls from this sloping cascade.
3.0View of El Capitan straight ahead.
Photo 4. The Yosemite Valley from Valley View. At left is the great cliff of El Capitan. at the right the Cathedral Racks and Bridalveil Fall. In the foreground is Bridalveil Meadow, which is underlain by almost 1,000 feet of Pleistocene glacio-lacustrine deposits. Photo by U.S. National Park Service.

Photo 5. Ribbon Fall and its alcove. The edges of cracks along which great sheets have spoiled can be seen near the head of the alcove. Photo by U.S. National Park Service.

Photo 6. Road cut through the terminal moraine on the south side of Yosemite Valley at the base of Cathedral Rocks. The large boulder at the right-hand end of the cut is of Cathedral Peak granite, the nearest exposure of which is 13 miles away. Photo by U.S. National Park Service.

Photo 7. Southeast face of El Capitan. The irregular boundaries of an intrusive body of diorite form a crude map of North America.
3.9Junction at north end of El Capitan Bridge: continue straight ahead. Brief stop for view, across valley, of Cathedral Rocks, Cathedral Spires, and Taft Point. On the faces of North and Middle Cathedral Rocks can be seen nearly horizontal light-colored dikes of Bridalveil granite cutting a maze of other granitic and dioritic rocks.
4.6STOP 3. (At V-7 sign.) The low ridge to the left is the easternmost of a series of Wisconsin terminal moraines which held in the former Lake Yosemite. The lake was filled with as much as 300 feet of silt and sand deposited on advancing deltas by the Merced River and Tenaya Creek; this debris was possibly supplied by glaciers of Tioga age, which reached only as far downstream as the lip of Nevada Fall and the upper part of Tenaya Canyon (see Wahrhaftig, herein, fig. 5). Walk south along the old road at the east side of the moraine to the Merced River, a distance of about 800 feet. In the moraine at the river are exposed boulders of Cathedral Peak granite, Half Dome quartz monzonite, Sentinel granodiorite (probably Cretaceous), and Bridalveil granite. The nearest exposure of Cathedral Peak granite is 12 miles east of here. Return to the main road and continue west.
4.9Turnout. Brief stop for view of Bridalveil Fall, Leaning Tower, and Cathedral Rocks across valley.
5.8Valley View turnout. Brief stop for view of El Capitan, Clouds Rest, and Sentinel Rock to the east and Cathedral Rocks and Bridalveil Fall to the southeast. The western most of the Wisconsin terminal moraines lies a few hundred feet west of here.
6.0Turn left across Pohono Bridge.
6.5View of Rockslides across valley.
6.7Road again crosses the westernmost Wisconsin moraine.
7.0Junction with Wawona Road; turn right.
7.1STOP 4. Turn left into parking lot. Walk along trail (about 1,000 feet) to viewpoint at base of Bridalveil Fall. Near viewpoint are fallen blocks of Leaning Tower quartz monzonite (probably Cretaceous), Bridalveil granite, diorite, and El Capitan granite. On the cliff face at the lip of Bridalveil Fall can be seen a thick horizontal sheet of smooth-weathering Bridalveil granite. Underneath is reddish rough-weathering Leaning Tower quartz monzonite, and to the east is dark diorite. Note that the lower part of Bridalveil Fall is in a slight recess flanked by buttresses. The borders of the recess are marked by the edges of slabs parallel to the surface, apparently the original continuations of these slabs across the face of the fall have dropped from the cliff and are represented by the cone of talus extending up the cliff to an apex just west of the base of the fall.

Looking across the valley to the recess in which Ribbon Fall lies, one can see similar spalling in the upper part of this recess. Return to parking lot and drive east along the south side of the valley past the turnoff to Pohono Bridge. Bank on south side of road exposes bouldery terminal moraine.
7.9Road crosses the moraine examined at Stop 3.
8.4View of El Capitan across valley.
8.8Junction with road to El Capitan Bridge. View of Sentinel Rock to the east, directly up the road. The forms of Sentinel Rock, Glacier Point, the north face of Half Dome, etc., are controlled by vertical joints trending east to northeast.
9.0Brief stop at V-33 sign for view of El Capitan and Three Brothers across the valley. On the face of El Capitan the irregular boundaries of an intrusive body of diorite form a crude map of North America. This cuts an inconspicuous westward-sloping dike of gray rock, probably Leaning Tower quartz monzonite. The form of the Three Brothers is controlled by joints dipping obliquely westward.
Photo 8. The Three Brothers. The west-dipping joints that give these monuments their characteristic shape are emphasized by a light fall of snow. Photo by U.S. National Park Service.

Photo 9. Half Dome from Glacier Point. The precipitous northwest face is bounded by the wall of a nearly vertical fissure. The rounded back of the dome was farmed by exfoliation of massive quartz monzonite. Photo by U.S. National Park Service.

Photo 10. Glacial polish and slickensides on north wall of Yosemite Valley near Mirror Lake.
10.6View of Yosemite Falls across the valley.
11.1Road junction at south end of Sentinel Bridge. Continue east, not crossing bridge. South of here are remnants of the Old Village, the center of commercial activity in the valley between the late 1850's and 1917.
11.2Brief stop for a view to the east of North Dome, Half Dome, Royal Arches, Washington Column, and Glacier Point. The form of the Royal Arches, Half Dome, and North Dome is controlled by exfoliation of the Half Dome quartz monzonite, resulting from expansion due to unloading brought about by denudation.
11.5Road junction; continue straight ahead.
12.2Road crosses Happy Isles Bridge.
12.7Road crosses Wisconsin moraine. View of North Dome straight ahead.
12.8Road junction; bear right.
13.1Road junction; bear right.
13.5Road crosses rock avalanche that dammed Tenaya Creek, forming Mirror Lake.
Photo 11. The Royal Arches, North Dome, Washington Column, and Basket Dome, from near Glacier Point. These monuments are carved from a nearly joint-free mass of Half Dame quartz monzonite. Thin exfoliation slabs can be seen near the top of North Dome. The Royal Arches can be seen in this photograph to be the edges of giant exfoliation sheets farmed on the southwest side of this joint-free mass. Photo by U.S. National Park Service.
13.8STOP 5. Parking lot at Mirror Lake. View of Mirror Lake, and of Tenaya Canyon to the east. Walk along trail to the northeast about 1,000 feet to see glacial polish and striae on Half Dome quartz monzonite. Note well-formed books of biotite in this rock.

Return to parking lot and drive west.
14.5Road junction; bear right.
14.7"Indian Cave," north of road, is in coarse talus at the foot of the cliff.
14.9View ahead of sheeting in the Royal Arches, formed by exfoliation.
15.3Sugar Pine Bridge.
15.7Road junction; bear right.
15.8View ahead of flat-lying dikes of Half Dome quartz monzonite and pegmatite in Sentinel granodiorite.
15.9Entrance to Ahwahnee Hotel; turn right.
16.2Ahwahnee Hotel.

References

Gutenberg, Beno, Buwalda, J. P., and Sharp, R. P., 1956, Seismic explorations of the floor of Yosemite Valley, California: Geol. Soc. America Bull., v. 67, p. 1051-1078.

Matthes, F. E., 1930, Geologic history of Yosemite Valley: U. S. Geol. Survey Prof. Paper 160, 137 p.


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Last Updated: 03-Aug-2009