Characterizing Boulder Deposition to Assess Rock Fall Hazard in Upper Pines Campground, Yosemite National Park, Ca

CHARACTERIZING BOULDER DEPOSITION TO ASSESS ROCK FALL HAZARD IN UPPER PINES CAMPGROUND, YOSEMITE NATIONAL PARK, CA

HUMBOLDT STATE UNIVERISTY

Shaun Emmett Cordes

A Thesis Presented to The Faculty of Humboldt State University

In Partial Fulfillment Of the Requirements for the Degree Master of Science in Environmental Systems, Geology Option

May, 2012

Original signatures on file in the office of Graduate Studies.

ABSTRACT

CHARACTERIZING BOULDER DEPOSITION TO ASSESS ROCK FALL HAZARD IN UPPER PINES CAMPGROUND, YOSEMITE NATIONAL PARK, CA

Shaun Emmett Cordes

Large boulders exceeding 10 m3 in exposed volume are widely scattered throughout the Upper Pines Campground in eastern Yosemite Valley, Yosemite National

Park, California. These boulders rest 130-330 m from the base of adjacent talus slopes and lack typical geomorphic expressions of a rock fall deposit in Yosemite Valley. Three modes of transport were considered for deposition of these boulders: (1) glacial deposition during retreat, approximately 15-17 ka, (2) rock fall postdating 15 ka, and (3) fluvial deposition during large scale flooding such as might occur from glacial outburst.

The boulder deposit was characterized by field mapping, cosmogenic 10Be exposure ages, x-ray fluorescence (XRF) analysis, and spatial analysis to determine mode(s) of transportation and source. A mean cosmogenic exposure age of 9.6 ± 1 ka (derived from four samples) considerably postdates the Last Glacial Maximum (LGM), suggesting that boulder emplacement does not result from glacial deposition during retreat of ice. XRF results identify Glacier Point Granodiorite boulders transported from ~500 m above

Upper Pines on Glacier Point. Based on measured boulder attributes, I suggest that partial ii

burial by fan development conceals the original geomorphic expression of the deposit.

Discharge and bed stress modeling indicate that flooding was not capable of moving the boulders to their current location. Thus, I interpret that the boulders were deposited as the result of a single rock fall (or possibly rock avalanche) event that originated ~500 m above Upper Pines Campground on Glacier Point at approximately 9.6 ± 1 ka.

iii

ACKNOWLEDGMENTS

First and foremost, thank you Greg Stock for your support and guidance. Thank you Bud Burke, Andre Lehre, and Brandon Schwab for your time and advice; Allen

Glazner and Tom Chapman for adopting my XRF samples and providing me with the analytical geochemical results that were needed to complete this study; Harvey Kelsey and Sue Cashman for the off the clock discussions and field visit; fellow graduate students Dylan Caldwell, Tyler Ladinsky, and Paul Sundberg for the frequent discussions, ramblings, and brainstorming; Codie LaPoint, Cassie May, and Jenny

Scholfield for opening up their floor space for extended periods of time; and finally my number one field minion Casey Cordes.

Gerald Wieczorek of U.S. Geological Survey collected the radiocarbon samples at

Stoneman Bridge, and Jack McGeehin of the U.S. Geological Survey radio carbon laboratory preformed the radiocarbon dating at the Accelerator Mass Spectrometry

Laboratory at the University of Arizona. X-ray fluorescence analysis was conducted by

Professor Allen Glazner at the University of North Carolina Chapel Hill.

DEDICATION

I would like to dedicate this project to the faculty and staff of the Humboldt State

University Department of Geology: Bud Burke, Sue Cashman, Lori Dengler, Mark

Hemphill-Haley, Harvey Kelsey, Andre Lehre, Bob McPherson, William Miller, Brandon

Schwab, Steve Tillinghast and, Colin Wingfield. Thank you for your years of continued support and providing me with the opportunity to pursue this endeavor at Humboldt

State. You have all played a significant role in allowing me to discover the passion I have for geology.

TABLE OF CONTENTS

ABSTRACT ii

ACKNOWLEDGMENTS iv

DedicatioN v

TABLE OF CONTENTS vi

LIST OF TABLES viii

LIST OF FIGURES ix

LIST OF FIGURES CONTINUED x

INTRODUCTION 1

DEFINITION OF THE PROBLEM AND HYPOTHESES 5

GEOLOGIC SETTING 9 Overview 9 Cretaceous Intrusive Rocks and Field Relationships 9 Quaternary History of Yosemite Valley 15

METHODS 19 Field Mapping and Spatial Analysis 19 Cosmogenic 10Be Exposure Ages 21 X-Ray Fluorescence Spectrometry (XRF) 24

RESULTS 27 Field Mapping and Spatial Analysis 27 Cosmogenic 10Be Exposure Ages 30 X-Ray Fluorescence Analysis 32

TABLE OF CONTENTS CONTINUED

DISCUSSION 37 Aggradation of and Degradation of Yosemite Valley 37 Discharge and Flow Velocity Modeling 45 Rock Avalanche 50

CONCLUSION 52

REFERENCES 53

APPENDICIES 58 Appendix A - Detailed lithologic descriptions 58 Appendix B - Photo locations correlated to figure numbers 59 Appendix C - Gray et al. (2008) major element compositions for in situ Khd 60 Appendix D - Additional XRF sample data 61

vii

LIST OF TABLES

Table 1 – Sierra Nevada Glacial Nomenclature and Ages 16

Table 2 - Cosmogenic Beryllium-10 Data and Exposure Ages 31

Table 3 - Major Element Compositions 34

Table 4 - Discharge and Bed Stress Results from the Manning’s Equation. 48

viii

LIST OF FIGURES

Figure 1. Hillshade location map of Yosemite Valley 2

Figure 2. Hillshade location map of Eastern Yosemite Valley 4

Figure 3. P1 boulders located in Upper Pines Campground 7

Figure 4. P2 boulders located throughout Upper Pines Campground 8

Figure 5. Quaternary Geomorphologic Features. 10

Figure 6 - Geologic map of eastern Yosemite Valley 11

Figure 7. Kgp/Khd Contact 13

Figure 8. Kgp/Khd Hand Samples 14

Figure 9. Kcp Field Samples 14

Figure 10. El Capitan Terminal Recessional Moraine 18

Figure 11. Upper Pines Campground Mapped Boulder Locations 20

Figure 12. Cosmogenic 10Be Exposure Age Sample Locations. 22

Figure 13. XRF Sample Locations 26

Figure 14. Mapped Boulder Cumulative Frequency Curve 28

Figure 15. Spatial Analysis of Mapped Boulders. 29

Figure 16. Harker Variation Diagrams of XRF Analysis 35

Figure 17. Spatial Analsysis of XRF Results 36

LIST OF FIGURES CONTINUED

Figure 18. Stoneman Bridge Stratigraphic Section 38

Figure 19. Terrace Profiles 41

Figure 20. Additional Spatial Analysis 42

Figure 21. Conceptual Fan Model 43

Figure 22. Discharge Model Cross Section Location 46

Figure 23. Bed Load Shear Stress vs. Water Depth 48

Figure 24. Photos of Fluvial Deposits 49

Figure 25. Geologic Map with Rock Fall Extent 51

CHARACTERIZING BOULDER DEPOSITION TO ASSESS ROCK FALL

HAZARD IN UPPER PINES CAMPGROUND, YOSEMITE NATIONAL PARK, CA

INTRODUCTION

Yosemite Valley in Yosemite National Park, California, is a glacially carved U- shaped valley that has undergone multiple episodes of glacial erosion. Located in the central portion of the Sierra Nevada batholith, granitic plutons ranging from Middle

Cretaceous to Late Cretaceous dominate the ~1000 m of exposed wallrock (Fig. 1).

Rock fall is an ongoing phenomenon in Yosemite Valley, but has only been documented for the past one hundred and fifty years (Stock et al., 2012). The Last

Glacial Maximum (LGM) in the Sierra Nevada ended ~15 ka (Birkeland and Burke

1988; Bursik and Gillespie 1993; Phillips et al. 2009; Rood et al. 2010) and cleared previous talus fields and older glacial deposits. Since the LGM, accumulation of boulders and debris have created extensive talus fields at the base of Yosemite Valley’s

~1000 m tall cliffs and alluvial fans that now extend far into the valley. Remnant terraces on the valley floor record a maximum aggradation level prior to breaching of a terminal moraine that lowered the base level of the Merced River.

Figure 1. Hillshade location map of Yosemite Valley produced from a 1 m LiDAR Digital Elevation Model (DEM) provided by National Park Service (NPS). Red box indicates the field area and the tent indicates the location of Upper Pines Campground. Note the location of El Capitan and

Curry Village for future figures. 2

Since 1857, more than 900 rock falls and other slope movements have been documented in Yosemite National Park with most rock falls originating from the glacially-steepened walls of Yosemite Valley (Stock et al., 2012). In 2008, two rock falls of ~6000 m3 originated above Curry Village on Glacier Point and caused significant damage to park infrastructure and prompted the permanent closure of 246 structures in the Curry Village area. The 2008 Glacier Point rock falls highlighted the importance of rock-fall hazard assessment in Yosemite Valley, including that associated with large but infrequent prehistoric rock falls. A widely scattered group of boulders at the site of Upper Pines Campground in eastern Yosemite Valley may represent one of these large prehistoric rock falls.

Upper Pines Campground is located in the eastern extent of Yosemite Valley on a north-dipping alluvial fan and bordered to the east by the Merced River (Fig.2). The fan extends outward from Happy Isles just downstream of a prominent change in river gradient, and projects past a recessional moraine (adjacent to the campground) and into the eastern portion of Yosemite Valley. The campground consists of 240 campsites with granitic boulders >1 m3 scattered throughout the entire campground and west of the campground. Previous mapping compiled by Wieczorek et al. (1999) identified these boulders as a prehistoric debris flow and an uncharacterized talus deposit.

Figure 2. Hillshade location map of Eastern Yosemite Valley. Qlm = Quaternary lateral moraine.

DEFINITION OF THE PROBLEM AND HYPOTHESES

I identified two boulder populations in Upper Pines Campground based on visual estimation of exposed volume and angularity during field reconnaissance. The first boulder population (P1) is in the southern portion of the campground and consists of a higher frequency of boulders >5 m3 (Fig. 3). Due to the magnitude of boulder size,

P1 was hypothesized as being the result of a rock fall, despite a general lack of geomorphic resemblance to other mapped rock fall deposits within Yosemite Valley

(e.g., Wieczorek et al., 1998, 1999; Stock and Uhrhammer, 2010). P1 is over ~130 m from the talus adjacent to the west side of Glacier Point but appears to be isolated.

The second population (P2) occupies the majority of the campground with no noticeable trend in boulder size distribution (Fig. 4). The lack of concentrated boulders

>5 m3 and the deeply inset appearance of P2 boulders within the fan surface separate it from P1 (Fig. 4). Large boulders are still present in P2 but very infrequent when compared to P1. The relative infrequency of large P2 boulders led to the hypothesis that P2 may be the result of glacial deposition (till and/or ground moraines) subsequently partially buried by alluvial infilling.

Recognizing that these boulder populations may result from multiple modes of deposition and age led to the following multiple hypotheses, which are not necessarily

6 mutually exclusive. Possible hypotheses of modes of transportation and deposition include: (1) rock fall originating from Glacier Point, (2) glacial deposition during the

LGM, and (3) glacial outwash or large scale flood event. These hypotheses were tested in order to characterize mode of deposition, age, and boulder source.

Figure 3. P1 boulders located in Upper Pines Campground. See appendix B for photo locations. 7

Figure 4. P2 boulders located throughout Upper Pines Campground. See appendix B for photo locations. 8

GEOLOGIC SETTING

Overview

Retreat of Tioga ice (LGM) ~15 ka presumably left the valley free of debris

(Wieczorek and Jäger, 1996) with prominent recessional moraines documenting retreat of ice (Matthes, 1930; Huber et al. 1987) from west to east (Fig. 5). Ongoing valley infilling has left a low gradient alluvial fill now occupied by the modern day meandering Merced River, and talus accumulations adjacent to cliff faces. I used previous mapping by Calkins (1985), Alpha et al. (1987), Wieczorek et al. (1999), and

Peck (2002) in conjunction with a 1 m LiDAR DEM to create a detailed geologic map of the eastern extent of Yosemite Valley focusing around Upper Pines Campground

(Fig. 6).

Cretaceous Intrusive Rocks and Field Relationships

Glacier Point Granodiorite (Kgp), Half Dome Granodiorite (Khd), and Cathedral

Peak Granodiorite (Kcp) are the three lithologic units that will play a significant role in characterizing the boulder deposits within Upper Pines Campground. The topographic feature of Glacier Point consists of Kgp and Khd with a contact between the two units

Figure 5. Hillshade map of glacial features and prominent terraces. (a) Terminal recessional moraines (Qrt) in western Yosemite Valley. The moraine nearest El Capitan (outlined in red) dammed Yosemite Valley creating Glacial Lake Yosemite. (b) Lateral moraine (Qlm) adjacent to Upper Pines Campground and terraces (Qat) long Tenaya Creek and the Merced River. Dotted lines infer moraine crests.

Figure 6 - Geologic map of eastern Yosemite Valley overlain on a 1 m LiDAR DEM produced hillshade. Modified from Peck (2002), see appendix A for detailed unit descriptions. Dashed line represents the moraine crest.

~500 m above Upper Pines Campground (Fig. 7). Kgp occupies the summit of Glacier

Point and the contact between Kgp and Khd roughly runs in a NW-SE trend intersecting the valley west of Curry Village and extending up Illiloutte Creek. As described by

Peck (2002), Kgp contains 15-25% anhedral mafic minerals that have a foliated texture and Khd contains 8-12% euhedral mafic minerals (Fig. 8). The majority of eastern

Yosemite Valley consists of Khd which also extends up the Little Yosemite Valley tributary ~13 km to Merced Lake before terminating into Kcp. Kcp is readily distinguishable due to its potassium feldspar megacrysts (Fig. 9).

The contact between Kgp and Khd was significantly higher than the LGM trimline and was therefore not subject to LGM glacial erosion (Alpha et al. 1987).

Finding Kgp boulders within Upper Pines Campground would therefore be indicative of rock fall originating above the Kgp and Khd contact, or indicative of fluvial processes transporting and altering rock fall deposits that were up valley from Upper Pines. Khd occupies the surrounding wallrock of Upper Pines as well as the Little Yosemite Valley tributary and would therefore be found in glacial, rock fall, and fluvial deposits.

Because the Kcp contact is ~13 km up valley from Upper Pines, large >1m3 Kcp boulders would be an indicator for glacial deposition (as seen in Yosemite Valley

moraines) whereas smaller boulders would be an indicator for fluvial deposition.

Figure 7. (a) Glacier Point viewed from Dinner Ledge located on Washington Column. The dashed red line is the approximate location of the contact between Khd and Kgp ~500 m above the valley floor. (b) Hillshade with the approximate location of Khd and Kgp contact dashed in red. The black

line indicates the extent of talus surrounding the campground.

Figure 8. In situ hand samples collected from wallrock surrounding Upper Pines Campground. (a) Khd from the base of Glacier Point, and (b) Kgp from the summit of Glacier Point.

Figure 9. Cathedral Peak Granodiorite potassium feldspar megacrysts (circled in red). (a) Kcp boulder located in a road cut through the Tiago recessional lateral moraine north of Upper Pines Campground. (b) Kcp boulder located within the Upper Pines Campground.

Quaternary History of Yosemite Valley

Eastern Sierra Nevada glacial stratigraphy recognizes a series of Pleistocene glaciations with the original nomenclature published by Blackwelder (1931) and consisting of (oldest to youngest) McGee, Sherwin, Tahoe, and Tioga (Rood et al., 2010).

Blackwelder’s original nomenclature was based on relative dating and lacked numerical ages. Correlating glacial stratigraphy locally along the eastern Sierra Nevada and across the Sierra crest has been an ongoing debate fueled by a general lack of absolute dating

(Rood et al., 2010).

Recent work by Rood et al. (2010) and Phillips et al. (2009) reconfigures previous nomenclature into two events correlated with global marine oxygen isotope stages (MIS) and defines these events with ages. Phillips et al. (2009) sampled 124 boulders at the crest of mapped moraines in Bishop Creek, California and used cosmogenic 36Cl to correlate advances of MIS 2 and 6 to previous nomenclature. Rood et al. (2010) sampled

115 boulders from moraine crests in 9 drainages (including Bishop Creek), recalculated

Phillips et al. (2009) 36Cl data set, and used Phillips et al. (2009) input parameters to correlate 10Be results to 36Cl results. The end product of both studies is numerical ages of

229 boulders in agreement of MIS 2 and 6 events from 9 drainages covering ~260 km of eastern Sierra Nevada glacial deposits (Table 1). The older McGee and Sherwin mapped deposits did not yield conclusive results and were relatively dated to numerical ages based on positions noted in Phillips et al. (2009) and Rood et al. (2010) studies.

Table 1. Correlation of glacial nomenclature with 10Be and 36Cl constrained ages and Marine Oxygen Isotope stages.

Nomenclature Numerical Age Constraints Marine Oxygen Isotope Stage Tioga (Rood et al. 2010) 18.6 ± 1.9 ka 2 Tioga (Phillips et al. (2009) 28 ka - 14.5 ka 2 Tahoe (Rood et al. 2010) 144 ± 14 ka 6 Tahoe (Phillips et al (2009) 165 ka - 135 ka 6 Note that Rood et al. produced a mean age for MIS 2 and 6 whereas Phillips et al. produce a range.

Glacial ice has occupied Yosemite Valley at least three to four times with the earliest advances overtopping the valley walls and the youngest only partially filling the valley (Matthes, 1930; Wahrhaftig 1962; Huber, 1987). As the focus of this study is latest Pleistocene and Holocene time, older glaciations will not be addressed. I assume that older glacial deposits were either removed or buried by younger Tioga-aged deposits within Yosemite Valley. The Tioga advance peaked between 28-14.5 ka yrs (Bursik and

Gillespie, 1993; Phillips et al. 2009; Rood et al. 2010) and three samples collected by

Stock and Uhrhammer (2010) from a recessional moraine near El Capitan range from

28.12-14.32 ka. Previous work by Clark (1976) suggests that Tioga ice melted rapidly with Yosemite Valley totally deglaciated by 15 ka (Wieczorek and Jäger, 1996).

Matthes (1930) reported that the most prominent moraine near El Capitan was responsible for damming melting ice water and creating Glacial Lake Yosemite (Fig 10).

However, in 1878 this moraine was subject to demolition by early settlers to alleviate ponding of the Merced River near El Capitan (Glazner and Stock, 2010). As ice fully retreated out of Yosemite Valley, Glacial Lake Yosemite was fed by the Merced River and Tenaya Creek. Infilling of sediment advanced a delta from east to west and

17 eventually completely infilled the valley (Matthes, 1930). Subsequent incision by the

Merced River lowered the original alluvial fill ~5 m to the present day base level

(Matthes 1930). Terraces that are remnants of the maximum alluvial fill surface are preserved on the periphery of the valley, and distinguishably on the LiDAR DEM (unit

Qat in Figure 5b). As a result of infilling and erosion, the preserved moraines are most likely only a fraction of their original height.

Figure 10. El Capitan terminal recessional moraine (identified in Figure 4) colored to identify location within tree line. This moraine was responsible for damming the Merced River and forming Glacial Lake Yosemite and was breached sometime after 9.6 ± 1 ka causing local base level change of the Merced River which resulted in local incision of Yosemite Valley. The moraine was blown out by settlers in 1878 to alleviate ponding in El Capitan Meadow (Glazner and Stock, 2010).

METHODS

Field Mapping and Spatial Analysis

519 boulders were mapped by hand and GPS throughout the campground (Fig.

11). Criteria for mapping boulders consisted of exposed boulders visually estimated to be

>1 m3, inset with the fan surface, and undisturbed (some boulders had been moved during construction and were not mapped). There are countless boulders <1 m3 scattered across the entire campground but these boulders were not mapped.

The GPS location of boulders had a ± 7 m accuracy and was left uncorrected after determining that of the accuracy had little impact on the wandering of points outside the vicinity of the campground. Furthermore, boulders mapped using GPS were field checked for accuracy after importing data into ArcMap GIS. A 1 m LiDAR DEM and

GIS dataset of Yosemite Valley Infrastructure, both provided by NPS, and used to create hillshades, base maps, and eventually led to remote sensing and spatial analysis.

The angularity and approximate exposed volume of boulders was measured and compiled. Angularity was visually determined by the number of abrupt facets (definitive and traceable vertex between planes) using the Powers (1953) guide to roundness.

Figure 11. Mapped boulders within the Upper Pines Campground overlain upon the Yosemite Valley Infrastructure GIS dataset (provided by NPS). Rectangles protruding from campground loops are assigned parking spots for campsites. Note the concentration of boulders around loop 6.

Boulders were found to predominantly range from sub-angular to sub-rounded, with few rounded or angular boulders. Sub-angular boulders consist of fifty percent or more abrupt facets and sub-rounded consists of less than fifty percent abrupt facets. Because of the overall small percentage of rounded and angular boulders mapped, these boulders were included into the dataset as sub-rounded and sub-angular, respectively. Boulders were treated as cubic in shape and dimensions were measured by laying a tape measure flat upon the ground with length and width perpendicular to each other in an "L" shape. The highest point of a boulder was used to represent boulder height and measured perpendicular to length and width.

Cosmogenic 10Be Exposure Ages

Five boulders were sampled for cosmogenic 10Be exposure dating (Fig. 12). This technique utilizes the accumulation of the cosmogenic isotope 10Be in quartz exposed within ~1 m of the Earth surface by a known cosmic ray bombardment rate (Lal, 1991;

Gosse and Phillips, 2001). As cosmic rays interact with quartz within the rock, neutrons are spalled from the nucleus of oxygen atoms creating the cosmogenic nuclide 10Be. Rock fall deposits are well suited for 10Be exposure ages because rock fall is an instantaneous occurrence that excavates rocks from within a cliff face and exposes them on the ground surface (Ballantyne and Stone, 2004; Ivy-Ochs et al, 2009; Stock and Uhrhammer, 2010).

Vertical cliffs receive relatively low doses of cosmic rays (due to the incidence angle of incoming cosmic rays), and most rock falls >1 m in thickness produce rocks where

Figure 12. Cosmogenic 10Be exposure sample locations (yellow) overlain upon boulder locations (blue). UPC-1, 2, 3, and 4 were chosen due to their close proximity to the loop 6 boulder concentration. UPC-5 was selected because it is ~300 m from loop 6.

23 faces were likely shielded within the cliff prior to failure, thereby reducing the possibility of 10Be inheritance (10Be accumulation prior to failure). Other complicating factors include surface erosion of a boulder, rotation of a boulder after deposition, spallation from forest fires, and topographic, vegetation, and snow shielding of cosmic rays (Gosse and Phillips, 2001; Ivy-Ochs et al., 2009).

To address those issues, I collected samples from the top of large boulders generally positioned above the influence of fire-induced spallation. I accounted for topographic shielding (a significant concern due to the ~1 km tall cliffs of Yosemite

Valley) by making detailed measurements of the angle to the skyline from the exact sample location on a boulder. Snow and vegetation shielding was assumed to be negligible following the rational of Stock and Uhrhammer (2010) and a boulder erosion rate of 0.00065 cm/yr (Small et al., 1997; Stock et al., 2005) was assumed.

To test for multiple modes of deposition (e.g. glacial vs. rock fall) four out of the five (UPC-1, 2, 3, and 4) boulders sampled were taken from the notable concentration identified during reconnaissance in the southern portion of the campground. The fifth boulder sampled (UPC-5) was in the northern most extent of the second population where exposed volumes appeared smaller than in the first population. All boulders sampled were undisturbed, partially buried, were boulders (visually >3 m3). Chemical preparation of 10Be samples was performed at the Georgia Institute of Technology following standard methods (Kohl and Nishiizumi, 1992). Mass spectrometer analysis of 10Be

24 concentrations was performed by the Center for Accelerator Mass Spectrometry at

Lawrence Livermore National Laboratories.

X-Ray Fluorescence Spectrometry (XRF)

Thirty-eight hand samples were collected from boulders located within the Upper

Pines Campground, and surrounding wallrock and analyzed by XRF spectrometry in order to determine chemical affinity between Kgp and Khd (Fig. 13). Samples were broken up and cut to create unaltered surfaces free from chemical weathering and vegetative matter. These samples were ground to pea sized gravel then powdered into rock flour. Samples were then shipped to the University of North Carolina, Chapel Hill for XRF analysis on a Rigaku Supermini wavelength-dispersive XRF spectrometer.

Eight in situ samples of Kgp were collected to create a geochemical database to identify variability within this unit for comparison with samples collected within the

Campground. Six of the samples were collected along the summit of Glacier Point and two were collected near where Kgp intersects the valley floor, west of Curry Village near

Le Conte Gulley (Fig. 13). These samples encompass the lateral variation along the summit of Glacier Point, and the vertical variation from the summit to the valley floor.

Nine in situ samples of Khd wallrock surrounding Upper Pines were collected to identify local variability within Yosemite Valley. These Khd data were compiled with previous

XRF analysis of Khd by Gray et al. (2008) and were then plotted in Harker diagrams.

Comparing the geochemistry of in situ wallrock samples to boulder samples in the

25 campground allows for identification of transported boulders as Kgp and Khd which was inconclusive in the field. More importantly, the comparisons show the spatial distribution of sampled boulders on the fan; potentially identifying the relative contribution to boulder deposition of rock fall versus fluvial processes.

Figure 13. XRF sample locations of in situ wallrock (yellow), and sampled boulders (green).

RESULTS

Field Mapping and Spatial Analysis

The dataset for 519 mapped boulders was imported into ArcMap GIS and overlain on various maps including hillshade, 3 m and 10 m contours, slope and aspect.

In order to test for notable geomorphic trends or characteristics, mapped boulders were plotted using magnitude of exposed volume, height, and occupied surface area.

Cumulative frequency plots for mapped boulders show that sub-angular boulders throughout the campground are larger than sub-rounded boulders (Fig 14). Overlaying boulders on a hillshade and displaying by angularity (Fig. 15a) shows that the P1 concentration of boulders next to loop 6 is mostly sub-angular and the majority of all sub- angular boulders in the campground are located in this concentration. Sub-rounded boulders are scattered throughout the campground and show no dominate clustering in contrast to the clustering of sub-angular boulders in P1.

Angularity is a subjective measurement and this is especially the case when working with sub-angular to sub-rounded boulders which grade from one to the next within the Powers (1953) guide to roundness. One person’s interpretation may not agree with another’s which can yield discrepancies. To minimize reliance on angularity alone, these data were plotted by angularity as well as exposed volume (Fig. 15b). Using

Figure 14. Cumulative frequency curve of mapped boulders plotted individually by angularity. Note that sub-angular boulders are larger in exposed volume then sub-rounded.

Figure 15. (a) Mapped boulders plotted by angularity. Note the large concentration of sub-angular boulders directly to the south of Loop 6. (b) Boulders plotted by angularity as well as magnitude of exposed volume std. dev. (c) Boulders plotted by exposed volume std. dev.

30 angularity as well as exposed volume, the concentration of sub-angular boulders around loop 6 is still apparent. Exposed volume increments were created based on the standard deviation (11 m3) from the mean (4.08 m3). Magnitude of exposed volume starts at <

4.08 m3 and increases by 1 standard deviation ending at > 48 m3 or > 4 standard deviations. Removing angularity and looking strictly at exposed volume removes any subjectivity associated with angularity and separate populations can still be identified

(Fig. 15c). Therefore using all three approaches yield the same results that there is an apparent concentration of large boulders within the southern portion of the campground and directly surrounding loop 6.

Cosmogenic 10Be Exposure Ages

Four out of the five boulders sampled (UPC-1, 2, 3, and 5) for exposure ages yield ages between 8.06-11.94 ka with these four individual ages overlapping within analytical uncertainty (Table 2). The mean age of the four boulders is 9.6 ± 1 ka. The fifth boulder

(UPC-4) is an outlier with an age of 5.7 ± 0.5 ka, considerably younger than the other four. All ages postdate the LGM indicating that boulders present are not the result of glacial deposition. Furthermore, there is no correlation between relative dating (based on exposed volume or angularity) and 10Be exposed age because both sub-angular and sub- rounded boulders were dated and overlap in age within analytical uncertainty. As previously stated, UPC-5 was sampled because it was visually estimated as the largest

Table 2. Cosmogenic beryllium-10 data and exposure ages for Upper Pines Campground boulders

Sample Lat/Long Elevation Thicknessa 10Be Production rate Shielding Erosion Mass Be 10Be/9Bef,g 10Be Ageg,j, (˚N/˚W) (m) (cm) (atoms g-1 yr-1) factord rate quartze carrier (x 10-13) concentrationg k (cm yr-1) (g) (mg) ,h,i (104 atoms (ka) b c -1 Spallation Muons g SiO2) UPC-1 37.7327/ 1226 4 9.69 0.269 0.9180 0.00065 99.780 0.3897 3.95 ± 0.08 10.23 ± 0.23 10.90 119.5608 ± 1.04

UPC-2 37.7330/ 1225 3.5 9.85 0.269 0.9295 0.00065 99.500 0.3913 3.54 ± 0.11 9.21 ± 0.33 9.59 ± 119.5606 0.95

UPC-3 37.7342/ 1222 3 9.76 0.269 0.9197 0.00065 100.06 0.4090 3.25 ± 0.06 8.80 ± 0.19 9.23 ± 119.5617 8 0.87

UPC-4 37.7336/ 1223 4.5 9.44 0.268 0.8995 0.00065 100.49 0.4060 2.02 ± 0.04 5.37 ± 0.12 5.71 ± 119.5620 5 0.53

UPC-5 37.7356/ 1218 2 9.82 0.269 0.9200 0.00065 94.103 0.4080 2.98 ± 0.06 8.55 ± 0.19 8.90 ± 119.5636 0.84 aThe tops of all samples were exposed at the boulder surface. bConstant (time-invariant) local production rates based on Lal (1991) and Stone (2000). A sea level, high-latitude value of 4.8 10Be g-1 quartz was used. cConstant (time-invariant) local production rate based on Heisinger et al. (2002a, 2002b). dGeometric shielding correction for topography and sample surface orientation calculated with the Cosmic-Ray Produced Nuclide Systematics (CRONUS) Earth online calculator (Balco et al., 2008) version 2.2 (http://hess.ess.washington.edu/).. eA density of 2.7 g cm-3 was used based on the granitic composition of the samples. fIsotope ratios were normalized to 10Be standards prepared by Nishiizumi et al. (2007) with a value of 2.85 x 1012 and using a 10Be half-life of 1.36 x 106 years. g hA mean blank value of 53,540 ± 10,845 10Be atoms (10Be/9Be = 3.33 x 10-15 ± 8.74 x 10-16) was used to correct for background. iPropagated uncertainties include error in the blank, carrier mass (1%), and counting statistics. jPropagated error in the model ages include a 6% uncertainty in the production rate of 10Be and a 4% uncertainty in the 10Be decay constant. kBeryllium-10 model ages were calculated with the Cosmic-Ray Produced Nuclide Systematics (CRONUS) Earth online calculator (Balco et al., 2008) version 2.2 (http://hess.ess.washington.edu).

32 boulder in P2 and ~300 m from P1, making it ideal for 10Be sampling in order to test for two different periods of deposition (deposition during the LGM vs. postdating the LGM).

Given that UPC-1, 2, 3, and 5 have essentially the same 10Be exposure age, I interpret these boulders as originating during the same event. More specifically, boulder populations P1 and P2 originated from the same event despite inconsistencies of the exposed volume trend identified by field mapping and GIS analysis (Fig.15). Several interpretations as to why these four boulders yield a similar age can be made: (1) exhumation of boulders (with ages reset by burial) from the maximum aggradation surface, (2) rock fall, (3) rock fall subsequently modified by flooding. These interpretations will be addressed in detail in the discussion section.

X-Ray Fluorescence Analysis

Locating Kgp boulders within the Upper Pines Campground is an indicator for slope processes and potentially fluvial processes. As previously stated Kgp contains 15-

25% anhedral mafic minerals that have a foliated texture and Khd contains 8-12% euhedral mafic minerals (Peck, 2002). Kgp wallrock readily displays the described foliated texture and higher mafic mineral percentage making it easily distinguishable with little variation from Peck’s description. However, the observed Khd wallrock surrounding

Upper Pines was found to have locally foliated mafic minerals and increases in mafic mineral percentages; variations which created an appearance similar to Kgp. In the context of what is seen within the boulders of Upper Pines Campground, boulder mafic

33 minerals range from 8- 25% and contain foliated and non-foliated mafic minerals which made field identification of Kgp within Upper Pines Campground inconclusive and only

Kcp readily distinguishable.

Results from thirteen of the thirty-eight hand samples collected have been analyzed and the concentrations of major elements are compiled (Table 3). These data are plotted as Harker variation diagrams (Fig. 16). In situ Kgp wallrock yielded a 57.5-

58.15 wt% SiO2 contrasting with in situ Khd wallrock from Gray et al. (2008) 64.5-68.9 wt% SiO2. A compositional wallrock population’s gap between 61.9-64.9 wt% SiO2 can be seen in (Figure 16). Two out of the four sampled exposure age boulders (UPC-2 and

5) with a mean age of 9.6 ± 1 ka yrs fall outside of the population’s gap and incredibly close to the in situ Kgp trend. These boulders are most likely Kgp in origin, indicating they were moved from above the Kgp/Khd contact (~1828 m elevation) by slope processes. The other two boulders (UPC-1 and 3) fall directly within the Gray et al.

(2008) Khd trend. Together these four boulders provide an age and lithologic constraint on modes of transportation for boulders within Upper Pines Campground.

Boulders sampled for XRF analysis throughout the Upper Pines Campground

(UP series) fall within both geochemical trends. Kgp boulders are found to be scattered throughout the campground and not concentrated in a single location (Fig. 17). Two interpretations of the origin of boulders exposed in Upper Pines can be drawn from these results: (1) rock fall originating across the Kgp/Khd contact, or (2) fluvial modification of an existing rock fall deposit up valley from Upper Pines Campground.

Table 3. Major element compositions from in situ Kgp and Upper Pines Campground boulders sampled

Sample Unit Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Sum UPC-1 Khd 3.55 1.52 15.51 67.98 0.14 3.61 3.42 0.43 0.07 4.43 100.66 UPC-2 Kgp 3.69 2.4 16.78 61.89 0.18 2.9 5.05 0.7 0.09 6.08 99.76 UPC-3 0 UPC-4 Kgp 3.56 2.99 16.7 61.25 0.21 2.2 5.78 0.81 0.1 6.9 100.5 UPC-5 Kgp 3.72 2.62 16.66 61.42 0.25 3.34 5.24 0.73 0.1 6.36 100.44 GP-1 Kgp 3.35 3.00 16.80 58.15 0.17 2.61 5.90 0.82 0.11 8.00 98.909 GP-2 Kgp 3.40 3.39 16.95 56.77 0.20 2.03 6.22 0.93 0.13 8.77 98.796 GP-3 Kgp 3.43 3.26 17.41 56.18 0.20 1.92 6.54 0.87 0.12 8.55 98.492 GP-4 Kgp 3.12 2.61 15.98 60.78 0.16 2.77 5.19 0.81 0.10 7.68 99.174 GP-5 Kgp 3.41 2.94 16.76 59.23 0.19 2.18 6.07 0.78 0.11 7.93 99.587 GP-6 Kgp 3.42 3.10 17.41 57.50 0.20 2.37 6.00 0.95 0.11 8.21 99.274 GPB-1 Khd 3.54 1.51 16.19 65.17 0.10 3.61 3.59 0.46 0.09 5.33 98.77 UP-9 Khd 3.29 1.39 15.10 68.17 0.10 3.59 3.44 0.47 0.08 4.22 98.74 UP-10 Khd 3.46 1.14 15.04 69.26 0.13 3.94 3.11 0.4 0.06 3.51 99.28 UP-18 Kgp 3.05 3.04 16.75 59.75 0.18 3.29 5.59 0.85 0.11 7.31 99.13 UP-20 Kgp 3.59 3.37 17.42 59.56 0.2 2.94 5.47 0.91 0.11 7.51 99.51 UP-22 Kgp 3.23 3.23 16.68 59.86 0.18 3.29 5.6 0.86 0.11 7.32 99.21 UP-34 Khd 3.62 1.82 15.79 64.88 0.19 3 4.52 0.68 0.09 5.44 99.22 UP-35 Kgp 3.42 2.09 16.44 61.89 0.21 3.20 5.15 0.93 0.09 5.87 98.28 See appendix C for Gray et al. (2008) in situ Khd data used and appendix D for additional XRF sample data.

Figure 16. Harker variation diagrams of XRF analysis. Note the compositional wallrock population gap between in situ Kgp and Gray et al. (2008) Khd. 10Be exposure age series (UPC) falls in both populations as do samples collected in the Upper Pines Campground (UP). Note that UPC-3 is not plotted on this figure and that the lithology of UPC-3 is known through preliminary XRF results.

Figure 17. Mapped boulders overlain with XRF data showing the location of Khd and Kgp boulders in relation to all mapped boulders. UPC-1, 2, 3, and 5 yield a mean age of 9.6 ± 1 ka.

DISCUSSION

The results from field mapping and spatial analysis, cosmogenic 10Be exposure ages, and XRF analysis can be used to draw several interpretations of the origin of boulders within Upper Pines Campground. As previously stated, there are three primary hypotheses to test: (1) rock fall originating from Glacier Point, (2) glacial deposition during the last LGM, and (3) glacial outwash or large scale flood event.

Mapping and spatial analysis shows no geomorphic signature of rock fall beyond the concentration of large boulders near loop 6 (P1). Cosmogenic exposures ages of select boulders indicate that the second hypothesis is unlikely to be correct as all ages significantly postdate the LGM. XRF analysis of major elements identifies two geochemistry population trends for Kgp and Khd with sampled boulders (in Upper Pines

Campground) falling within both populations. This suggests that Kgp boulders may have been moved by either slope processes (rock fall) or fluvial processes.

Aggradation of and Degradation of Yosemite Valley

Two radiocarbon samples were collected along the cutbank of a terrace located between Stoneman Bridge and House Keeping Camp by Gerald Weizoreck of the U.S.

Geological Survey (Fig. 18). The samples were located ~1.5 m above the modern

Figure 18. Stratigraphic section of the Merced River left cutbank between Stoneman Bridge and House Keeping Camp, showing calibrated radiocarbon age ranges for samples from the cutbank. ~2.5 m of aggradation occurred after deposition of radiocarbon samples and prior to degradation. Red star = sample site location, HCK= House Keeping Camp, SB = Stoneman Bridge, CV = Curry Village Figure modified from Gerald Wieczorek by NPS Park Geologist Greg Stock

39 channel and ~2.5 m below the terrace surface. The two samples were taken within ~0.5 m of each other and have overlapping calibrated radiocarbon ages (Fig. 18) that collectively span 10,500-11,700 yr BP. These data suggest that >2.5 m of alluvial aggradation occurred near Stoneman Bridge after sample burial and prior to incision of the river channel. Based on the fine silts and clay layers, I infer that the sediment was deposited into a low energy environment with interbedded sand layers representative of storm peak discharge events capable of transporting coarser material. The absence of gravel, cobbles, or boulders within the stratigraphic section at Stoneman Bridge further supports a low energy depositional environment at this location.

The mean 10Be exposure ages (9.6 ± 1 ka) of boulders sampled within the campground overlap within analytical uncertainty of the radiocarbon samples collected near Stoneman Bridge. This suggests that boulders within the Upper Pines Campground were most likely deposited before incision of the maximum aggradation surface commenced. If this is correct, then some boulders may have been subjected to partial or total burial depending on their location on the original depositional surface. For example, boulders near the toe of the fan would be subjected to partial or total burial whereas boulders near the head of the fan would be subjected to less burial. This is assuming that the fan was originating from a single point such as an abrupt slope change or small cliff as seen up valley where an ice fall occurred during the LGM.

Slope profiles of terraces near Stoneman Bridge and Washington Column show a cross valley relationship of abandoned terraces ~4 m higher than the modern day river

(Fig. 19). There are no age constraints on terrace surfaces and it is unknown if these terraces represent the maximum aggradation surface prior to valley incision. However, both terraces were subjected to the same amount (~4 m) of incision and slope profiles across the Upper Pines Campground fan reveal the same ~4 m elevation difference between the fan and modern river. Assuming that all surfaces indicate the same amount of aggradation prior to incision, then 2.5-4 m of aggradation occurred, subjecting boulders to partial burial at the time or shortly after the time of boulder deposition. A further consideration to be addressed below is the change in gradient from the open valley to the head of the fan and how it would impact the distribution of aggraded sediment.

UPC-5 is the shortest in exposed height whereas UPC-1 is the tallest. Height, occupied area, and exposed volume increase from north to south (Fig. 20). The inset and relatively small appearance of boulders in the northern portion of the campground may best be explained by partial burial of boulders during fan development subsequent to boulder deposition. A simple conceptual model of this process is presented in Figure 21. I assume that the fan is originating and propagating from a single point of origin such as an abrupt slope change or cliff as seen up valley of Upper Pines Campground at Vernal and

Nevada Falls (both were part of a series of ice falls during the LGM). As time (t) increases from the retreat of glacial ice, a postglacial surface is created (t=0). A talus slope similar to the angle of repose is created at the base of a cliff or abrupt slope change

(t=1). As time increases additional debris propagate outward to the north depositing

Figure 19. Cross valley terrace (Qat) height correlation. Both terraces are ~4 m above the modern day channels. The terrace near House Keeping camp was the source of the radiocarbon samples which collectively yield an age of 10,500-11,700 Yrs BP and are located ~2.5 m below the terrace surface in the cutbank.

Figure 20. (a) Boulders plotted by standard deviation of exposed volume. (b) Boulders plotted by standard deviation of area occupied (length of boulder times width). (c) Boulders plotted by standard deviation of height. Plotting boulders by attributes side by side shows boulders increasing in orders of magnitude of standard deviation from the northwest to southeast. Note that exposed volume is plotted by four standard deviation magnitude increments as opposed to five.

Figure 21. Aggradation model of the Upper Pines Campground fan surface forming from a fixed point of origin. Boulders are represented by gray triangles. Deposition of boulders occurred ~9.6 ± 1 ka onto the depositional fan surface (t=5) before subsequently being buried by aggradation of the fan (t=6), and then degraded forming the modern fan surface (t=7).

44 sediment, decreasing gradient, and burying older surfaces. At ~9.6 ± 1 ka (t=5) boulders are deposited and boulder height, occupied area, and exposed volume are constant from north (fan toe) to south (fan head). As aggradation continues, boulders are subjected to partial burial (t=6) resulting in increasing boulder height, occupied surface area, and exposed volume from north (fan toe) to south (fan head). Degradation of the fan occurs and the modern fan surface is created (t=7), leaving boulders near the fan toe with relatively smaller appearance compared to boulders near the fan head. In this scenario the alluvial fan that now occupies Upper Pines Campground, would have propagated outward from a single point of origin due to the extreme decrease in gradient moving from the confined portion of the Yosemite Valley directly south of Happy Isles, to the open valley.

Figures 20b and 20c show a notable trend of boulders 0.5-2.5 standard deviation arcing around loop 6 that was not apparent in Figure 20a. This is likely due to a small seasonal channel that has incised into the fan and exposed boulders >3 m3 (Fig. 4d). The location of Figure 3c and Figure 3d is separated by ~20 m with the seasonal stream channel located between the two. The difference of appearance between these two photos identifies that large boulders of similar size are located across both sides of the stream and are either partially buried or fully buried. In the model presented in Figure 20 a rock fall or fluvial deposit would have its geomorphic expression almost entirely buried with only the largest boulders continuously exposed on the surface to continue 10Be production.

Discharge and Flow Velocity Modeling

Due to the lack of geomorphic expression of a rock fall, I used the Manning’s

Equation (equation 1 and 2) to estimate discharge and flow velocity for three different water depths at a confined portion of the valley where the gradient abruptly changes (Fig

22). I then calculated the bed stress (equation 3) generated by water depths of 5 m, 10 m, and 15 m, and the critical shear stress (equation 4) required to entrain boulders ≤1.4 m and ≤3.5 m in diameter. The diameters of boulders used were determined by the height of

UPC-1 and UPC-5 because of their known numerical age and lithology on the fan. If a large scale flood or outwash event had occurred, both boulders must have been entrained to reach their current locations. Slope was calculated from the Merced River at Happy

Isles Bridge to the cross section. Cross sectional area was calculated as a trapezoidal uniform open channel.

Equation (1) Discharge (m3/s):

Cross sectional area (m2):

Equation (2) Mean velocity (m/s):

Depth: d = (m), slope: s = 0.029, Manning’s resistance coefficient: n = 0.1, conversion factor (SI units): k = 1

3 Equation (4) Bed load shear stress (N/m ):

3 Equation (3) Critical shear stress (N/m ):

2 Shield’s parameter: ɳ = 0.06, specific weight of water: γ = 9810 (N/m ), density of quartz: ρQtz = 2650 (kg/m3), density of water: ρ = 1000 (kg/m3), gravitational acceleration: g = (N/s2), diameter of boulder: D = (m), slope: S = 0.029, depth: d = (m)

Figure 22. Cross section profile and location (A to A’) in relation to exposure age boulder heights used in discharge modeling. Note the increase of fan width downstream from the cross section. UPC-5 is ~1 km from the cross section and UPC-1 is ~600 m.

Discharge and shear stress modeling indicates that it is possible to generate the necessary discharge to entrain boulders with a diameter ≤1.4 m and ≤3.5 m (table 4).

However, it is important to note that bed load shear stress is dependent upon water depth.

Moving from the confined portion up stream of the cross section to the open valley of the fan would significantly increase cross section width thus significantly decreasing water depth. Therefore, it is unlikely that boulders could remain entrained to reach their current locations. The largest flood event ever recorded in Yosemite occurred on January 2, 1997 with a peak discharge of ~283 m3/s and depth of ~ 4m at Happy Isles and did not inundate Upper Pines Campground.

Intact moraines in Little Yosemite Valley support that flood events similar to the modeled event have not occurred since the LGM, ruling out boulder deposition from a large scale flood event. Furthermore, it is well established in the literature that the Tioga glaciation ended ~15 ka (significantly older then exposure ages) ruling out an outwash event. Throughout Upper Pines Campground gravel, cobble, and boulder deposits are found and indicate that fluvial influence over the fan has occurred (Fig. 24a and b).

However, a cut bank near Happy Isles Bridge exposes unsorted inner fan materials of sizes up to ~1 m3 (Fig 22c), significantly smaller then boulders seen on the surface of fan in Upper Pines Campground. .

Table 4. Discharge and bed stress results from the Manning’s equation.

Boulder Mean Shear Diameter Bottom Top Width XS area Velocity Discharge Stress Critical Shear (m) Width (m) (m) Depth (m) (m2) Manning’s (m/s) (m3/s) (N/m2) Stress w1 w2 d a n v1 Q τ0 τcr 1.4 50 70 5 300 0.1 5.0 1494 1422 1360 1.4 50 95 10 725 0.1 7.9 5731 2845 1360 1.4 50 140 15 1425 0.1 10.4 14760 4267 1360 3.5 50 70 5 300 0.1 5.0 1494 1422 3399 3.5 50 95 10 725 0.1 7.9 5731 2845 3399 3.5 50 140 15 1425 0.1 10.4 14760 4267 3399 See equation 1-4 for detailed parameters.

Figure 23. Bed load shear stress is a linear function dependent upon depth. Moving from the confined upper reach of the Merced River to the open fan in Upper Pines Campground would increase cross section width and decrease cross section depth causing a decrease in bed load shear stress ( ).

Figure 24. (a) Fluvially deposited gravels and cobbles. (b) Fluvially deposited cobbles and boulders. (c) River right cutbank near Happy Isles Bridge exposing the incised fan. See appendix B for photo locations

50 Rock Avalanche

Prehistoric flooding events modified the fan surface and contributed to boulder deposition <1 m3 in size, but did not deposit boulders >1 m3. This likely explains why only small (<0.5m3) Kcp boulders, transported over 13 km, are found on the fan surface

(Fig. 9b). Large scale outwash events did not occur based on the mean exposure age and intact moraines in Little Yosemite Valley. Rock fall is a more likely explanation of why large boulders, both Kgp and Kkd in origin with a mean 9.6 ± 1 ka, are found within

Upper Pines Campground. Because both lithologies are present, I interpret that the detachment zone must have cross the contact between both units. The extent of Kgp boulders is ~300 m from the adjacent talus field and ~500 m from the cliff face which identifies the potential run out of a single rock fall event (Fig. 25). The extent of these boulders would likely classify this event as a rock avalanche and the geomorphic expression of this event was most likely concealed by aggradation along the fan surface.

Inflection points between adjacent talus slopes may be the only geomorphic expression besides mapped boulders of this rock avalanche. Red arrows indicate possible trajectory as inferred from slope and aspect of Glacier Point Apron, from the detachment zone to the approximate boundary of the deposit (Fig. 25). 10Be exposure ages indicate that an event of this scale has happened only once in Upper Pines Campground since the

LGM producing a recurrence interval of 1 in 15 ka.

Figure 25. Geologic map of Upper Pines Campground with 10Be exposure ages, XRF sampled boulders, and mapped boulders. Red arrows are approximate rock fall trajectories inferred from aspect and slope of Glacier Point Apron.

CONCLUSION

Of the three hypothesis evaluated in this study, rock fall is the most likely origin for boulders in Upper Pines Campground. Four out of the five cosmogenic exposure ages of sampled boulders are within analytical uncertainty of each other and yield a mean age of 9.6 ± 1 ka (Table 2), significantly postdating the LGM and ruling out glacial deposition. Although the coincidence of ages could result from synchronous exhumation from beneath sediment cover, XRF results indicate Kgp and Khd boulders of the same age which further supports simultaneous rock fall deposition. Discharge and shear stress modeling indicates that fluvial deposition of boulders is unlikely due to the unrealistically large discharge required to maintain water depth to sustain boulder entrainment (Fig.5).

Based on XRF analysis I interpret the rock fall as originating from Glacier Point across the Kgp and Khd contact, and I infer based on the extent of Kgp boulder ~500 m from the cliff face, that this event was a rock avalanche. Exposed boulder volume, occupied surface area, and boulder height suggest that this deposit was subject to partial burial during alluvial fan propagation (Fig. 20). The fifth boulder has an exposure age of

5.7 ± 0.53 ka, is Kgp, and significantly postdates the other four exposure ages. I provisionally interpret this age as resulting from a younger rock fall event that occurred above the Kgp contact and postdates aggradation of the valley and the 9.6 ± 1 ka event.

REFERENCES

Alho, P., Baker, V.R., and Smith, L.N., 2010. Paleohydraulic reconstruction of the largest Glacial Lake Missoula draining(s). Quaternary Science Reviews, vol. 29, no. 23- 24, p. 3067-3078.

Alpha, T.R., Wahrhaftig, C., and Huber, N.K., 1987. Oblique map showing maximum extent of 20,000-year-old (Tioga) glaciers, Yosemite National Park, central Sierra Nevada, California. U.S. Geological Survey Miscellaneous Investigations Series Map I-1885.

Birkland, P.W., and Burke, R.M., 1988. Soil catena chronosequences on eastern Sierra Nevada moraines, California, U.S.A. Arctic and Alpine Research, vol. 20, p. 473- 484.

Ballantyne, C.K., and Stone, J.O., 2004. The Beinn Alligin rock avalanche NW Scotland: Cosmogenic 10Be dating, interpretation and significances. The Holocene, vol. 14, p. 448-453.

Bischoff, J.L., and Cummins, K., 2000. Wisconsin Glaciation of the Sierra Nevada (79,000–15,000 yr B.P.) as Recorded by Rock Flour in Sediments of Owens Lake, California. Quaternary Research, vol. 55, p. 15-24.

Blackwelder, E., 1931. Pleistocene glaciations in the Sierra Nevada and basin ranges. Geological Society of America Bulletin, vol. 42, p. 865-922.

Braun, J., Zwartz, D., and Tomkin, J.H., 1999. A new surface-processes model combining glacial and fluvial erosion. Annals of Glaciology, vol. 28, 282-290.

Brocklehurst, S.H., and Whipple, K.X., 2001. Glacial erosion and relief production in the Eastern Sierra Nevada, California. Geomorphology, vol. 42, p. 1-24.

Burkins, D.L., Blum, J.D., Brown, K., Reynolds, R.C., and Erel, Y., 1999. Chemistry and mineralogy of a granitic, glacial soil chronosequence, Sierra Nevada Mountains, California. Chemical Geology, vol. 162, p. 1-14.

Bursik, M.I. and Gillespie, A.R., 1993. Late Pleistocene glaciation of Mono Basin, California. Quaternary Research, vol. 39, p. 24-35.

Burke, R.M., and Birkeland, P.W., 1979. Re-evaluation of multiparameter relative dating techniques, and their application to the glacial sequence along the eastern escarpment of the Sierra Nevada, California. Quaternary Research, vol. 11, p. 21- 51.

Calkins, F.C., Huber, N.K., and Roller, J.A., 1985. Bedrock geologic map of Yosemite Valley, Yosemite National Park, California. U.S. Geological Survey Miscellaneous Investigation Series Map I-1639, scale 1:24 000, 1 sheet.

Clark, D.H., and Gillespie, A.R., 1997. Timing and significances of late-glacial and Holocene cirque glaciations in the Sierra Nevada, California. Quaternary International, vol. 38-39, p. 21-38.

Clark, M., 1976. Evidence for rapid destruction of latest Pleistocene glaciers of the Sierra Nevada, California. Geological Society of America Abstract with Programs, vol. 8, no. 3, p. 361-362.

Coleman, D.S., Gray, W., and Glazner, A.F., 2004. Rethinking the emplacement and evolution of zoned plutons: Geochronologic evidence for incremental assembly of the Tuolumne Intrusive Suite, California. Geology, vol. 32, no. 5, p. 433-436.

Dühnforth, M., Anderson, R.S., Ward, D., and Stock, G.M., 2010. Bedrock fracture control of glacial erosion processes and rates. Geology, vol. 38, no. 5, p. 423–426.

Economos, R.C., Memeti, V., Peterson,S.R., Miller, J.S., Erdmann, S., and Žák, J., 2010. Causes of compositional diversity in a lobe of the Half Dome granodiorite, Tuolumne Batholic, Central Sierra Nevada, California. Earth and environmental science transactions of the Roal society of Edinburgh, vol. 100 issue 1-2. p. 173- 183.

Gillespie, A.R., and Molnar, P., 1995. Asynchronous maximum advances of mountain and continental glaciers. Reviews of Geophysics, vol. 33, no. 3, p. 311-364.

Glazner, A.F., and Stock, G.M., 2010, Geology Underfoot in Yosemite National Park: Mountain Press, Missoula, MT, 300 pp.

Gosse, J. C., Phillips, F.M., 2001. Terrestrial in situ cosmogenic nuclides: theory and application. Quaternary Science Reviews, vol. 20, p. 1475-1560.

Gray, W., Glazner, A.F., Coleman, D.S., and Bartley, J.M., 2008. Long-term geochemical variability of the Late Cretaceous Tuolumne Instrusive Suite, central Sierra Nevada, California. Geological Society of London, special publication 304, p. 183-201.

Guzzetti, F., Reichenbach, P., and Wieczorek, G.F., 2003. Rockfall hazard and risk assessment in the Yosemite Valley, California, USA. Natural Hazards and Earth System Sciences, vol. 3, p. 491-503.

Hallet, B., Hunter, L., and Bogen, J., 1995. Rates of erosion and sediment evacuation by glaciers: A review of field data and their implications. Global and Planetary Change, vol. 12, p 213-235.

Hostetler, S.W.m and Clark, P.U., 1997. Climatic controls of western U.S. glaciers at the last glacial maximum. Quaternary Science Reviews, vol. 16, p. 505-511.

Huber, N.K., 1987. The geologic story of Yosemite National Park. U.S. Geological Survey Bulletin, Report: B 1595, p. 64

Ivy-Ochs, S., Heuberger, H., Hubik, P.W., Kerschner, H., Bonani, G., Frank, M., and Schluchter, C., 1998. The age of the Koefels event: relative 14C, and cosmogenic isotope dating of an early Holocene landslide in the Central Alps (Tyrol, Austria). Zeitschrift fur Gletscherkunde und Glazialgeologie, vol. 134, p. 57-70.

Ivy-Ochs, S., Poschinget, A.V., Synal, H-A., and Maisch, M., 2009. Surface exposure dating of the Flims landslide, Graubünden, Switzerland. Geomorphology, vol. 103, p. 104-112.

Kohl, C.P., and Nishiizumi, K., 1992, Chemical isolation of quartz for measurement of in-situ produced cosmogenic nuclides. Geochimica et Cosmochimica Acta, vol. 56, p. 3583-3587.

Lal, D., 1991. Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models. Earth and Planetary Science Letters, vol. 104, p. 424-439.

Matthes, F.E., 1930. Geologic history of the Yosemite Valley. U.S. Geological Survey Professional Paper 160.

Morrissey, M.M., Savage, W.Z., and Wieczorek, G.F., 1999. Air blasts generated by rockfall impacts: Analysis of the 1996 Happy Isles event in Yosemite National Park. Journal of Geophysical Research, vol. 104, no. B10, p. 23,189-23,198.

Peck, D.L., 2002. Geologic map of the Yosemite Quadrangle, central Sierra Nevada, California. U.S. Geological Survey Geologic Investigations Seires Map I-2751, scale 1: 62 000, sheet 1.

Phillips, F.M., Zreda, M.G., Benson, L.V., Plummar, M.A., Elmore, D., and Sharma, P., 1996. Chronology for fluctuations in Late Pleistocene Sierra Nevada glaciers and lakes. Science, vol. 274, no. 5288, p. 749-751.

Phillips, F.M., Stone, W.D., and Fabryka-Martin, J.T., 2001. An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land/atmosphere interface. Chemical Geology, vol. 175, p. 689-701.

Phillips, F.M., Zreda, M.G., Plummer, M.A., Elmore, D., and Clark, D.H., 2009. Glacial geology and chronology of Bishop Creeek and vicinity, eastern Sierra Nevada, California. Geological Soceity of America Bulletin, vol. 121, no. 7-8, p. 1013- 1033.

Powers, M.C., 1953. A new roundness scale for sedimentary particles. Journal of Sedimentary Petrology, Vol. 23, No. 2, p. 117-119.

Ratajeski,K., Glazner, A.F., and Miller, B.Y., 2001. Geology and geochemistry of mafic to felsic plutonic rocks in the Cretaceous intrusive suite of Yosemite Valley, California. Geological Society of America, vol. 113, no. 11, p. 1486-1502.

Rood, D.H., Burbank, D.W., and Finkel, R.C., 2010. Chronology of glaciations in the Sierra Nevada, California, from 10Be surface exposure dating.Quaternary Science Reviews 30 (2011), p. 646-661.

Small, E.E., Anderson, R.S., Repka, J.L., and Finkel, R., 1997. Erosion rates of alpine bedrock summit surfaces deduced from in situ 10Be and 26Al. Earth and Planetary Science Letters, vol. 150, p. 413-425.

Smith, S.J. and Anderson, R.S., 1992. Late Wisconsin paleoecologic record from Swamp Lake, Yosemite National Park, California. Quaternary Research, vol. 38, no.1, p. 91-102.

Stock, G.M., and Uhrhammer, R.A., 2010. Catastrophic rock avalanche 3600 years BP fromEl Capitan, Yosemite Valley, California. Earth Surf. Process. Landforms, vol. 35, p. 941–951.

Stock, G.M., Collins, B.D., Santaniello, D.J., Zimmer, V.L., Wieczorek, G.F., and Snyder, J.B., 2012, Historical rock falls in Yosemite National Park, California (1857-2011): U.S. Geological Survey Open-File Report (in review)

Stock G.M., Anderson, R.S., and Finkel, R.C., 2005. Rates of erosion and topographic evolution of the Sierra Nevada, California, inferred from cosmogenic 26Al and 10Be concentrations. Earth Surface Processes and Landforms, vol. 30, p. 985- 1006.

Stuiver, A., Reimer. P.J., and Reimer, R., 1986. CALIB Radiocarbon Calibration: http://calib.qub.ac.uk/calib/

Wahrhaftig, C., 1962. Geomorphology of the Yosemite Valley region, California. Bullitin - California, Division of Mines and Geology, p. 33-46.

Wieczorek, G.F., and Jäger, S., 1996. Triggering mechanisms and depositional rates of postglacial slope-movement processes in the Yosemite Valley, California. Geomorphology, vol. 15, p. 17–31.

Wieczorek G.F., Morrissey, M.M., Iovine, G., and Godt, J.W., 1999. Rock-fall potential in the Yosemite Valley, California. U.S. Geological Survey, Open-file Report 99- 578.

APPENDICIES

Appendix A - Detailed lithologic descriptions

Appendix B - Photo locations correlated to figure numbers

Appendix C - Gray et al. (2008) major element compositions for in situ Khd

Sample Unit Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Sum HD01-2 Khd 1.41 14.46 68.61 0.15 3.71 3.18 0.53 0.07 3.89 99.45 1.41 HD01-6 Khd 1.78 15.38 66.50 0.20 2.84 3.76 0.46 0.09 4.38 99.28 1.78 HD01-29 Khd 0.99 14.87 70.29 0.12 3.73 2.87 0.32 0.06 2.66 99.51 0.99 HD01-30 Khd 1.73 16.17 65.23 0.21 3.19 4.04 0.59 0.08 4.28 99.21 1.73 HD01-43 Khd 1.13 14.01 71.10 0.10 3.91 2.57 0.37 0.06 2.78 99.22 1.13 HD01-49 Khd 1.43 15.15 67.50 0.13 3.86 3.06 0.42 0.08 3.48 98.77 1.43 HD01-74 Khd 1.64 15.31 66.40 0.15 4.19 3.43 0.57 0.07 3.95 98.74 1.64 HD-0175 Khd 1.61 15.99 66.18 0.14 4.16 3.56 0.45 0.07 3.75 99.13 1.61 HD02-113 Khd 1.82 16.32 64.54 0.14 2.96 4.37 0.50 0.08 3.81 98.28 1.82 HD01-32 Khd 1.10 15.58 68.82 0.14 3.59 3.14 0.46 0.06 2.98 99.51 1.10 HD01-10 Khd 1.60 15.94 66.16 0.20 3.90 3.35 0.57 0.09 4.11 99.59 1.60 Hd01-80-P Khd 0.87 15.57 68.07 0.16 3.99 3.01 0.43 0.05 2.82 98.85 0.87 HD01-64-P Khd 1.19 15.80 66.98 0.19 3.59 3.38 0.49 0.06 3.18 98.77 1.19 HD01-77A-P Khd 0.94 15.35 68.90 0.19 3.14 3.25 0.50 0.05 2.95 99.31 0.94 HD01-84-P Khd 1.29 15.08 68.29 0.15 3.22 3.20 0.60 0.08 3.46 99.14 1.29 HD02-109-P Khd 1.06 15.58 68.37 0.15 3.95 2.96 0.44 0.08 2.75 99.27 1.06 HD02-102-P Khd 1.03 15.41 67.83 0.19 3.56 3.14 0.49 0.05 2.99 98.57 1.03 HD02-111-P Khd 1.08 15.87 67.82 0.16 3.99 3.01 0.41 0.07 2.71 99.13 1.08

Appendix D - Additional XRF sample data Coordinate system: Nad 83 UTM zone 11

Map Unit Sample Unit Sample (Peck Location Notes Northing Easting Type (XRF) 2002) ~ 3 m to the left of Church Bowl Lieback climbing route, CB-1 wallrock Kgp? - 4181135.38287 272632.924940 Yosemite Valley Base of "the one pitch way" Bishops Terrace climbing route pg. CB-2 wallrock Kgp? - 4181108.85101 272688.199651 77 Rock Climbing Yomite's Select, Yosemite Valley Glacier Point near Peck's 2002 Geologic Map of Yosemite Khd GP-1 wallrock Kgp Kgp 4178889.39752 273733.366977 & Kgp contact GP-2 wallrock Kgp Kgp Glacier Point near Le Conte Gulley 4179360.27766 272670.306035 GP-3 wallrock Kgp Kgp Glacier Point Four Mile Trail 4179266.74399 272499.553428 GP-4 wallrock Kgp Kgp Glacier Point parking lot 4178710.96178 273072.326177 GP-5 wallrock Kgp Kgp Glacier Point parking lot 4178706.04292 272914.307944 GP-6 wallrock Kgp Kgp Washburn Point 4177929.11029 273241.758323 GPB-1 wallrock Khd Khd Base of Glacier Point 4179386.59919 273638.315316 HD-1 wallrock Khd - NW of Half Dome on valley floor, Yosemite Valley 4180211.01351 274935.505307 Base of North Dome and adjacent to Mirror Lake, Tenaya ND/ML-1 wallrock Khd - 4181347.62509 275335.124195 Canyon RA-1 wallrock Khd - Base of western rib of Royal Archers, Yosemite Valley 4180800.58395 273535.940985 RA-2 wallrock Khd - Base of middle rib of Royal Archers, Yosemite 4180761.42527 273851.430036 Union-1 wallrock Kgp - NE side of Union Point near Le Conte Gulley, Yosemite Valley 4179977.12905 272437.584225 Union-2 wallrock Kgp - NE side of Union Point near Le Conte Gulley, Yosemite Valley 4180062.38922 272518.472071 UP-09 float Qal Khd Upper Pines Campground, Yosemite Valley 4179408.40073 274254.022353 UP-10 float Qal Khd Upper Pines Campground, Yosemite Valley 4179475.04821 274319.912474 UP-14 float Qal - Upper Pines Campground, Yosemite Valley 4179317.70252 274342.480958 UP-15 float Qal - Upper Pines Campground, Yosemite Valley 4179286.44341 274351.918093 UP-18 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179510.18346 274212.258713 UP-20 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179346.18209 274365.878781 UP-22 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179486.88785 274043.221453 UP-23 float Qal - Upper Pines Campground, Yosemite Valley 4179427.91125 274265.310385 UP-24 float Qal - Upper Pines Campground, Yosemite Valley 4179285.95354 274332.208495

Map Unit Sample Unit Sample (Peck Location Notes Northing Easting Type (XRF) 2002) UP-26 float Qal - Upper Pines Campground, Yosemite Valley 4179300.28771 274370.219634 UP-27 float Qal - Upper Pines Campground, Yosemite Valley 4179301.05561 274363.308518 UP-28 float Qal - Upper Pines Campground, Yosemite Valley 4179316.92558 274342.063235 UP-29 float Qal - Upper Pines Campground, Yosemite Valley 4179337.24938 274330.288741 UP-34 float Qal Khd Upper Pines Campground, Yosemite Valley 4179556.10140 274357.677238 UP-35 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179558.91704 274352.045958 UP-36 float Qal - Upper Pines Campground, Yosemite Valley 4179501.58037 274218.175079 UPC-1 float Qal Khd Upper Pines Campground, Yosemite Valley 4179268.24736 274341.458361 UPC-2 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179278.75718 274333.572896 UPC-3 float Qal - Upper Pines Campground, Yosemite Valley 4179432.46345 274254.722393 UPC-4 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179356.13954 274235.791248 UPC-5 float Qal Kgp Upper Pines Campground, Yosemite Valley 4179577.37555 274105.547605 Note that only thirteen of the thirty-eight hand samples have been analyzed. UPC-3 is known to be Khd in origin from preliminary XRF results run at Humboldt State University.