EARTH SCIENCES CENTRE GÖTEBORG UNIVERSITY B193 1999

THE GEOMORPHOLOGICAL EVOLUTION OF TILTED BLOCK MOUNTAINS

- a case study from Sierra Nevada, California

Kerstin Ericson

Department of Physical Geography GÖTEBORG 1999

GÖTEBORGS UNIVERSITET Institutionen för geovetenskaper Naturgeografi Geovetarcentrum

THE GEOMORPHOLOGICAL EVOLUTION OF TILTED BLOCK MOUNTAINS

- a case study from Sierra Nevada, California

Kerstin Ericson

ISSN 1400-3821 B193 Projketarabete Göteborg 1999

Postadress Besöksadress Telefo Telfax Earth Sciences Centre Geovetarcentrum Geovetarcentrum 031-773 19 51 031-773 19 86 Göteborg University S-405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN

“The Ninth Symphony is the Yosemite of music! Great is Granite And Yosemite is its prophet!”

Rev. Thomas Starr King 2 Abstract

Landscape evolution is complex and extends over a long geological time period where temporal variations in crustal movement, eustatic levels and climate are established factors. Hence, these parameters will determine whether deep weathering, pedimentation, glacial, fluvial or eolian erosion etc. will be the dominating exogenic process. The aim of this study is to describe crystalline bedrock forms in order to genetically classify landforms and evaluate the relative importance of different processes for the present morphology according to this form-process concept.

The westward tilted Sierra Nevada batholith was studied owing to its well known and remarkable geological history. The batholith started to form during the early Mesozoic, 210 Ma, and the intrusions took about 130 Ma to complete. At the end of the Cretaceous period Sierra Nevada began its uplift from the sea. Hence periods of orogenic activity such as faulting, uplift and westward tilting succeeded each other during late Cretaceous-early Tertiary. In connection with the orogenic activity deep weathering and subsequent stripping and fluvial incision have remodeled the landscape. During Pleistocene major parts of Sierra Nevada was exposed to heavy glaciation.

The gross morphology in Sierra Nevada suggest a structurally controlled landscape exposed to deep weathering. Remnants of saprolites cover parts of the western side of Sierra Nevada and the slopes towards the Central Valley of California. The relief on exposed plateaus suggest that the major landforms in Sierra Nevada are inherited and preserved predating the Pleistocene glaciation. The glacial erosion has been confined to the more elevated High Sierra and has only remodeled the valleys. The study points at important steps in the geomorphic evolution of tilted block mountains which may be applicable to the evolution of formerly glaciated continental margins such as the Scandinavian peninsula and the North American Cordillera.

3 Sammanfattning

Landskapsutveckling är komplex och verkar över en lång tidsperiod där temporala variationer i jordskorpans rörelser, eustatiska nivåer samt klimatförändringar är etablerade faktorer. Följaktligen bestämmer dessa parametrar om djupvittring, pedimentation, glacial, fluvial eller eolisk erosion etc. kommer att bli den dominerande exogena processen. Ändamålet med denna studie är att beskriva de kristallina berggrundsformerna för att klassificera landformernas genes och värdera den relativa betydelsen de olika processerna har för den nuvarande morfologin enligt detta form-process begrepp.

Den västligt lutande Sierra Nevada batoliten studerades på grund av dess välkända och anmärkningsvärda geologiska bakgrund. Batoliten började formas under tidig Mesozoikum, 210 Ma, och intrusionerna tog cirka 130 Ma att fullfölja. Vid slutet av Krita påbörjade Sierra Nevada upplyftningen från havet. Under sen Krita-tidig Tertiär följde orogenisk aktivitet såsom förkastningar, uplift och västlig lutning på varandra. I samband med den orogena aktiviteten omformade bland annat djupvittring med påföljande stripping och fluvial nedskärning landet. Under Pleistocen var stora delar av Sierra Nevada utsatt för glaciation.

Stormorfologin i Sierra Nevada tyder på ett djupvittrat strukturkontrollerat landskap. Saprolitrester täcker delar av den västra sidan av Sierra Nevada samt sluttningarna ner mot Great Valley of California. Reliefen på exponerade platåer påvisar att de huvudsakliga landformerna i Sierra Nevada är nedärvda samt antedaterar den Pleistocena glaciationen. Den glaciala erosionen har varit begränsad till det högre liggande High Sierra och har endast omformat dalarna. Studien påvisar flera viktiga steg i den geomorfologiska utvecklingen av lutande bergsmassiv vilket kan appliceras på utvecklingen av tidigare nedisade kontinentala gränser såsom den Skandinaviska halvön och den Nordamerikanska Cordilleran.

4 CONTENTS

1 INTRODUCTION ...... 7

2 THE STUDY AREA - SIERRA NEVADA, CALIFORNIA...... 8

2:1 TOPOGRAPHY ...... 8 2:2 GEOLOGIC SETTING...... 8 2:3 PHANEROZOIC EVOLUTION...... 9 2:3.1 The origin and history of the present geomorphology ...... 9 2:3.2 Faulting, uplift and westward tilting...... 10 2:4 QUATERNARY EVOLUTION AND THE GREAT ICE AGE...... 11 2:5.1 Glacial reshaping of the landscape ...... 12 2:6 FORM-PROCESS RELATIONS...... 14 2:6.2 Bedrock forms related to deep weathering processes and stripping...... 14 2:6.3 Conditions favorable for glacial erosion...... 15 2:6.4 Bedrock forms related to glacial erosion...... 15 3 METHODS ...... 16

3:1 DEMARCATION OF STUDY AREAS...... 16 3:2 MAPS AND TOPOGRAPHIC PROFILES...... 16 3:3 FIELD MAPPING...... 17 3:4 MANUAL FOR FIELD MAPPING...... 18 4 RESULTS ...... 19

4:1 TOPOGRAPHICAL PROFILES OVER TIOGA PASS QUADRANGLE...... 19 4:2 TOPOGRAPHICAL PROFILES OF YOSEMITE VALLEY...... 21 4:3 YOSEMITE NATIONAL PARK – FIELD MAPPING...... 25 4:3.1 Yosemite Valley ...... 26 4:3.2 Yosemite Valley - weathering forms...... 26 4:3.3 Yosemite Valley - glacial forms ...... 27 4:3.4 Tamarack...... 27 4:3.5 Tamarack - weathering forms...... 27 4:3.6 Foresta...... 28 4:3.7 Foresta - weathering forms ...... 28 4:3.8 Foresta - glacial forms...... 30 4:3.9 Road sections in Yosemite National Park...... 30 4:4 HETCH HETCHY...... 31 4:4.1 Weathering forms ...... 32 4:4.2 Glacial forms...... 33 4:4:3 Roadside geomorphology between Hetch Hetchy reservoir and Poopenaut ...... 33 4:5 KINGS CANYON & SEQUOIA NATIONAL PARKS...... 34 4:5.1 Kings Canyon...... 35 4:5.2 Generals Highway ...... 35 4:5.3 Generals Highway - Silliman Creek...... 36 4:5.4 Moro Rock...... 37 4:6 KERN RIVER VALLEY ...... 38 4:7 BIG PINE LAVA FIELDS...... 39 4:8 STRUCTURE OF DIFFERENT FORMS...... 41 5 DISCUSSION...... 43

5:1 YOSEMITE VALLEY...... 43 5:2 STRUCTURE OF DIFFERENT FORMS...... 44 5:2.1 Valleys with vertical joint controlled walls: Yosemite Valley, Hetch Hetchy Valley, Kings Canyon and Kern River...... 44 5:2.2 Forms on plateaus outside Yosemite Valley, Hetch Hetchy Valley, and Kings Canyon ...... 45 5:2.3 Forms in and about Foresta: landscape with no recent glaciation ...... 45 5:2.4 Pediments east of the Sierra Nevada fault - Big Pine lava fields...... 46

5 5:2.5 Relative importance of different processes for the present forms...... 46 6 CONCLUSIONS ...... 49

ACKNOWLEDGMENTS ...... 50

REFERENCES ...... 51

FURTHER READING...... 53 APPENDIX ...... 54

6 1 Introduction

The evolution of denudational landforms/landscapes developed in hard crystalline rocks is often complex and extends over a long geological time period. In the long term perspective landscape evolution depends on temporal variations in crustal movement, eustatic levels and climate. Those parameters will in turn determine whether sediment burial, river incision, pedimentation, deep weathering, glacial erosion etc. will be the dominant process.

In unglaciated shield areas, such as Australia, the long term development of landforms can be quite well explained and dated by the existence of saprolites and cover rocks of different ages (Young 1983; Bird and Chivas 1993; Ollier 1995; Twidale & Campbell 1995). In glaciated shield areas, on the other hand, the preglacial regolith as well as landforms have been more or less completely eroded as a consequence of repeated glaciations during Pleistocene. However, the significance of glacial erosion has been re-evaluated during the last decade in formerly glaciated areas such as Scotland (Hall 1986; Hall & Sugden 1987), Canada (Bouchard et al. 1995) and France (Battiau- Queney 1997).

The evolution of passive margins formed by Mesozoic and Cenozoic uplift leading to rejuvenation and fluvial incision in the newly created continental scarp has been in focus during the last decades (Ollier 1982, 1991; Summerfield & Thomas 1987; Thomas 1995). Landforms that have been preserved in the scarp hinterland form elevated paleoplains where the sub continent of southern Africa as well as eastern Australia are excellent examples.

In contrary the evolution of similar features such as tilted block mountains has been less studied, however, their geomorphological uplift geometry share features with passive margins. Ollier (1991) points at the characteristically geomorphological evolutionary trend of tilted block mountains leading to major landscape rejuvenation in the uplifted parts and preservation in the less uplifted parts of the block. Hence, both passive margins and tilted block mountains may contribute to the understanding of the dynamic geomorphic system and denudation rates and patterns through time.

This work focus on the advantage of studying bedrock forms in order to genetically classify landforms and evaluate the relative importance of different processes for the present morphology in crystalline rock. This kind of work has been appreciated during the last decade in Sweden (Lidmar- Bergström 1989, 1997; Lidmar-Bergström et al. 1997; Olvmo et al. in press) as a useful tool in understanding the long term evolution of landforms as well as to understand the magnitude of glacial erosion.

In this study the geomorphological evolution of the Sierra Nevada batholith, which constitute of one of the most well known granite landscapes on earth, is studied by mapping characteristic landform assemblages. The study has taken place along a transect crossing the tilted block from west to east perpendicular to the uplift axis. The purpose is to define different landform zones, ranging from previously glaciated areas to areas with no connotations of ice ever operating on them. The study takes place on different elevations along the transect which in turn can be used to outline the main steps of landform development in this particular area. Furthermore an attempt is made to evaluate the role different processes has played for the present landscape.

7 2 The study area - Sierra Nevada, California

2:1 Topography Running half the length of California, Sierra Nevada is the largest single mountain range in the contiguous U.S. It is 80-130 km wide and runs almost 650 km in the north-south direction, covering nearly as much area as the French, Swiss and Italian Alps combined (Matthes 1930; Shelton 1966). The mountain range is strongly asymmetric with a steep eastern escarpment, with a dip of 25° at parts, and a gentle westward slope with a minor dip of 3° towards the Central Valley of California (fig. 1).

Fig. 1. Diagram of the tilted Sierra Nevada, Owens arrows show the direction of movement. Valley The height and slant of the range are exaggerated and streams are shown flowing in the general direction that Sierran streams flow. West of the range is the Central Valley of California filled with sediments derived from the mountains. Owens Valley is marked on the east side. (Modification after Matthes 1930). Regarding altitude the Sierra Nevada out-rivals all the other mountain ranges in the U.S. Not only is Mount Whitney, 4,417 m, the highest summit in the Lower 48 but the range as a whole stands higher above its immediate base than any other range. The peaks at the east base stands about 3,300 m above Owens Valley and at the west base they stand about 4,200 m above the Central Valley of California (fig. 1). This can be compared with the Rocky Mountains which stand only 2,700 m above the Great Plains (Shelton 1966).

2:2 Geologic setting Sierra Nevada is one of Earth’s grandest examples of granitic terrain. The batholith is formed by hundreds of granite intrusions, 70-200 Ma, ranging from less than 2 km2 to 1,300 km2. Furthermore, the granite plutons have sharp crosscutting relationships which indicate multiple injections (Bateman et al. 1963). The plutonic rocks of Sierra Nevada is built of five minerals: quartz, potassium feldspar, plagioclase feldspar, biotite, and hornblende (Huber 1989). This leads to a rock composition ranging from diorite and gabbro, quartz-monzodiorite, quartz-diorite, tonalite, granodiorite to granite.

Throughout most of the area the granitic rocks are jointed, commonly spaced from 0.6 to more than 3 m apart. At outcrop scale, three sets of joints are generally present, two nearly vertical, almost perpendicular to each other, and a third nearly horizontal creating approximately rectangular blocks. The joints are zones of weaknesses, a gateway for water and humic acids which may weather these hard and usually erosion-resistant rocks.

Remnants of metamorphic rocks can be seen along the eastern margin in the summit area and while approaching the western edge of the foothills (fig. 2), moreover, they are considered to be the oldest rocks within the Sierra, 440 million years old (Huber 1989). The rocks consist of schist, slate, quartzite, marble, calc-silicate hornfels, amphibolite and serpentine. Bedded and foliated bodies of

8 these rocks, 1.5-225 km wide and 7.5-400 km long, generally strike north-west and dip steeply to being almost vertical (Wahrhaftig 1965). The metamorphic rocks underlie approximately 15-20 per cent of the western slope of the southern Sierra Nevada below 2,700 meters. Furthermore, they are more resistant than the granitoids and generally rise above the immediately adjacent granitic terrain to form rugged, sharp-crested mountains with long even side slopes (Wahrhaftig 1965).

Fig. 2. Schematic section over the present geologic setting in California from the Pacific Plate defined by the San Andreas fault in the west to the Basin and Range area in the east. The Coast Range and the western edge of the foothills is built by metamorphic rocks. Remnants of metamorphic rocks cover some Sierran summits. Sediments derived from the mountains cover the Central Valley of California. Sierra Nevadas uplift and westward tilt is clearly defined via the steep east escarpment demonstrated by the Sierra-Mono fault on the eastern side of the mountain range. To be followed by the Basin and Range most westerly outpost, Owens Valley, where volcanoes still are active. Not to scale. (Modification after Hill 1975; Uppsala excursion guide 1980).

2:3 Phanerozoic evolution

2:3.1 The origin and history of the present geomorphology The geologic time-scale used in this study is presented in the appendix, page 53. In the Paleozoic the continents were joined together as one landmass and throughout most of the Paleozoic sea covered the surface that was going to become the Sierra Nevada. Consequently, thousands of meters of sediments such as clay, mud, sand as well as volcanic ash from submarine volcanoes settled on the bottom of the sea. In the early Mesozoic, about 210 Ma, the continents began to drift apart and the batholith that was going to become Sierra Nevada started to form. Hot, molten magma pushed the sediments aside transforming them into metamorphic rocks. However, it took about 130 million years to complete the granite intrusions and the subsequent cooling and the land did not begin its uplift from the sea until the end of the Cretaceous period, 65 Ma, (Bateman et al. 1963; Shelton 1966; Hill 1975).

Owing to its latitudinal position the Sierran landscape was subjected to subtropical climate with mean temperatures of 20° C during the latter part of Cretaceous until late Oligocene, 80-25 Ma, (Hill 1975). Geomorphic processes such as weathering and erosion could operate more swiftly and gradually the upper parts of the Sierran batholith was exposed. Due to the uplift parts of the granite core were probably still hot while the top parts were denuded. The result of the Paleogene denudation is a flat denudation surface with low topographical relief, today appearing as a summit level surface.

9 2:3.2 Faulting, uplift and westward tilting During the Miocene, 22-5 Ma, the movement between the North American plate and the Pacific plate turned to dextral strike-slip faulting, the San Andreas fault. At the same time began the western margin of the American landmass uplift (Hill 1975; Huber 1989). Parts of the Pacific plate slid beneath the North American plate in a subduction zone commencing partial melting of the Pacific plate which in turn caused melting of the lower parts of the North American plate. Extensive volcanism began and large areas in northern Sierra Nevada were covered by lava, ash and lahars. Due to subduction of the Pacific plate volcanoes have continued to erupt in Sierra Nevada to the present day.

During the Neogene (latter part of Tertiary) up to the present day Sierra Nevada has gone through repeated uplift events and westward tilting which has formed the present mountain range with the gently sloping west side and steep east escarpment (figs.1 and 2). Owing to the uplift and subsequent tilting, weathering and stripping accelerated during Neogene and Quaternary which in turn initiated renewed deep weathering and fluvial incision. Furthermore, most of the major streams in the Sierra Nevada follow their ancient river channels, incisions started prior to the uplift. The rivers flow westward into the San Joaquin or Sacramento rivers and hence to the sea while just a few rivers flow eastwards into Nevada.

Matthes (1930) argued that Sierra Nevada had gone through three major Tertiary uplifts intervened by pauses. Each uplift initiated a new cycle of erosion and produced a more pronounced landscape incision with a greater topographical relief (fig. 3).

A B CC B

Fig. 3. Bird’s-eye view over the development in Yosemite Valley. A. the first uplift during the Paleocene caused headworth growth of the Merced River and by the Miocene the broad-valley stage with meandering streams and a rolling surface of rounded hills was developed; B. the second uplift throughout Pliocene, the mountain-valley stage, steepened the stream gradients and deepened the valleys; C. the third and greatest uplift during late Pliocene-early Pleistocene developed the canyon stage and Merced River deepened the valley further. (After Matthes 1930).

Selected landforms identified: E Echo Peak ND North Dome LC Liberty Cap CR Cathedral Rocks C Clouds Rest TC Tenaya Creek SD Sentinel Dome LT Leaning Tower SM Sunrise Mountain IC Indian Creek G Glacier Point DP Dewey Point M Mount Maclure HD Half Dome SR Sentinel Rock RC Ribbon Creek L Mount Lyell BP Bunell Points SC Sentinel Creek BV Bridalveil Creek F Mount Florens LY Little Yosemite Valley EP Eagle Peak MR Merced River MW Mount Watkins B Mount Broderick YC Yosemite Creek R Royal Arches

10 BD Basket Dome CC Cascade Cliffs EC El Capitan W Washington Column By choosing Yosemite Valley as reference Matthes explanations of the stages are; The first uplift during the Paleocene epoch was slow moving, raising the Sierran crest about 1,200 m. Merced River drainage system evolved and by the beginning of the Miocene the broad-valley stage was developed (fig. 3A); The second uplift took place in late Miocene, adding 900 m to the crest. The Merced River deepened the valley with 210 m and the mountain-valley stage was developed by late Pliocene (fig. 3B); The third and greatest uplift during late Pliocene-early Quaternary added 1,800 m to the crest and the current heights were achieved. The Merced River deepened the valley further by about 390 m and the canyon stage was fulfilled (fig. 3C). The canyon-stage may also give a probable view over Yosemite Valley prior to the great Ice Age.

Matthes explanation over Sierra Nevada’s development has been challenged over the years, nevertheless, his thoughts remain and are still used to point out a possible course of event. Wahrhaftig (1965) demonstrated that the three surfaces are better explained by stepped topography, which could have developed simultaneously during late Cenozoic time primarily in the last 10 million years. Schaffer (1997) came to the same conclusion. However, he argued that the topography was initiated during the Late Cretaceous epoch and was pronounced by early Cenozoic time. Other studies (Christensen 1966; Huber 1981) have pointed out that the uplift began slowly and accelerated over time. Indeed the major uplift was followed by a continuous slower moving uplift that still proceeds with an approximate rate of 4 cm (1½ inch) per 100 years (Huber 1989). Anyhow, most present studies point out that Sierra Nevada’s long-term evolution includes two or three phases of uplift and a succession of late Cenozoic volcanism.

2:4 Quaternary evolution and the Great Ice Age During the last 2 million years throughout the Pleistocene until early Holocene, 10,000 years ago, parts of Sierra Nevada were repeatedly glaciated. It should be pointed out that these mountain glaciers had no connections with the great continental ice sheets to the north and east even though they acted at the same time. The conception of the Quaternary evolution in Sierra Nevada is built upon stratigraphy of glaciogene sediments as well as glacial geomorphology.

Table 1. Approximate timing of the mentioned major Quaternary glaciations of the western Sierra Nevada. Sherwin is one of pre-Tahoe glaciation’s advances. Modifications after Matthes 1930; Huber 1981; Ericsson & Gembert 1991 and Schaffer 1997.

Sierran glaciation Approximate age in years European correlation Tioga 35,000-13,000 BP Late Weichselian glacial maximum about 15,000-20,000 BP Tahoe glacial maximum about 60,000-75,000 BP Mid Weichselian (?) Sherwin, pre-Tahoe older than 700,000 years Menapia (?) Pre-Tahoe initiation 2.5 Ma BP Pretiglia

It is still unknown how many glaciations the Sierras went through owing to fragmentary remnants of older glacial deposits. Nevertheless, extensive field studies (Whitney 1869; Matthes 1930; Huber 1981; Schaffer 1997) have demonstrated that Sierra Nevada has undergone at least three major periods of glaciation, i. e. pre-Tahoe (oldest), Tahoe and Tioga (youngest) (Table 1 and fig. 4).

11 Fig. 4A shows how Yosemite Valley might have appeared during Sherwin, the largest pre-Tahoe glaciation, when the valley was completely filled with ice some 700,000- 800,000 years ago. Fig. 4C gives a probable limelight over the Tioga glaciation which had its maximum between 15,000- 20,000 years ago. Fig. 4B gives a glimpse of the valleys’ possible appearance after Sherwin, 700,000 years ago. When Sherwin withdrew it probably left behind the largest, deepest, postglacial Lake Yosemite ever, about 11 km long and up to 600 m deep (Matthes 1930; Schaffer 1997).

A B C

Fig. 4. Bird’s eye view of Yosemite Valley as it might have appeared during different ice ages; A. during Sherwin, the largest pre-Tahoe glaciation, 700,000-800,000 BP; B. after the Sherwin glaciation, 700,000 years ago; C. during the Tioga glaciation, 15,000-20,000 BP. See fig. 3 for identification of landforms. (After Schaffer 1997).

2:5.1 Glacial reshaping of the landscape There have been several discussions over the development in Sierra Nevada and Yosemite Valley in particular. Muir’s research from the middle of the 19th century and onward proposed that the glaciers had done most of the excavating in and about Yosemite Valley (1912, placing together earlier research from the19th century). Moreover, Muir suggested that glaciers once had completely covered the Sierra Nevada to the Central Valley and beyond. Whitney (1869) claimed that the overriding process creating Yosemite Valley was down-faulting followed by long-term stream erosion and that a glacier merely had occupied the valley, not widening it. Thus, glaciers may have transported debris out of the valley. Matthes (1930) gave both right to a certain extent claiming that the area had been glaciated, however, neither as heavy or vast as Muir stated nor as little as Whitney proposed. The glacial transformation of valleys in the central part, like Yosemite Valley and Tuolumne Valley, is highly variable and make a strong contrast to the undulating higher sierras. Glacial striae and other glacial forms indicate glaciation on some parts of the higher elevations, however the transformation has not been to the same extent as the valleys (Huber 1989; Schaffer 1997).

In fig. 5 present-day Yosemite Valley is illustrated (Huber 1989). Bear in mind Matthes earlier drawings of the valley (fig. 3), especially the canyon-stage, and compare them with Hubers’. It is clear that the succeeding glaciers have widened the pre-glacial valley by removing all the weathered material as well as mass wasted rocks. The glaciers have, as Whitney proposed, been acting as a cleansing agent sweeping the valley clear of all debris as well as having an erosive effect on the mountain sides, leaving a U-formed valley behind. However, the glaciers were relatively powerless when dealing with the massive granite monoliths in Sierra Nevada. Grand examples are El Capitan, Half Dome and Cathedral Rocks in Yosemite Valley. Matthes (1930) suggested that a large pre-

12 Tioga advance carried out most of the Quaternary glacial erosion in the lower and upper Yosemite Valley, 150 m and 460 m respectively. Huber (1990) concluded that the longitudinal profile of the Tuolumne Valley east of the Grand Canyon of the Tuolumne was primarily due to glacial erosion.

The U-formed valleys make a sharp contrast to the ”High Sierra”, a term coined by Whitney regarding the higher region of the Sierra Nevada most of it laying above the timberline. It is possible that most of the High Sierra was glaciated during pre-Tahoe stages and partly glaciated during the Tioga and Tahoe stages, however the relief features show hardly any signs of glacial erosion. Instead the High Sierran landscape is characterized by bare mountain areas where large scale sheeting plays a great part for the geomorphology, giving the multiple granite domes a character of undulating rounded hills, see the back parts on fig. 5. This Paleogene denudation surface share similarities with the “Paleic surface” in Norway thought to reflect weathering in a tropical climate (Gjessing 1967). In this thesis the High Sierras may at some occasions be referred to as the Paleic surface.

Fig. 5. Bird’s-eye view of present-day Yosemite Valley, selected landforms identified (Huber, 1989).

13 2:6 Form-process relations

2:6.1 Form-process relations This study is based upon the concept of form-process relations in landscape evolution. In order to understand the involved weathering and stripping processes a brief explanation over deep weathering and glacial erosion is taken in.

2:6.2 Bedrock forms related to deep weathering processes and stripping The climate plays a significant role in rock weathering because decomposition of rocks are favored in warm and humid climates. High temperature as well as good supply of water, e. g. high precipitation, favors biologic activity producing organic acids. The water transports humic acids through the cover soil towards fresh bedrock where the acids leach metal bases from the silicate lattice. Hence the weathering front, the advance of weathering into fresh bedrock (Mabbutt 1961), may proceed more rapidly in the humid tropics due to the high precipitation and high temperatures in that environment. It is suggested that the chemical weathering rate is four times higher in humid tropical conditions compared to higher latitudes or higher altitudes (Thomas 1994).

Deep weathering products such as clay rich saprolites (weathering mantles), core-stones, sheeting with straight joint controlled bedrock surfaces, and smoothly rounded sweeping forms, are used as connotations of chemical weathering in a former warm and tropical climate (Twidale 1993; Lidmar- Bergström et al. 1997). This can be compared with saprolites with a high gruss contents, and coarser rock surfaces which are believed to belong to colder but yet humid climates (Twidale 1993; Lidmar-Bergström et al. 1997).

Solid rock The different stages of weathering in heavily jointed granite bedrock is illustrated in fig. 7. Solid granite rock being exposed to warmth and chemical solution is susceptible to moisture Joints Weathered attack and decomposes between joints. This rock opens up pathways for more moisture and weathering can accelerate. Weathering does Core stones not advance universally, but may attack divided rock masses from all sides transforming sharp corners to rounded edges. Three surfaces weathering When the granite has transformed into saprolite corners rounded it will remain with the same structure as the

Fresh rock, no visible fresh bedrock had before weathering as long Two surfaces sign of rock material as the vegetation above is intact and protect weathering weathering the rock from erosion. The transition to fresh edges rounded One surface rock may also be gradual and no identifiable weathering surface will then separate the unaltered rock

Fig. 7. Weathering of joint blocks and stages in the from the weathered mantle (Thomas, 1994). formation of core stones (Huber, 1989). The duration of weathering in connection with the fracture system in the bedrock are the main causes to which core stones owe

14 their shape and size. The time deep weathering can act on the bedrock is also crucial for the saprolite formed. Thus, the top parts of the saprolite is more transformed into clay than the parts closer to the bedrock.

If the vegetation cover vanish the possibilities of marine, fluvial and/or glacial erosion of the saprolite increase. The weathering front can under these circumstances be exposed, i. e. stripped, leaving tors and core stones behind. Dismantling will inhibit further deep weathering since precipitation will drain off the exposed rock surface, besides, the water does not contain the humic acids necessary for continued deep weathering. Furthermore, dry granite is very stable and upon stripping of an area the landforms created will remain relatively unaltered because sub-aerial weathering is now the dominant factor until a vegetation cover, no matter how thin, has been reestablished and deep weathering can be revived (cf. Ollier 1991; Thomas 1994).

Saprolites are seldom found in former glaciated landscapes mainly depending on the powerful stripping effect of glacial erosion. There have been no deep weathering since the beginning of the Great Ice Age nor since the Tioga ice left Sierra Nevada (Hill 1975). Hence, the expansion or lack of deep weathering profiles or clay-rich saprolites in Sierra Nevada may give some answers regarding the ice’s effectiveness.

2:6.3 Conditions favorable for glacial erosion The variation in the intensity of glacial erosion is highly depending on the thickness of the ice, time and rate of glacial movement and the ground surface. Whether the ice has reached pressure melting point leading to a more erodable ice, or if the ice is frozen to the base with a less erodable ice as a result. The nature of the ground surface; topography; rock composition; jointing; chemical transformation of the rock; permeability as well as the composition of the regolith are all main factors of the surface beneath the glacier. Other erodable factors are the shape, abundance and hardness of the rock fragments contained in the ice at the base of the glacier. Thus, glacial erosion is more effective in the beginning of glaciation when the ground is softer and covered with saprolite. (Sugden & John 1976).

2:6.4 Bedrock forms related to glacial erosion The most prominent and eye-catching feature of glacial erosion in hard rock is abrasion and plucking of fractured and loosened rock fragments leading to the creation of roche moutonnées. However, glacial striae is of more vital importance since the loosened debris from a highly fractured rock have been detached for glacial transport and used as abrasive tools.

Sub-glacial melt water is a great manufacturer of softly rounded concave or trough shaped cross sections such as p-forms, longitudinal furrows and flutes (Dahl 1965; Benn & Evans 1998). However, the erosive power from sub-glacial melt water can above all be seen in the outstanding potholes.

15 3 Methods

3:1 Demarcation of study areas Field areas within Sierra Nevada were chosen from legal accessibility such as; National Parks: Yosemite, Sequoia and Kings Canyon (fig. 6); National Forests: Sierra, Sequoia and Inyo; as well as several road cuttings near state highways; 120, 140, 41, 49, 180, 198, J22, 155, 178, 190. In Yosemite National Park the areas Yosemite Valley; Tamarack; Foresta; and Hetch Hetchy areas were chosen for a more detailed survey (fig. 6). In Sequoia and Kings Canyon National Parks the survey was defined to Kings Canyon, Generals Highway and Moro Rock. In Sierra National Forest the study was restricted to road cuttings while the Kern River Valley was studied more closely in Sequoia National Forest. Big Pine lava fields, an area between the two towns Big Pine and Independence, was studied in Inyo National Forest.

Fig. 6. Location of the principal study areas in California, USA.

3:2 Maps and topographic profiles All data regarding maps are based upon United States Geological Survey, U.S.G.S., 15 inch quadrangle topographic maps at a scale of 1:24,000 and 1:62,500 with contour intervals of 40 feet and 80 feet, respectively. The maps used in this study are: Wawona, Mariposa Grove, Kinsley, El Portal, El Capitan, Half Dome, Tamarack Flat, Yosemite Falls, Tenaya Lake, Lake Eleanor, Hetch Hetchy Reservoir, Muir Grove, Giant Forest and Lodgepole.

Large scale maps were created to give a more lucid picture over the study areas. While converting elevation data from feet to meter together with the subsequent rounding accurate elevation might have been lost. However, the redrawn maps in this study will by no means claim to be appropriate for further work, they are just a pointer over the research areas locality. Thus, it is not advisable to reconvert the figures for use with the topographic maps.

In order to achieve an appropriate picture over Yosemite Valley’s change from U-valley shape in former glaciated areas to V-valley shape in areas with minor or none glacial influence fourteen cross sections over Merced River canyon were drawn at different locations. The distance on the profiles was set to five kilometers and notes in feet were taken down regarding the elevation above

16 sea level on every 100 meter. Hence conversions from feet to meter were calculated upon which the profiles were drawn.

To elucidate an area in the High Sierras that had been heavily glaciated during the Pleistocene glaciation topographical profiles, 10 km long, as well as a map over Tioga Pass Quadrangle were drawn. The procedure followed the above description.

While constructing the profiles the surface elevation of lakes and rivers has been used. Larger lakes (fig. 8-9 p. 19-20), as well as areas with meandering rivers (fig. 10-12 p. 21-23), will then appear as flat surfaces. This may be in bad congruity with reality regarding the lakes, however it was a simple way to illustrate the valley glaciers sculpturing effect.

3:3 Field mapping The intention was to study different morphological regions within the Sierra Nevada i. e. to evaluate the role of different processes for the development of major valleys, top surfaces and batholith margins. With respect to the Yosemite Valley, it was chosen for a more detailed study of former valley glaciers work on the granite domes. Upon it, Hetch Hetchy and Kings Canyon were used as comparative sites.

The original intention was also to make a comparison of landforms in former heavy glaciated areas on higher elevations. Unfortunately this could not be carried out owing to the extreme bad weather where three meters of snow covered the ground on elevations above 2,000 m. Instead Big Pine in Mono Valley, east of Sierra Nevada, was studied. Hence, a comparison of deep weathered landforms in an area exposed to volcanism could be accomplished.

The eight main areas with surroundings were mapped during 15 field days. Yosemite Valley, Hetch Hetchy and Foresta pretty thoroughly while the other locations were mapped more perspicuously. See paragraph 3:4, Manual for field mapping, regarding mapped landforms. Small, 0.1-10 m, to medium scale, 10–100 m, bedrock forms were classified either as glacial or non-glacial. The non- glacial forms were subdivided according to process of formation, e. g. surface weathering, etch forms or water induced forms. The glacial surfaces were used as a time reference and the other forms were classified as older or younger than the glacial surface.

In pursuit of weathered material, e. g. decomposed granite, road cuttings and primary rock with easy accessibility were examined. Road cuttings, core stones and other geomorphologic phenomena were documented by several photographs and drawings.

The found saprolites have just been visually inspected during field study. In order to facilitate dating of the weathered material a more thorough grain analysis is recommended. Upon it a comparison regarding the composition of other saprolites produced at different time periods and/or environments may be made.

A following-up field study regarding heavy glaciated areas on higher elevations is also recommended in order to get a full picture over glacial vs. deep weathered influence in the area.

17 3:4 Manual for field mapping During field work a modification of a manual for mapping landforms worked out by Lidmar- Bergström 1986, revised by Lidmar-Bergström and Olvmo 1995, was used (unpublished manuscript).

Following forms were mapped during field survey: Substratum: · Primary rock · Glacial till

Bedrock-forms: · Flat rock face · Rock hill

Primary forms - caused by rock type or cleavage: · Sheeting - pressure release fractures · Dome formations: cliff domes, dome formed rock hills · Block formations: cliff mounds · Columnar formations: (pseudo)rauk, tor(mounting)

Secondary forms - caused by weathering, glacial erosion or other erosion: · Sharp edges due to frost action or other cracking · Weathering induced forms: · edge rounded convex forms · rounded, gently sweeping forms · bulging, bending outwards, flared slope · mushroom forms · Glacial forms: · roche moutonnées · Wind polish, direction · Abrasion by running water/waves, potholes, polished surfaces, p-forms

Deep weathering: · Gruss weathering, with core-stones · Clay weathering, with core-stones

Surface weathering - above all exfoliation with rough surfaces: · Exfoliation, small-scale => large-scale · Spherical weathering, block weathering · Enlarged joints · Differentiated weathering: · weathered path-ways, depressions or dikes · rough pitted surfaces, stand-up quartz and feldspar crystals · Alveoli weathering: · tafoni, weathering hollows, tubular hollows · handle and knob forms · Frost weathering and transport of sharp edged blocks: · short transfer or clitter, widespread blocks · talus · sheeting-pressure release fractures

18 4 Results

The presentation of the results follow the evaluated different landform zones. The zones range from elevated areas with heavy glaciation, Tioga Pass, to areas with local glaciation, Yosemite, Kings Canyon and Sequoia National Parks, and finally leading to areas lacking glacial influence, Kern River Valley and Big Pine.

4:1 Topographical profiles over Tioga Pass Quadrangle The locations of the profiles over Tioga Pass Quadrangle are presented in fig. 9. The interpretation of the topographical profiles was made from larger profiles in juxtaposition with accurate topographical maps. Hence, differences in height and distance between the profiles appeared more clearly than presented in this study.

Mount Conness NW SE 4000 Mt Conness cirque

3700 Sheep Peake cirques

3400 trough valley

3100 Roosevelt

elevation, in meter Lake 2800 asl 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 distance, in meter

Roosevelt

NNE SSW 4000

3700 cirques cirque

3400 Ragged Peak cirque Roosevelt Lake Conness 3100 I I Creek elevation, in meter 2800 asl 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 distance, in meter

Fig. 8. Profiles covering an area north of Tuolumne Meadows, Sierra Nevada, showing how glaciers have transformed the landscape, see fig. 9 for location. ”Mount Conness” demonstrates how glaciers on different elevations have had a disparate influence on the landscape. ”Roosevelt” emphasize one valley glaciers length within an area, namely Roosevelt Lake, which is a former ice lake. The riverbed of Conness Creek, a recipient of glacial meltwater, can be noticed by the dip in the profile’s 6,500 m distance.

19 RP RP

LD

Fig. 9. Location of drawn profiles on Tioga Pass Quadrangle map. The straight line shows the ”Roosevelt” profile and the dotted line shows the ”Mount Conness” profile. Major glacial incisions are marked.

Selected landforms identified: SP Sheep Peake MC Mount Conness RP Ragged Peak LD Lembert Dome

The area has been heavily glaciated where glaciers have worked their way little by little into the mountain sides, the last ice leaving the area 10,000 years ago. What is striking is the disparate influence but yet congruent of the glaciers incisions. By following the contour intervals on the map one can tell how ancient valley glaciers have transformed the area. The general trend seem to be basins all along a northwestern-southeastern line which indicates an ice flowing from northeast towards south-west. Several cirque glaciers on the northeastern hillsides occupy former glacial

20 basins. There are about 70 tiny glaciers in the sierra today, almost all being cirques’, living wholly within basins carved by the giant valley glaciers of the Pleistocene glaciation. However, these tiny glaciers are not, as one might believe, remnants of glaciers from the Great Ice Age but remnants from glaciers that started to grow during the Little Ice Age, 1500-1850 (Hill 1975).

The profile Mount Conness (fig. 8) visualize how several glaciers on different elevations have transformed the landscape. The incision in the landscape has varied which may be depending on the bedrock’s mineral contents, the glacier’s size, and the elevation it was working on. The profile has contact with several ancient glacier basins of which the valley glacier by Roosevelt Lake has carved its way into the mountainside while more southeastern glaciers have made minor incisions on higher elevations. One possible cause could be that the western side of the line received the main portion of precipitation fallen while the eastern side lay in rain shadow. Hence, northeastern laying glaciers were just as cold but had less snow and therefore might have been frozen fast to the ground leading to minor incisions.

The profile Roosevelt (fig. 8) shows the extension of a former valley glacier which basin is now filled with the water of Roosevelt Lake. The glacial sculpturing of the landscape has been disparate and occurred during different glaciations. One glacier has deepened the canyon immediately south of Roosevelt Lake. Conness Creek’s riverbed, a recipient of glacial melt water, can be noticed by the dip in the profile’s 6,500 m distance. Following the profile from south to north reveals how an ice has carved its way around Ragged Peak. North of Ragged Peak the ice has filled the lowland as high as the base of the vertical cliffs. Conness Creek is the main recipient of melt water from higher elevations, the creek’s deep incision may indicate a pre-glacial riverbed. Some of the dip in the landscape may also be due to powerful tapping of melt water from former cirques, now ice-dammed lakes Young Lakes, northeast of Ragged Peak south of Roosevelt Lake.

4:2 Topographical profiles of Yosemite Valley

Fig. 10. Location of drawn cross sections/profiles in Yosemite Valley, the name of each profile is placed by the corresponding line.

21 The locations of the following profiles are presented in fig 10. The profiles in fig. 11 and 12 reveals how glacier tongues via two valley-beds, Tenaya Creek and Merced River, worked their way down and into Yosemite Valley (Tenaya Canyon, Half Dome and Sierra Point). Royal Arches shows where the two tounges joined and how they step by step transformed the valley to U-valley shape (Glacier Point, Eagle, Sentinel, El Capitan and Bridalveil Meadow). The ice seems to have ceased and become less erosive by Rainbow View, and Merced River’s incision could once again determine the shape of the valley floor, V-shape (Turtleback Dome, Elephant Rock, Big Meadow and Foresta). The westward tilting and Merced Rivers incision may be illustrated by following the elevations from east to west.

Tenaya Canyon Half Dome SSE NNW SE NW 2750 2750 2250 2250

1750 Creek 1750 Tenaya River

l Half Dome Merced 1250 1250 l 750 750 elevation, in meters elevation, in meters asl asl 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Royal Arches Sierra Point NE SW S N 2750 2750

2250 2250 Point Sierra 1750 1750 Merced River Merced 1250 l River 1250 l l 750 elevation, in meters

elevation, in meters 750 asl 0 1000 2000 3000 4000 5000 asl 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Glacier Point Eagle

SSW NNE SSE NNW 2750 2750 Point 2250 Glacier 2250 1750 1750 North Dome River Merced River 1250 l 1250 Merced 750 750

elevation, in meters asl elevation, in meters asl 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Fig. 11. Cross sections of Yosemite Valley, Sierra Nevada, see fig. 10 for location. The cross sections are drawn from east, Tenaya Canyon, where a glacier tongue came down into the valley, to Eagle, where the valley is perfectly U-shaped. Crossing l in the profile indicates Merced River’s location. Furthermore, by looking at the elevation the mountains gentle slope westward can also be notified.

22 Sentinel El Capitan

SE NW S N 2750 2750 El Capitan

2250 2250 Rocks Cathedral 1750 1750 River Merced River Merced 1250 l 1250 l El Capitan Meadow 750 750

elevation, in meters asl elevation, in meters asl 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Bridalveil Meadow Rainbow View S N SW NE 2750 2750 Point Crocker

2250 2250 View Rainbow 1750 1750 Merced River Merced River 1250 l 1250 l Bridalveil Meadow 750 750 elevation, in meters asl elevation, in meters asl 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Turtleback Dome Elephant Rock

SSE NNW SE NW 2750 2750 2250 2250

Turtleback Dome 1750 1750 The Cascades Elephant Rock

1250 Merced River 1250 Merced River l l 750 750

elevation, in meters asl elevation, in meters asl 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Big Meadow Foresta

ESE WNW SSE NNW 2750 2750 2250 2250 1750 1750 Big Foresta

1250 Merced River 1250 Meadow Merced River l 750 750 l elevation, in meters

elevation, in meters asl asl 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 distance, in meters distance, in meters

Fig. 12. Cross sections of Yosemite Valley, see fig. 10 for location. The cross sections continue from east, visualizing the canyon’s change from U-valley shape in former glaciated areas, Sentinel, to a more V-valley shape in areas with minor or none recent glacial influence further west, Foresta. Furthermore, by looking at the elevation the mountains gentle slope westward can also be notified. Crossing l in the profile indicates Merced River’s location.

23 Several glaciations have worked their way through Yosemite Valley eroding the valley sides filling the floor with glaciofluvial debris. The maximum extent of the Pleistocene ice cover (fig. 13) filled the valley to its brim reaching west of El Portal (Matthes 1930) further than fig. 10 goes. This is also the point where Merced River starts to meander more readily, indicating a river free to make its own incision in the bedrock i. e. not depending on an earlier over-deepening by glaciers.

The elevation on the profiles (fig. 11 & 12) reveal some of the relative relief and the westward tilt. A comparison between Tuolumne River and the inferred average trend of pre-glacial Merced River in fig. 13 will better illustrate the latter event. The longitudinal profile of Merced River (fig.13) propose that Yosemite Valley is almost entirely the result of earlier excavating glaciers. The valley floor stand in direct connection with the sudden steps the hanging valleys Vernal and Nevada Falls make up as well as Pywiack Cascade by Tenaya Creek. This should be compared with Tuolumne River which in spite of its recent heavy glaciation, Tioga, proceed with a more gradual climb to the higher Sierras.

The last glaciation, Tioga, was rather thin and did not reach further than Bridalveil Meadow during its maximum extent about 15,000 years ago (Huber 1989, 1990). While comparing the profiles with the maps and field studies the glaciers greatest erosive power has been between Royal Arches and Bridalveil Meadow (fig. 10). The glacial erosion west of Bridalveil Meadow ceased gradually until Foresta (fig. 10) where an almost perfect V-shaped canyon appear (limit of Tioga glaciation fig. 13). The valley is once again widened west of Arch Rock to form a meadow at El Portal (fig.13).

Yosemite Valley is cut 750-1,050 m below the older upland surface, the contrast between the two is particularly overwhelming by El Capitan which can be gathered by the steep cliff shown by the namesake profile (figs. 10 and 12). While standing by the rim of the valley on the top surface the view of the undulating Paleic top surface is revealed.

Fig. 13. Longitudinal profile of Merced River and its tributary Tenaya Creek, Tuolumne River is outlined for comparison. All profiles follow stream courses but ignore minor meanders. The Tuolumne plot is superimposed to intersect at the 1,200-m elevation which is the approximate elevation of the Merced-Tenaya rivers junction at the head of Yosemite Valley. The dotted line indicates the bedrock basin in Yosemite Valley. The dashed line indicates the inferred average trend of the pre-glacial Merced River without the Yosemite Valley basin, although some excavation below the line probably resulted from stream erosion prior to glaciation. (Modification after Huber, 1990).

24 4:3 Yosemite National Park – field mapping

Fig. 14. Geomorphologic map over Yosemite Valley and vicinity.

25 While reading the result of field mapping it is advisable to look at the field maps over Yosemite Valley and vicinity (fig. 14) and over the Hetch Hetchy area (fig. 26 p. 32) which shows the whereabouts of localized forms. A follow-up on U.S.G.S. 15 inch quadrangle topographical maps in scale 1:24,000 covering the study areas may also be recommended.

The field study over Yosemite National Park is divided into five parts namely Yosemite Valley, Tamarack, Foresta, Road sections and Hetch Hetchy (fig. 6 p. 16).

4:3.1 Yosemite Valley Yosemite Valley is geomorphologically characterized by an over deepened trough valley with sheer cliff walls. Truncated valley spurs bear evidence of a massive transformation and straightening by former glaciers which have cleansed the valley from weathered and spalled material. The adjoining top surfaces are structurally controlled exfoliation domes and selective erosion has determined the paths of glacier tongues forming hanging valleys from which waterfalls now cascade.

4:3.2 Yosemite Valley - weathering forms The east side of the valley by Mirror Lake and Tenaya Creek show no or minor signs of deep weathered profiles, still some not to deep rounded joints are noticed. The area surrounding the trail from Mirror Lake to Happy Isles, west of Glacier Point, is framed by an extensive boulder field with blocks in various sizes ranging from 0.3-3 m in diameter. Some of the larger boulders look like overthrown tor-like pillars with rounded edges and corestone like compartments. Others are erratics laying scattered on the valley floor whereas the main part are rocks spalled off from the walls. Quite a few of the larger blocks show thin exfoliation as well as differentiated postglacial weathering and some have p-forms on the top surface.

While following the trail from Happy Isles leading to Vernal and Nevada Falls (fig. 14) the bases and sides of the cliff walls are covered with large boulders creating a talus reaching Merced River. Higher up closer to Vernal Falls the almost vertical wall show signs of deep weathered joints. Between Vernal and Nevada Falls several cliffs have edge rounded convex forms. Below the waterfalls glacio-fluvial p- forms and potholes were found. The cliff walls in the valley are sheer but where fractured the corners are sharp showing signs of extensive spalling due to freeze-thaw actions. While trailing by the rock walls boulders in various sizes make up magnificent small Fig. 15. Bridalveil Falls, a hanging valley above and large talus where the Rockslides, west of El sheer cliff walls. Merced River with glaciofluvial Capitan, is the most powerful one. debris creating graded bedding in the foreground.

26 4:3.3 Yosemite Valley - glacial forms Apart from the valleys U-shape glacial influence is evident when observing all the hanging valleys, of which Bridalveil Falls is the most magnificent (fig. 15). Furthermore, Tioga-age terminal moraine is exposed in a road cut west of Bridalveil Meadow. Areas in and about Tenaya Creek and Merced River consist of glacio-fluvial debris and large erratics lay in both creeks. The river bank by Bridalveil Meadow has a characteristic graded bedding (fig. 15) and cobblestones are seen southeast of El Capitan. Owing to the extensive spalling glacial evidence on the bedrock are hard to find but some glacial striae on the base of the wall northeast of Mirror Lake indicates glaciation. However, Tioga, was rather thin and did not reach high up on the walls. Personal communication with climbers gave more information about glacial polish on the apron of Glacier Point. Even so glacial marks are scarce on the upper parts of the valley’s walls which implies active slope processes.

Glacial erosion has played a major role in remodeling Yosemite Valley, however the top parts of the valley as well as the Paleic surface show minor signs of deep weathering. Hence, the deep weathered connotations are older than the glacial ones.

4:3.4 Tamarack Northeast of Foresta and Big Meadow is a 0.5 km2 site with pronounced sheeting stooping east towards Tamarack Creek. Geomorphologically the whole area shows landforms induced by deep weathering on a stripped etch surface (fig. 16) on a slightly slanted plateau. No glacial forms are to be seen.

4:3.5 Tamarack - weathering forms Rectangular blocks formed by intersecting joints cover a small part of the site, joints are rinsed exposing edge rounded convex forms. Displacement along sheet fracture has taken place on other parts of the site (fig. 17). The area closer to the rim of Yosemite valley is more grussified, the granite being decomposed and surfaces covered with a 1-5 cm thick exfoliation. The latter area show minor pitting, saucer shaped depressions weathering hollows, several mushroom weathering landforms (fig. 18) and tafoni. The found tafoni varies in size from 2-40 cm in diameter and 1- Fig. 16. Stripped etch surface showing deep 20 cm deep. weathered joint pattern system, Tamarack Creek, elevation 1,500 m.

27 Fig. 17. Sheet fracture on edge convex rounded granite block formations. Tamarack Creek, elevation 1,500 m.

Former weathering pathways have created shallow gutters in the bedrock. The smallest ones being 0.5 m wide 0.1 m deep, 10 m long respectively. A larger gutter, 2 m wide, 2-3 dm deep, 13 m long, may be a rock levee initiation as explained by Twidale (1993) in terms of the bedrock surface close to the channel being exposed. The exposed channel is dry and sheds water, whereas weathering can continue on the soil covered surroundings.

Tamarack was glaciated during Sherwin according to Matthes 1930 and Huber 1989. However, there are no glacial signs just deep weathered landforms.

Fig. 18. Mushroom weathering, Tamarack Creek. 4:3.6 Foresta Foresta is a c. 4 km2 situated southwest of Tamarack. It is geomorphologically characterized by a stripped etch surface dominated by a joint system running NNE-SSW. The granite is fine to medium grained but very dry and rotten, and it can be broken apart very easily while being held in hand.

4:3.7 Foresta - weathering forms North of the study site Foresta (fig. 14) between Tamarack and Big Oak Flat Road an abrupt hill slope reveals a partly stripped c. 2.5 km2 large area with a multitude of edge rounded core stones covered with a relatively thin vegetation cover, 0.5 m in places (fig. 19). Glacial till can be seen in the lower parts of the slope.

Partly stripped slope Foresta etch surface

Corestones Tors Dome formed hill, weathered within joints

Glacial till in places Weathering zone with incipient corestones

Fig. 19. Schematic profile over Foresta and surroundings suggesting a dome formed hill weathered within joints followed by a stripped etch surface partially reshaped by glacial erosion and slope processes. Core stones and tors are seen resting on the slope and on the etch surface. Not to scale.

28 Foresta is dominated by numerous core stones and tors in various sizes, ranging from 0.5-5 m high, and tor-like pillars with a maximum height of 12 m covering the ground (fig. 20a). Quite a few of the tors are bedrock-rooted and have depressions on the surface, indicating pathways for water and deep weathering (fig. 20b), others have weathering hollows or tafoni closer to the ground (fig. 21a,b,c).

1 m b a

Fig 20, a, b. Stripped tors, exposed to deep weathering in Foresta. b) with a 0.6 m weathering depression. Elevation 1,300 m.

Some core stones make up inselberg-boulders or nubbins, about 20-50 m in diameter, 10-20 m high. The exposed boulders are often round and smooth but rough pitted surfaces are also very common. Indeed, pitting can be seen on almost every surface in all directions. When tapped on some rocks sound fresh, others sound hollow indicating continued weathering behind a thin layer of exfoliation. Stepwise and flake exfoliation is very common. Connotations of differentiated weathering were above all albeit veins protruding 1-3 cm, they are easily loosened but do not crumble up as the granite does.

a

b c

Fig. 21 a, b, c. Stripped tor-like pillar exposed to deep weathering 6 m high, 20 m wide facing west (a). East side of the tor only 3 m high with weathering hollow facing northwest (b). Close-up picture of the 0.6 meter deep weathering hollow (c). Foresta, elevation 1,300 m.

A mature well developed red clay-rich saprolite is found in a section east of Big Meadow. The section is moist and core stones in situ show signs of advanced stages of weathering having red clay

29 in fractures and on the surface. Following the surface of the unraveled core stones a thin, 2-6 cm deep, but fractured layer of flaky polygonal, mural, exfoliation weathering is conspicuous. Mural weathering suggests that the exposure of the boulder exposed an external slightly altered rock shell to sub-aerial environments which led to solutional loss and shrinkage (Thomas 1994).

4:3.8 Foresta - glacial forms Foresta was last glaciated during Sherwin, 800,000 BP, (Matthes 1930; Huber 1989). When the ice withdrew Big Meadow was a lake which has choked-up over the succeeding years to its present condition of a marsh land. No connotation of glaciation or glacio-fluvial debris were to be found apart from glacial till on the bottom of the meadow. The till is underlain by silty, sandy material at 3 m depth, personal communication with a local person.

Both Tamarack and Foresta share landform similarities According to Matthes (1930) and Huber (1989) the areas were glaciated during Sherwin glaciation. Whether Matthes and Huber are right in their assumptions is hard to judge from this study since exposed landforms have been subjected to post- glacial weathering. Thus, all eventual glacial marks, striaes etc. have weathered away during later Millennia. However, both stripped etch surfaces reveal deep weathering within joints.

4:3.9 Road sections in Yosemite National Park Several road cuttings within the national park and its surroundings present deeply weathered profiles, 5-15 m high, consisting of clay as well as coarser material with incipient core stones or core stones in situ. The weathered material goes up to the surface but the depths of the profiles are hard to determine since they go further down than the settings do. Some localities are overlain by glacial till. Within the west side of the park the most developed profiles can be seen on higher elevations, above 1,500 m, while further west, beyond the parks limits, deep weathered profiles are found on lower elevations depending on the westward tilt.

Road 41, Wawona Road, Big Oak Flat Road and Hetch Hetchy Road (fig. 14 p. 25) gives plenty of beautiful deep weathered examples (figs. 21- 24). The weathered granite is mainly medium grained with blond to light red color.

The core stones range from 0.3-2.5 m in diameter and on most sites they lay in situ imbedded in gruss. Some road cuttings show settings with core stones in situ and tor-like pillars in the fore-ground. One of the sites is situated west of El Portal, right by Merced River, about 500 m a.s.l. Here a narrow granite pluton has intruded in the metamorphic layer. The exposed granite is very rotten, decomposed into gruss (Wagner, 1991) and core stones in situ are seen like protruding rock eggs, 0.5- 1.5 m in diameter (fig. 23). According to Matthes (1930) and Huber (1989) no Fig. 21. Deep weathering along joints and incipient core stones. Core stones are also seen resting on the top surface. glacier has ever reached this far west. Road cut at Wawona Road, elevation 1,650 m. James Dean providing scale.

30 Different types of deep weathered core stones are seen on several locations. A perfect example of spheroidal weathering which via exfoliation show incipient unraveling of core stones can be seen by Big Meadow overlook (fig. 24). Mariposa Grove, the southern part of the park, exhibit several deep weathered convex edge rounded granite blocks (fig. 25). Owing to the snow cover glacial marks could not be seen apart from a 0.6 m pothole on a rock east of Crane Flat, however this rock showed deep weathered features as well. A few road cuttings exposed till and scattered erratics lay above or close to examined road cuttings.

Fig. 22. Deep weathered and protruding core stones in Fig. 23. Deep weathered and protruding core stones situ. Big Oak Flat Road, elevation 1,650 m. in situ. West of El Portal, elevation 600 m.

Fig. 24. Spheroidal weathering around core stone, 3 m Fig. 25. Deep weathered convex edge rounded wide 2 m high. Road cut at Big Meadow overlook, granite blocks, 3.5m wide 3m high. Mariposa Big Oak Flat Road, elevation 1,650 m. Grove, elevation 1,900 m.

4:4 Hetch Hetchy The geomorphological map over Hetch Hetchy and surroundings is presented in fig. 26. The Hetch Hetchy area resemble Yosemite Valley’s geomorphology in that it is an overdeepened canyon with sheer cliff walls. Some truncated valley spurs and hanging valleys with cascading waterfalls indicate the valleys reshaping of a fluvial valley by former valley glaciers. The structural control of the adjoining top surface is also very obvious in this area. Since the valley is dammed, acting as reservoir for San Francisco, it is impossible to judge the valleys complete appearance.

Immediately south of the dam’s head gate is a large roche moutonnée with its pluckside facing west. Despite being plucked the whole side show signs of deep weathered softly rounded edges where

31 most joints are deep and enlarged. The majority of the joints are between 0.1-0.9 m wide, 2-4 m deep, and 4-10 m long. One incipient flared slope is facing southwest. The major part of the surface on the roche is thinly exfoliated and the rock sounds hollow where exfoliation is 0.5-3 cm. Pitting is seen on almost all surfaces as well as differential weathering and weathering hollows. On the top surface a large pothole, 1.5-2 m in diameter, and some minor pothole initiations can be seen. Glacial striae from northeast is mapped on several places. Several boulders, 0.5-1.5 m in diameter, lay scattered on the top surface. A lot of boulders of the local lithology have been moved by the ice southwest of the roche. Parts of the roche show signs of p-forms as if glacial melt water has formed it.

Fig. 26. Geomorphologic map covering studied areas in Hetch Hetchy and surroundings.

4:4.1 Weathering forms Many deep weathered edge rounded convex forms occur in between sheeting joints (fig. 27).

Differential weathering in the form of a diorite vein, 0.3 m wide about 15 m long, protrudes 0.06 m on the

0.5 m

32 Fig. 27. Laminated weathering in joints, between Tueeulala and Wanama Falls, elevation 1,200 m. bedrock stooping towards the dam. Glacial striae facing southwest were found next to the vein. The rock surface is coherent and the vein protruding too much to be considered post-glacial weathering. In the gap between Kolana Rock and Kolana Rock Hetchy Dome Hetch Hetchy Dome the evidence of the three major uplifts with succeeding erosion and sheeting is outstanding (fig. 28). The relative relief between Hetch Hetchy Reservoir and Hetchy Dome and Kolana Rock is about 730 and 620 meter respectively.

Hetch Hetchy Reservoir N Vertical sheeting due to offloading are seen on both valley walls. On the south- Fig. 28. Drawing over the gap between Kolana Rock and west side of the canyon a deeply fractured Hetchy Dome. The gap gives evidence of the three major uplifts vertical joint controlled cliff is perceptible, and the subsequent erosion and sheeting. The border between some talus cover the base of the rock. each uplift is seen as darker lines on both domes.

4:4.2 Glacial forms By following the trail north of Hetch Hetchy Reservoir a wall with rounded edge surfaces is revealed, some plucking or spalling is noticeable. The granite is medium grained with a pink color indicating a more recent decomposition than in Yosemite Valley where the granite is having a more pale color. Moreover, decomposition has not been as heavy as in Yosemite Valley in that the granite is more coherent and does not crumble as easily as in Yosemite Valley. At the base of the rock wall parts of the bedrock is overlain by glacial till and on some sites gruss overlay till. The latter case might be downfall due to mass wasting.

West of Tueeulala Falls a couple of roche mouttonnées following sheeting planes are documented. One roche has thin exfoliation on top, no glacial striae is seen. Small erratics pushed by the ice lay beside the roche. Talus cover the bedrock above the roches. Crescentic gouge marks in N-S direction are found south of Tueeulala Falls. Glacio-fluvial connotations such as potholes in various sizes and pothole initiations were seen below and east of Wanama Falls. By the same place large erratics were placed upon pedestal rocks.

4:4:3 Roadside geomorphology between Hetch Hetchy reservoir and Poopenaut Upon leaving the Reservoir the road leading to the entrance is steep and narrow. The landscape form a stepwise appearance, divided into levels with steep slopes followed by plateaus. The steps are at this

33 Fig. 29. Corestone with incipient mushroom weathering, Hetch Hetchy, elevation 1,200 m. location probably a visible evidence of the synergy between uplift, sheeting, deep weathering and erosion indicating the evolution of different valley generations. At other places in the Sierras this is explained as stepped topography (Wahrhaftig 1965) considering the contrasted weathering of wet and dry sites.

Exposed rock domes laying closer to the mountainside show a weathered surface documented by enlarged weathered joints. Closer to the rim of the valley glacial erosion is more obvious in that a few roche moutonnées with a southwest facing pluck side are exposed on smooth polished granite plateaus. Quite often are the joints on the plateaus slightly enlarged and have edge rounded convex forms. The joints have most likely been washed free of weathered material by glacio-fluvial meltwater.

The exposed road cuttings have no continuity, at some sites large erratics lay on top of glacial till, next gruss weathering with core stones in situ will be exposed, only to be followed by glacial till. As with the southern part of Yosemite the frequency of gruss and core stones in situ increases with altitude e. g. a couple of sites with spheroidal and Fig. 30. Vertically joint controlled wall, deep weathered within joints, Poopenaut, elevation 1,300m. mushroom weathering came on (fig. 29). Erratics in various sizes lay scattered on both glacial till and the plateaus.

The Poopenaut area (fig. 26) is an undulating plateau which is abruptly cut off by the steep valley side. The area is a salad bowl of all kinds of geomorphic forms. Displacement along sheeting fractures has taken place, small and large erratics lay on sheer polished granite, glaciofluvial melt water has created p-forms and tiny pothole initiations. The rock surfaces expose small scale weathering forms such as thin layered exfoliation, pitting, weathering pits of various sizes, and several handle and knob forms in a more fine grained composition. While standing on the rim of the valley a vertically joint controlled wall (fig. 30), deep weathered within joints, was unveiled.

Despite the fact that the area has been recently glaciated by Tioga (Huber, 1989, 1990) the deep weathered surfaces are definitely in favor.

4:5 Kings Canyon & Sequoia National Parks While approaching Kings Canyon driving through the foothills on road 180 (fig. 31) tors seem to pop up everywhere. Closer to the park, on higher elevations, road cuttings present about the same

34 deep weathered profiles, 5-15 m, with incipient core stones or core stones in situ seen in Yosemite. Road cuttings by the road leading to Kings Canyon have a thin layer of exfoliated rock with a sandy appearance covering fresh rock. The core stones range from 0.3-2.5 m in diameter and the granite is mainly medium grained with a blond to pale red color. For locations of mapped locations see fig. 31.

4:5.1 Kings Canyon At first sight the valley is V-shaped and does not look glaciated at all. However, it goes through different faces and further east it forms a soft U-shape, resembling Tenaya Canyon, east of Yosemite Valley. While looking on the mountain crest the excavation of quite a few cirque glaciers are exposed, mainly on the north side of the wall. The mountain crest in the west side of the canyon show sharp corners at places due to extensive spalling, further east the corners are more rounded. In road cuttings by the foot of the rock wall glacial till were exposed in one cutting, to be followed by gruss and core stones in situ in the next. Fig. 31. Map over researched areas in Kings Canyon Erratics of different sizes, 0.5-5 m in and Sequoia National Parks. diameter are to be seen scattered at various locations, even on top of deep weathered sites. The riverbed consist of glacio-fluvial debris.

4:5.2 Generals Highway Generals Highway runs between General Grant’s tree in Kings Canyon Natl. Pk. to General Sherman´s tree in Sequoia Natl. Pk. and beyond (fig. 31). At the junction between Kings Canyon and Sequia Natl. Parks a dome formed rock hill was examined (A, fig. 31). The soil contains a lot of gruss and cuttings belonging to the hill unveil incipient core stones and core stones in situ. Quite a few core stones lay scattered, however, some of them might be erratics of the local lithology. Several large basal knobs are standing on the hill, a couple by themselves but a few in a group about 0.5-1 m apart. One of the larger knobs, 6 m high, 4 m wide, and 10 m long, has a pronounced flared slope underlain by weathering hollows 0.1-0.3 dm deep, 0.2-0.6 dm in diameter (fig. 32). On the top surface it had a 5 cm deep pothole initiation and glacio-fluvial marks, the latter

35 Fig. 32. Flared slope with weathering hollows. Glacio-fluvial p-forms on top surface. Generals were also seen on other knobs and tors. A couple of smaller basal knobs and tors showed incipient and completely developed mushroom weathering. Small- scale surface weathering such as thin exfoliation, knob and handle forms, and the everywhere prevailing pitting as well as differential weathering were mapped on observed knobs and tors.

The same kind of small-scale forms were seen on gently rounded tors by Great Baldy viewpoint (B fig. 31), approximate elevation 2,000 m a.s.l. Here a tor like pillar, more likely a protruding deep weathered basal knob, 6 m high 20 m long and 4 m across, has pothole initiation and p-forms on the top surface. Some erratics lay below this pillar, probably descending from the same. Upon trailing beneath the viewpoint glacial meltwater has stripped a 10 m long deep weathered joint pattern system. The tors are 0.5-3 m high and 0.5-1 m wide with gently rounded surfaces. Fluvial depressions follow the same pattern as seen in Foresta.

Quite a number of large sheeting planes that may have been fluvial polished with big and small erratics on the surface were seen (fig. 33). One of them, a convex sheeting area facing north, 150 m wide and at least 300 m long, had several deep weathered core stones on top, of which at least two had tafoni weathering (fig. 34). Again the bedrock exhibits thin exfoliation, enlarged joints, pitting, differential weathering, and even a protruding feldspar vein, 6 cm high.

Fig 33. Large scale sheeting plane. Generals Highway, elevation 2,100 m. Fig. 34. Large tor with incipient donught formation on fluvial polished sheeting plane. Generals Highway, elevation 2,100 m.

4:5.3 Generals Highway - Silliman Creek Silliman Creek (fig. 36) is another extensive sheeting area having a slight concave form which may be a result of fluvial incision (C, fig. 31). Fluvial water has created numerous, at least 25, pot-holes in various sizes, 0.4-2.5 m in diameter, 0.1-1 m deep,

36

0.5 m within the creeks boundary. Boulders and granite sheets, deriving from a structural controlled convex sheeting area laying direct south-east of the creek, have slid or been pushed by the ice into the watercourse. The sheet joints follow the and are peeled off like an onion. On top of the sheeting plane is a common sight regarding deep weathered granite in the Sierras. Intersecting joints, 2 by 4 m, have created 0.5-1 m thick rectangular blocks with edge rounded convex forms (fig. 35). On top of the rectangular blocks lay erratics as an evidence of glacial activity. Another glacial evidence was seen just a couple of miles down the road from Silliman Creek in that glacial till was exposed in a road cutting.

Silliman Creek

potholes Fig. 36. Schematic profile over Silliman Creek. The creek is incised in the bedrock. Intersecting rectangular blocks lay on a structural controlled convex sheeting area. Some boulders derived from the local lithology lay on top of the blocks and in the creeks boundary. Not to scale

4:5.4 Moro Rock Moro Rock (fig. 37) gives a perfect example of a vertical joint controlled rock where sheet joints follow topographic surfaces. The dome-shaped monolith formed by extensive spalling due to unloading is a very common, yet grand, sight in the Sierras. A smooth cliff wall is exposed on sheeting surfaces, however, the remains from spalled rocks have sharp corners. Being exposed to sub-aerial weathering the wind plays a major part in creating ventifacts and wind flutes (fig. 38). Knob handles in various sizes 5-30 cm in diameter are seen protruding 1-12 cm.

37 Fig. 37. Moro Rock, elevation 2,050 m.

4:6 Kern River Valley Kern River Valley is cut into a deep weathered structural zone which has been successively incised by Kern River. South of Lake Isabella flows Kern River through a successively incised V-shaped canyon whose sides are fractured and deep weathered. At places the sides have a stepwise appearance and core stones are left resting on the slopes (fig. 39), some tors have protruding knob and handle forms and pedestal rocks on top (fig. 40). On other sites tors and core stones make up cone shaped nubbin mounds.

Kern River

Fig. 39. Schematic profile over Kern River Valley south of Lake Isabella. The canyon is deeply incised by Kern River in a stepwise manner. Deep weathering takes place within joints and core stones are seen resting on the slopes. Not to scale.

0.5 m

38 Fig. 40. Pedestal rock above Kern River.

Fig. 41. Torlike pillar c. 30 m high, 10 m across. Kern River Valley. Elevation 900 m.

At the southern end of the valley the walls are controlled by vertical joints and bedrock slabs are sliced off tumbling into Kern River. Tor like pillars, tilted towards the river, come in all sizes where one of the most magnificent is about 30 m high, 10 m across (fig. 41). Tors on higher elevations, closer to the crest, have a more rounded appearance than the core stones laying closer to Kern River, i e lower in the original weathering profile.

The slopes on higher elevations are covered by a patchwork of core stones resting on the slopes. Owing to slope processes lower elevations are mainly covered by saprolites and detached core stones are often fractured on account of fall from the heights above.

Exposed road cuttings show structurally controlled joint systems with remarkable weathering along joints up to 50 m deep. Above the road cuts a thin weathering cover is partly stripped exposing a convex etch surface with tors laying on top. Some road cuttings have an external shell of gruss resembling cement which crumble up when hacked on. Beneath the outer shell the decomposed rock is moist and at 0.3 dm depth the rock is strong, i. e. fresh enough to give resistance. Gruss weathering is also more evident on higher elevations. Weathering is more advanced on lower elevations and quite a few mudslides reveal a mature clayey sandy soil.

Several convex sheeting planes are covered with edge rounded rectangular blocks. Once the boulders are exposed slope processes take over and the boulders start to slide off the sheeting plane. Some tors with tafoni were seen closer to the river.

The river must at some point have exerted a high pressure on its bed since potholes, 1 m in diameter, were seen in the riverbed. However, these must be normal fluvial potholes since the most southern part of the system has not been subject to glacial transformation as the northern part of Kern River has. South of the valley flows Kern River through softly rolling hills.

4:7 Big Pine lava fields Big Pine is situated on Sierra Nevada’s eastern foothills bordering the structural basin and range zone. The relative relief is about 3,000 m between Big Pine and the crest of the east escarpment. The escarpment is outstanding with an almost straight mountainside completely fractured and cracked open with rocks having very sharp edges. Frost and thawing processes in accordance with strong slope processes acting on the cliffs have created massive taluses.

The foothills by Big Pine are characterized by tors, 2-4 m tall, and tor-like pillars, up to 10 m tall, surrounded by alluvium. Extrusive volcanic rocks of supposed Plio-Pleistocene origin rest

39 unconformably on the granite. This implies that the tors and boulders are exhumed pre-volcanic landforms, which suggest that also the pediment is of pre Plio-Pleistocene age (cf. Cooke et al, 1993). Pedestal rocks lay on top of some tors and a multitude of core stones cover the ground. Most of the rocks have a thin, 1-5 cm deep, exfoliated layer on the surface, some exfoliation is a kind of mural weathering, others are flaky or have a stepwise appearance (fig. 42). Knob handles protruding 4-10 cm, c. 4-15 cm wide and 5-15 cm long, were seen on several tors (fig. 43).

A couple of large boulder inselbergs, approximately 30 m high and 100 m in diameter were surrounded by lava outcrop (fig. 44). However, no connotations of lava could be seen beneath the tors nor the core stones on the inselbergs.

Fig. 43. Knob handle protruding 8 cm, 10 cm wide, Big Pine, elevation 1,400 m.

0.5 m Fig. 42. Bedrock-rooted tor-like pillar with stepwise exfoliation following the boulders surface. Big Pine, elevation 1,400 m.

Fig. 44. Boulder inselberg once surrounded by lava field, Big Pine. Elevation 1,400 m.

40 4:8 Structure of different forms An attempt to gather and structure the different forms found in the different landform zones was made. The result is presented in the following tables. In table 2 valleys with local glaciation as well as valleys lacking glacial erosion is taken under consideration. It is striking that the same kind of etch forms, sub-aerial weathering forms and fluvial forms were found in all landforms zones. However, to a various degree.

Tab. 2. Structure of different forms found in valleys with vertical joint controlled walls: Yosemite Valley, Hetch Hetchy Valley, Kings Canyon and Kern River Valley. Etch forms Sub-aerial weathering Fluvial forms Glacial and glaciofluvial forms forms core stone like rough pitted surfaces polished surfaces striae on bedrock walls compartments and rock floor tor-like pillars handle and knob forms potholes erratics convex edge rounded thin/thick exfoliation grooves potholes granite blocks 1-5 cm / 10-20 cm weathered joints with enlarged joints hanging valleys lineation moraine arête

Tab. 3. Structure of different forms found on plateaus outside Yosemite Valley, Hetch Hetchy Valley and Kings Canyon. Etch forms Sub-aerial weathering Fluvial forms Glacial forms forms core stone like rough pitted surfaces grooves erratics compartments tor-like pillars handle and knob forms polished surfaces roche moutonnée convex edge rounded thin/thick exfoliation glacial polish granite blocks 1-5 cm / 10-20 cm weathered joints with enlarged joints lineation

Tab. 4. Structure of different forms found in Foresta. Landscape with no recent glaciation. Etch forms Sub-aerial weathering Fluvial forms Glacial forms forms core stone like thin/thick exfoliation polished surfaces erratics compartments 1-5 cm /10-20 cm. tor-like pillars rough pitted surfaces convex edge rounded handle and knob forms granite blocks

41 weathered joints with lineation

42 5 Discussion

5:1 Yosemite Valley Owing to multiple glaciations, which has eroded the valley sides, overdeepened and filled the floor with glaciofluvial debris, Yosemite Valley has gone through radical changes which today is presented by a remarkable U-valley (figs.45, 46). The powerful U-shape has probably been accomplished by Sherwin, the largest pre-Tahoe glaciation, which during its maximum extent filled the valley to its brim reaching west of El Portal (Matthes, 1930) (fig. 13 p. 24). This is also the point where Merced starts to meander more readily. The latest glaciation, Tioga, was rather thin and did not reach further than Bridalveil Meadow during its maximum about 15,000 years ago according to Huber (1989). While comparing the profiles with the maps and field data the glaciers greatest erosive power has been between Royal Arches and Bridalveil Meadow. The glacial erosion west of Bridalveil Meadow cease gradually until Foresta where an almost perfect V-shaped canyon appear. The valley is once again widened west of Arch Rock to form a meadow at El Portal. Postglacial fluvial erosion must be considered insignificant seeing that no fluvial incision in the glacial valley is observed. Slight fluvial incision is restricted to the knick points between the main valley and the hanging valleys. In the thousands of years to follow glaciation freeze-thaw cycles have favored rock fall due to off-loading creating sheer and steep cliff walls with talus covered bases. Altogether this makes Yosemite Valley an unsatisfactory place to come across glacial evidence such as striae or other geomorphic landforms.

Fig. 45. View over Mirror Lake and Tenaya Creek. Fig. 46. View over Yosemite Valley looking east. El North Dome to the left, facing south and lower parts Capitan to the left and Bridalveil Falls to the right of Half Dome to the right. facing north. Half Dome is seen in the background.

While following the trail leading from the valley to Vernal Falls (fig. 14 p. 25) the change from boulders with sharp corners and post glacial weathered erratics to bedrock walls with sheeting joints with edge rounded surfaces is gradual. Close to the valley floor the steep wall facing west show no sign of glacial erosion nor deep weathering. On higher elevations and facing north the intersecting joints are more enlarged and have the same edge convex rounded forms found in road cuttings. This indicates that the wall must have been formed by weathering along vertical joints and at the time of glaciation the walls have probably been composed of grand tor-like pillars e. g. Like those seen in Kern River Valley. The valley glacier has either had a gentle approach just pushing the pillars off the walls and down into the valley. Or while the ice worked its way on the valley floor, cleansing the rock base free of debris, it provided the pillars with an unstable base support and upon melting the

43 pillars were overthrown. During later millennia the wall has continued to spall. This possible cause of event could mean that glacial erosion was less effective on higher elevations.

Merced’s riverbed at Vernal Falls was not subject to glacial erosion during Tioga. One assumption may then be that this area reveals partly how the walls in Yosemite Valley looked like prior to Sherwin glaciation. If so, there is no or little doubt that the valley has been deep weathered prior to glaciation. Since the weathered wall is facing north one possible thought regarding the deepened joints could be a dry cold based ice frozen fast to its bed preserving the landforms during glaciation (cf. Kleman 1994). Next glacial melt water cleansed the fractures setting them free of weathered material. The thousands of years which followed postglacial weathering has probably subsequently accentuated the established deep weathered forms.

5:2 Structure of different forms

5:2.1 Valleys with vertical joint controlled walls: Yosemite Valley, Hetch Hetchy Valley, Kings Canyon and Kern River. All rivers are entrenched in deep canyons on the west side of the range, the deepest one Kings Canyon (fig. 47) having a relative relief of 2,400 m which for instance is deeper than the Grand Canyon. Other canyons relative relief varies between 750 m, Hetchy Reservoir, 1,050 m, Yosemite Valley and up to 1,200 m, Kern River. Nearly all the major canyons lay roughly parallel to each other flowing west to the Central Valley of California. Faults and vertical fracturing have played a major part in the valleys evolution. The canyons resemble one another and are Fig. 47. Kings Canyon facing west. The canyon may be morphologically similar with joint aligned steep compared with Yosemite Valley figs. 45 and 46. bedrock walls.

The deep weathered profiles documented specially in Kern River Valley suggests that deep weathering along orthogonal joint system plays a significant role for the development of valleys. In the Kern River Valley partial stripping during fluvial incision has resulted in exhumation of core stones, tor-like pillars and extensive weathering along sheeting joints.

In formerly glaciated terrain stripping of the saprolite is almost complete and has resulted in for instance troughs, steep slopes, hanging valleys and local glacial over-deepening. Postglacial surface weathering is insignificant and has only influenced the landforms in establishing a layer of thin exfoliation and differential weathering. Postglacial slope processes, such as slab failures, are of local importance.

Hetch Hetchy valley is intermediate between these two extremes. It has been covered to its brim with ice, last by Tioga, and for that reason it exhibits several pieces of evidence of glacial erosion. On the other hand the ice has been less effective in some areas and large amount of deep weathered

44 landforms are detectable. Some weathered landforms found on the floor in Hetch Hetchy Valley have obviously survived repeated glaciations whereas some forms on higher elevations present weathering patterns. This phenomenon has also been noted in Scotland where the variations on a regional scale seemed to be related to topography with pre-glacial surfaces preserved on uplands and glacially-modified surfaces on lower grounds (Sugden 1989). In Hetchy the scale of variation at the valley bottom could also be related to selected zones of the ice sheet having experienced locally accelerated flow due to bed deformation (Boulton & Jones 1979; Sharpe & Shaw 1989). Another explanation could be an ice which in parts had a very low pressure melting point and as such suppressed glacial sliding and erosion or as Kleman (1994) argued, an ice frozen fast to its bed.

5:2.2 Forms on plateaus outside Yosemite Valley, Hetch Hetchy Valley, and Kings Canyon Glacial erosion on plateaus has stripped the weathered material and the remaining surface bear traces of deep weathered joints. On some localities a pre-existing basal weathering front with troughs, basins and gutters is exposed. According to Thomas (personal communication) this might be inherited tropical landforms where tropical pitting can initiate weathering and in turn create dambos, ill defined concavities in the landscape to which convergent water flow bringing sediment to the hollows (Thomas 1994). After erosion and stripping the dambos are cleansed and on some occasions water fill the dambos deepening the same creating lakes. The most prominent glacial forms, roche moutonnées, are structurally controlled in that they follow the bedrock’s dome shaped structure.

The bedrock on the slopes towards the central valley of California is normally covered by a more or less thick weathering cover. In the Mojave Desert, California, relations between modern pediment surfaces and potassium-argon-dated lava flows has recorded a four-million year history of pediment evolution (Dohrenwend et al. 1987). This study gives general information of the minor change pediment domes have gone through since early Pliocene time. The result from the Mojave Desert may also be compatible with the pediments applied to the Central Valley of California. In parts the weathering cover has been eroded and expose a weathering front showing characteristic landforms hailing from pre-existing patterns which can be compared with forms found in the humid tropics. The stripped surfaces follow convex sheeting planes on most locations and some etch surfaces are covered by a mosaic of rectangular core stone blocks. Such block systems are also a common sight on elevated plateaus within Sierra Nevada. Twidale & Romani (1994) imply that fractionation during cooling may give rise to core stone masses developing into boulders and Ollier (1988) stated that the ”retention of weathering products will be favored by flat topography amongst other things”. In combination and if applied to the Sierra Nevada this means that the joint pattern system is the oldest sheeting plane and it was formed during the granites cooling phase in the Mesozoic.

5:2.3 Forms in and about Foresta: landscape with no recent glaciation During Pleistocene Foresta was completely glaciated during the Sherwin glaciation. Glacial erosion partly stripped the area leaving an etch surface with saprolite remnants. Stripping has left nubbins, convex core stone like compartments, grand core stones and tor-like pillars to be exposed to exogenic processes. Whatever glacial evidence the rocks in this area have presented has been long gone owing to postglacial processes, irrespective of some erratics. Core stones have continued to weather post-glacially establishing a flaky form of layered thin exfoliation.

45 5:2.4 Pediments east of the Sierra Nevada fault - Big Pine lava fields The landforms in Big Pine, eastern Sierra Nevada, show the same deep weathering pattern as found in Kern River Valley and Mojave Desert (Oberlander 1972, 1974; Dohrenwend et al. 1986). The landforms emphasize further the establishment of deep weathered pre-existing landforms in granitic terrain.

Many studies in the semi-arid zone demonstrate that lowering of pediment surfaces takes place either simultaneously (Dohrenwend et al. 1987), or by alternate mantling and stripping (Mabbutt 1966; Oberlander 1974). By using the evidence of dated volcanic deposits on pediment surfaces and the weathered profiles beneath Oberlander (1974) concluded that the pediments in Mojave Desert were inherited exhumed remnants of a sub-Pliocene landscape established through etch planation. This result may also be applicable to landscapes laying further east and north of the Mojave Desert thus, Owens Valley.

5:2.5 Relative importance of different processes for the present forms The purpose of this paper is to use Sierra Nevada as an example to draw attention to the land- forming effect of different exogenic processes on granitic landforms. The landforms characteristic of the Sierran granite are above all controlled by different joint systems, just as in areas far away from Pleistocene glaciation such as Australia, Spain and South Africa (Twidale & Campbell 1995; Twidale et al. 1996). However, in the absence of exogenic processes structures will never become landforms. In most areas deep weathering and subsequent stripping alone is responsible for the joint exploitation which is decisive for the landform development in this region. By structuring the processes involved (tab. 5) an attempt is made to grade the processes.

Tab. 5. Relative importance of different processes for the present bedrock forms. Scale; X minor importance; XXX – major importance.

Yosemite Valley Foresta Upper surfaces Kern River Hetch Hetchy valley Hetch Hetchy Valley Kings Canyon Kings Canyon Sequoia Horizontal sheeting X XX XXX XX Vertical sheeting XXX - X XXX Deep weathering XX XXX XX XXX Surface weathering X XX XX XX Fluvial erosion XXX - - XXX Glacial erosion XXX X X - Glaciofluvial erosion X X X - Glacial stripping XXX XX XXX - Fluvial stripping XX - - XXX

At first sight and just by looking at the gross morphology of the studied areas horizontal and vertical fracturing suggest that the plutons intrinsically structural control in combination with glacial erosion are the dominating parameters. However, it is hard to determine how deep the ice has eroded into

46 fresh bedrock. Glacial erosion as a cleansing agent is dominating within valleys and on higher elevations such as Tuolumne Meadows. One could say that on a macro-scale or regional scale of landform assemblages (10-250 km2) they override the produce established by other processes. On a meso-scale or district scale of major landforms (1-10 km2) the structural control is a dominating factor but it does not diminish the importance from other processes. However, when looking at the parameters on a micro-scale or local scale of slopes and minor landforms (0.1-1 km2) it is evident that weathering, above all deep weathering, has had a dominating influence on the studied landforms and occur at all topographic levels. Hence, it is evident that glacial erosion is of minor importance when compared to landforms established by deep weathering. An event that Kern River Valley may illustrate in that the southern part of Kern River has never been glaciated and yet the deep weathered landforms found here may also be seen in former glaciated areas.

N

Paleo- saprolites

Fig. 48. Long term evolution of Sierra Nevada. Not to scale.

Sierran landforms are compatible with Scottish and Swedish landforms along with forms found in the humid tropics. In north-east Scotland weathering zones on a district-scale are recognized as patterns from humid tropical weathering systems of Neogene to early Pleistocene age (Hall 1985, 1986).

47 Lidmar-Bergström (1988, 1989, 1996, 1997) came to the main conclusion that the major landforms in south Sweden consists of exhumed sub-Cambrian and sub-Mesozoic paleo-landforms. The long term evolution of Sierra Nevada follow a complex and intriguing pattern (fig. 48). The major impression is that old saprolites in deep profiles are preserved on the western low angle slope of the tilted block. A common sight are mature and advanced weathering profiles containing large amounts of clay pointing at subtropical weathering conditions. Profiles, having escaped glacial erosion, lay further west and are probably remnants of older saprolites preserved by the western tilt. Furthermore, they have a higher clay contents than younger gravely saprolites situated on higher elevations and/or further east. However, erosion has been more effective on the eastern side where the tilting is more elevated.

The great variety of tors and core stones resting on the slopes and top surfaces as well as the light colored clay formation within joints points at weathering in a tropical climate. The etch forms and sub-aerial weathering forms are probably the result of prolonged deep weathering and subsequent stripping during late Tertiary. This was most likely initiated during late Mesozoic to early Tertiary. The eastern slope fault scarp is characterized by slope processes due to rapid uplift. The pediment east of the fault scarp may be remnants of a surface developed prior to uplift and is thus possible to correlate to the undulating relief of the uplifted plateaus. A similar kind of surface in Norway thought to reflect weathering in a tropical climate is described as the ”Paleic surface” (Gjessing 1987). The western uplifted part of the tilted block is a stripped etch surface formed by deep weathering and stripped by fluvial and glacial erosion during Plio-Pleistocene. The general conclusion is that a variety of erosion processes have occurred at Sierra Nevada but the major forms have resulted from the oblique uplift of the eastern rim in combination with stripping of old saprolites and renewed rim incision along old valley systems (fig. 49). Finally a glacial reshaping of landforms has taken place during Pleistocene. Hence, the study points at several important steps in the geomorphic evolution of tilted block mountains.

incision

Planation Tilting / Stripping Incision / Stripping

Fig. 49. Steps in the geomorphic evolution of tilted block mountains.

48 6 Conclusions

While driving through Sierra Nevada it seems like this is a mountain range slowly being decomposed from solid bedrock to gruss and clay. The western parts of Sierra Nevada is covered by sedimentary cover rocks and alluvium. Hence, the transition from a landscape exposed to deep weathering and subsequent, but partial, stripping to the more elevated and stripped etch surfaces in the east is gradual but yet overwhelming.

The gross morphology in Sierra Nevada suggest a structurally controlled landscape where horizontal and vertical sheeting have created domes. The relief on exposed etch surfaces, plateaus, propose that the major landforms are inherited and preserved even though they have been slightly reshaped by glacial erosion. Postglacial weathering is active but of minor importance for the major landforms depending on its slow moving process. On plateaus in the western part of the Sierra Nevada postglacial weathering has acted over a longer time span with more accentuated forms as a result.

The Pleistocene glaciation has only played a minor role in remodeling Sierra Nevada. In the eastern High Sierra where glaciation has been heavier it is seen in cirque glaciers, roche moutonnées, glacially polished surfaces and trough valleys. Glacial erosion in the western part of the batholith is detectable in the magnificent U-valleys, glacially polished surfaces and in the shaping of roche moutonnées. However, the majority of the roche moutonnées follow ancient horizontal sheeting planes and are therefore primarily structure controlled.

In summary the gross morphology suggest a structure controlled landscape where periods of deep weathering during the Mesozoic and early Tertiary has exerted strong influence on developed landforms. In combination with uplift and high denudation rate during late Cenozoic the surface we see today in Sierra Nevada has been extracted. The Pleistocene glacial influence has merely given the landscape its ultimate appearance. This study may also be applicable to the evolution of formerly glaciated continental margins such as the Scandinavian peninsula with fluvial incision in the west, Norway, followed by glacial over-deepening and the development of fiords and trough valleys.

49 Acknowledgments

This study was carried out at the Department of Earth Sciences, University of Göteborg, Sweden. The performed field study was supported by a grant awarded from Geografiska Föreningen i Göteborg. Financial support was also acknowledged from ICA-Allköp, Horred.

I would like to sincerely thank my supervisor Mats Olvmo1 whose great enthusiasm regarding long term landform development inspired me in writing this thesis. He also critically read the manuscript and gave several pieces of elucidative advice regarding its improvement.

Thanks also to Magnus Johansson1 for his enlightening comments and useful discussions while writing this thesis. Björn Holmer1 gave general and valuable advice for which I am much obliged.

A special thanks goes to Laura Coase who was a great company while assisting me during some field days and illuminating nights. All friendly and accommodating people I met while in California deserve a special thank. I am also very grateful to my friends at the department for valuable discussions and joyous acclamations during the progress of this work.

Above all I would like to thank my family who have put up with me and my absent-mind during this work. Without their love, support and patience this thesis would probably not have left its initial stage.

1 Department of Geosciences, Göteborgs Universitet

50 References

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53 Appendix

The geologic time scale used in this study (Huber, 1989).

54