U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1782

Deglaciation and Postglacial

Treeline Fluctuation in the

Northern San Juan Mountains,

Colorado
Cover photo: Site of former Lake Emma (foreground) and Emery Peak (4,057 meters) to the south, 

northern San Juan Mountains, Colo.
Deglaciation and Postglacial Treeline 

Fluctuation in the Northern San Juan 

Mountains, Colorado

By Paul E. Carrara

Professional Paper 1782

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Carrara, P.E., 2011, Deglaciation and postglacial treeline fluctuation in the northern San Juan Mountains, Colorado: 

U.S. Geological Survey Professional Paper 1782, 48 p. 
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Background and Purpose....................................................................................................................2

Geography and Geology .....................................................................................................................3

Vegetation Zones in the San Juan Mountains.................................................................................3

The Timberline-Treeline Ecotone and Controlling Factors.............................................................3

Climate....................................................................................................................................................5

Acknowledgments................................................................................................................................5

Glacial Geology of the San Juan Mountains.............................................................................................7

Extent of Pinedale Glaciers ................................................................................................................7

Timing of Deglaciation..........................................................................................................................8

Radiocarbon Ages ......................................................................................................................8

Cosmogenic Ages........................................................................................................................8

Cirque Moraines and Present-Day Snowfields .............................................................................10

Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains ...................17

Introduction .........................................................................................................................................17

Telluride Water Well (2,665 m asl) ...................................................................................................19

Molas Lake (3,205 m asl)....................................................................................................................19

Little Molas Lake (3,370 m asl)..........................................................................................................21

Black Mountain Lake (3,413 m asl) ..................................................................................................21

Placer Gulch Bog (3,600 m asl).........................................................................................................21

California Gulch Bog (3,615 m asl) ...................................................................................................22

Bog Near Hurricane Basin (3,660 m asl).........................................................................................22

Eureka Gulch Bog (3,665 m asl) ........................................................................................................27

Lake Emma site (3,740 m asl).............................................................................................................29

Radiocarbon-Dated Conifer Fragments ................................................................................29

Pollen Analysis...........................................................................................................................33

Fossil Insects .............................................................................................................................34

Picayne Gulch Bog (3,750 m asl) ......................................................................................................37

Discussion.....................................................................................................................................................37

Holocene Treeline and Climatic Fluctuations in the San Juan Mountains ..............................37

Early Holocene Intensification of the Summer Monsoon? ..........................................................39

Conclusions ..................................................................................................................................................40

References Cited..........................................................................................................................................41

Appendix A Evidence of Higher Treelines During the Holocene in Western 

North America, as Indicated by Radiocarbon Ages or Cross-Dated 

Conifer Remains..............................................................................................................................45
iv

Figures

1. Map showing northern San Juan Mountains and selected sites discussed 

 in this study....................................................................................................................................2

2. Map showing study sites near the headwater region of Animas River..............................4

3. Diagram showing trajectories of major air masses that cross the 

San Juan Mountains ....................................................................................................................6

4. Photograph of site of the former Lake Emma in 1979 .............................................................7

5. Map showing limits of Wisconsin (Pinedale) glaciation in the western 

San Juan Mountains ...................................................................................................................9

6. Photograph of Molas Lake area...............................................................................................10

7. Diagram of composite stratigraphic section at Lake Emma site........................................11

8. Diagram of stratigraphic sections and radiocarbon ages from bogs in 

 northern San Juan Mountains..................................................................................................12

9–12. Photographs of: 

 9. Highland Mary Lakes area ..............................................................................................15

 10. Yankee Boy moraine in Yankee Boy Basin....................................................................16

 11. Grenadier moraine, northern base of Grenadier Range.............................................16

 12. Snowfield at northern base of Trinity Peaks in Grenadier Range.............................18

13. Stratigraphic cross section and radiocarbon ages from Telluride water well ................20

14. Photograph of Placer Gulch bog exposure near Evening Star Mine.................................23

15. Photograph of California Gulch, looking upvalley.................................................................25

16. Map of Hurricane Basin area...................................................................................................26

17. Photograph of bog near Hurricane Basin...............................................................................27

18. Diagram showing changing apparent elevation of the Hurricane Basin 

 core on the basis of spruce/pine ratios ..................................................................................28

19–21. Photographs of: 

 19. Eureka Gulch bog study site............................................................................................29

 20. Lake Emma and adjacent Sunnyside Mine around 1900 AD .....................................30

 21. Cross section of spruce fragment from Lake Emma site............................................31

22–25. Diagrams of: 

 22. Percentages of various types of pollen from Lake Emma .........................................35

 23. Concentrations of various types of pollen from Lake Emma......................................36

 24. Radiocarbon ages of coniferous wood fragments from study sites.........................38

 25. Timeline of events in the San Juan Mountains ...........................................................41

A–1. Sites of Holocene conifer fragments above present day treeline in western

 North America............................................................................................................................45

Tables

1. Climatic data from stations in the northern San Juan Mountains, Colo. ............................6

2. Radiocarbon ages of coniferous wood fragments from Lake Emma site, northern 

San Juan Mountains, Colo. .......................................................................................................13

3. Radiocarbon ages from Telluride water well.........................................................................19

4. Radiocarbon ages from bogs in northern San Juan Mountains ........................................24
v

Abbreviations Used in This Report

°C/km degrees Celsius per kilometer

δD delta deuterium (change in deuterium concentration)

< less than

> greater than

AMS accelerator mass spectrometry

asl above sea level

Be beryllium

cal yr calibrated radiocarbon age given in calendar years before present 

(present set to 1950 A.D.)

cm centimeter, centimeters

cm/km centimeter per kilometer

cm/yr centimeter per year

ka kiloannum (thousand years ago)

kg kilogram, kilograms

km kilometer, kilometers

km2 square kilometer, square kilometers

m meter, meters

m/yr meter per year

per mil/°C per mil per degree centigrade

U.S. United States

yr B.P. radiocarbon year before present (present set to 1950 A.D.)

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in)

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 0.3861 square mile (mi2) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Conversion Factors
vi

A Laboratory of Isotope Geochemistry, University of Arizona, Tucson

AA National Science Foundation–Arizona AMS Facility, University of Arizona, Tucson

CAMS Center for Accelerator Mass Spectrometry, Lawrence Livermore National 

Laboratory, Livermore, Calif.

CURL Colorado University Radiocarbon Laboratory, Boulder, Colo.

DIC Dicarb Radioisotope Laboratory, Chagrin Falls, Ohio*

GaK Gakushuin University, Tokyo, Japan*

GX Geochron Laboratories, Cambridge, Mass.

I Teledyne Isotopes, Westwood, N.J.*

SI Radiation Biology Laboratory, Smithsonian Institution, Rockville, Md.*

St Laboratory of Isotope Geology, Swedish Museum of Natural History, Stockholm, 

Sweden*

USGS U.S. Geological Survey, Menlo Park, Calif.*

W U.S. Geological Survey, Reston, Va. (conventional radiocarbon age)*

WW U.S. Geological Survey, Reston, Va. (accelerator mass spectrometry

Y Yale University Radiocarbon Laboratory, New Haven, Conn.*

Radiocarbon Laboratory Abbreviations Used in This Report

[*no longer operating]

Definitions Used in This Report

Radiocarbon ages Reported in radiocarbon years before present (yr B.P.); “present” set to 

1950 A.D.

Calendar years Taken from Fairbanks and others, 2005 (cal yr); "present" set to 1950 A.D.

Cosmogenic ages Labeled as 10Be or 36Cl ka; set from the year the ages were determined
Abstract 

The San Juan Mountains of southwestern Colorado 

contain numerous lakes and bogs at and above treeline. In 

June 1978, Lake Emma, a tarn above present-day treeline, was 

suddenly drained by the collapse of underground mine workings. This study was initiated because the draining exposed a 

well-preserved archive of subfossil coniferous wood fragments 

that provided a unique opportunity to further our understanding of the paleoclimatic history of this region.

These paleoclimatic studies—coniferous macrofossil 

identification in conjunction with radiocarbon dating, deuterium analysis of the dated conifer fragments, as well as pollen 

and fossil insect analyses—yielded new information regarding 

Holocene climate and accompanying treeline changes in the 

northern San Juan Mountains. This report synthesizes previously published reports by the author and other investigators, 

and unpublished information of the author bearing on late 

Pleistocene and Holocene treeline and climate in this region.

Retreat of the glacier that occupied the upper Animas 

River valley from its Pinedale terminal position began about 

19.4±1.5 10Be thousands of years ago and was essentially 

complete by about 12.3±1.0 10Be thousands of years ago. 

Two sets of late Pleistocene cirque moraines were identified in the northern San Juan Mountains. The older set 

is widespread and probably correlates with the Younger 

Dryas (11,000–10,000 radiocarbon years before present; 

12,800–11,500 calendar years). The younger set is found only 

in the Grenadier Range and represents remnant glacier ice 

lying in well-shaded niches in a mountain range undergoing 

rapid deglaciation. A snowbank at the northern base of this 

range appears to be fronted by a Little Ice Age moraine.

Soon after deglaciation the average July temperature is 

estimated to have been about 5°C cooler and timberline about 

650 meters lower than at present. However, timberline (and 

treeline) responded rapidly to the postglacial warming and 

reached higher-than-present elevations by the early Holocene.

A comparison of recently obtained accelerator mass 

spectrometry radiocarbon ages of coniferous wood fragments 

from Lake Emma, previously dated by conventional radiocarbon methods during the 1980s, led to a slight modification of 

previously published ages of Holocene treeline fluctuations. 

As early as 9,200 radiocarbon years before present (about 

10,400 calendar years) and probably to about 5,400 radiocarbon years before present (about 6,200 calendar years), 

treeline was at least 80 meters higher than at present (about 

3,660 meters). Furthermore, a large conifer fragment with a 

complacent annual ring record suggests that timberline may 

have been at least 140 meters higher than present (about 

3,600 meters) about 8,000 radiocarbon years before present 

(about 8,900 calendar years). These past elevations of treeline 

and timberline suggest that growing-season temperatures 

were at least 0.5–0.9°C warmer than at present. Deuterium 

data from the Lake Emma wood samples suggests that the 

maximum average temperature change from about 9,000 

to 5,400 radiocarbon years before present (about 10,150 to 

6,200 calendar years) was about 4°C. Owing to these warmer 

temperatures the summer monsoon circulation, which currently brings a large part of the annual precipitation to the San 

Juan Mountains, probably was more intense during the early 

and middle Holocene than it is today. 

Between about 5,400 and 3,500 radiocarbon years before 

present (about 6,200 and 3,770 calendar years) it appears that 

treeline was near its present-day limit. After 3,500 radiocarbon 

years before present (about 3,770 calendar years), evidence of 

treeline position is very sparse, suggesting that treeline lay at, 

or below, its present-day elevation. However, a spruce krummholz fragment from the Lake Emma site provided two radiocarbon ages of about 3,100 radiocarbon years before present 

(about 3,300 calendar years). It is not clear whether this wood 

fragment represents a short-lived climatic amelioration or 

whether it was from a unique individual that grew above the 

general treeline at that time. 

Approximately 20 other studies at sites throughout 

western North America that have yielded records of conifer 

remains above present-day limits are presented in the appendix 

of this report. The results of many of these studies are similar to those obtained in this study and indicate a higher than 

present-day treeline during the early to middle Holocene.

Because this study is based on more than 100 radiocarbon 

ages, including 66 ages from 53 coniferous wood fragments 

from the Lake Emma site, as well as pollen and insect analysis 

and deuterium data from the wood fragments, this study represents one of the best-documented records of Holocene treeline 

fluctuations in North America.

Deglaciation and Postglacial Treeline Fluctuation in the 

Northern San Juan Mountains, Colorado

By Paul E. Carrara
2 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Introduction

Background and Purpose

The study area lies in the northern San Juan Mountains of 

southwestern Colorado (fig. 1). This mountain range contains 

numerous lakes and bogs, many of which are at and above 

treeline. Investigations of the sediment and organic material 

recovered in these bogs, including pollen and conifer macrofossils, in conjunction with radiocarbon dating, produced considerable new information regarding upper treeline position 

during the early to middle Holocene. In addition, on 4 June 

1978, Lake Emma (fig. 1), a tarn above present-day treeline, 

was suddenly and completely drained by the collapse of 

underground mine workings (Carrara and Mode, 1979; Marcus 

and Marcus, 1983; Carrara and others, 1984, 1991). The draining of Lake Emma provided a unique opportunity for paleoclimatic studies because the lake’s sediments were incised by 

the escaping waters, which exposed a well-preserved archive 

of subfossil coniferous wood fragments above present-day 

treeline. Fifty-three of these coniferous fragments were 

subsequently radiocarbon dated (some several times) (Carrara 

and others, 1984, 1991), yielding one of the best-documented 

records of Holocene treeline fluctuations in North America. 

Because of concerns about anthropomorphic causes of global 

warming, the study and documentation of the natural variability in ecosystems is important in order to understand the 

possible effects of future climate change.

The elevation of upper timberline (the upper elevational 

limit of large, upright trees) and treeline (the upper elevational 

limit of krummholz—the stunted, scattered, and commonly 

prostrate trees that grow in physically stressed sites above 

timberline) is controlled by temperature during the growing 

season (Fritts, 1976; Tranquilli, 1979). The dating of krummholz coniferous wood fragments indicated periods of warmer 

than present-day summer temperatures. In addition, Friedman 

and others (1988) and Epstein and others (1999) measured 

deuterium concentrations in the cellulose of many of these 

krummholz fragments, which they interpreted as indicating 

warmer summer temperatures or increased precipitation from 

southerly sources (or both). Together, these studies yielded 

Black Mountain Lake

Hurricane Basin

Molas Lake

Telluride

Ophir

Rico

Silverton

Ouray

Lake City

Rio Grande

 Gunnison River

 Animas River Henson Creek

YANKEE BOY BASIN

GRENADIER RANGE

(2,670)

(2380)

(2,842)

(2,640)

(2,690)

Mt. Wilson

 (4,342)

(4,282)

 Uncompahgre River

Rio Grande Pyramid

 (4,213)

Clear 

Creek

(4,278)

(4,361) (4,272)

(4,065)

(2,973)

(4,313)

CONTINENTAL

DIVIDE

COLORADO

 38 ̊00'

 37 ̊45'

0

0 10 MILES

Dolores River

(3,413)

(3,740)

(3,205)

(3,660)

Study area

Red Mountain Pass

Little Molas Lake

(3,370)

10 KILOMETERS

 San 

Miguel River

(3,415)

Highland

Mary Lakes

Emery Peak

(4,057)

108 ̊00'

 

107 ̊30'

Lake Emma

EXPLANATION

Elevation is in meters

Peak

Town

Site discussed in text

Figure 1. Northern San Juan Mountains and selected sites discussed in this study. Elevations in meters.
Introduction 3

much information about Holocene treeline and climatic fluctuations in the San Juan Mountains. 

The purpose of this report is to synthesize previously 

published reports by the author and by other investigators, and 

previously unpublished information bearing on late Pleistocene and Holocene treeline and climate in order to further 

our understanding of the paleoclimatic history of the northern 

San Juan Mountains of Colorado. The previously unpublished 

information consists of radiocarbon ages of coniferous wood 

fragments or other material associated with conifer remains 

from a water well drilled near the town of Telluride (fig. 1) in 

1991 and from exposed sediments of Lake Emma collected 

between 1978 and 1987. These climate-related data from sites 

in the northern San Juan Mountains are presented and their 

implications for postglacial vegetation, Holocene treeline, and 

climate are then discussed.

Geography and Geology 

The San Juan Mountains, situated along the Continental Divide, constitute a rugged mountainous region of about 

20,000 square kilometers (km2

). About 2,000 km2 lies in 

the alpine zone above timberline, and many peaks exceed 

4,200 meters (m) in elevation. The mountain range contains 

the headwaters of the various tributaries of the Colorado 

River, including the Animas, Gunnison, San Miguel, and 

Uncompahgre Rivers, as well as the headwaters of the Rio 

Grande (fig. 1).

The San Juan Mountains consist of a sequence of middle 

to late Tertiary lava rocks and pyroclastic rocks as much 

as 2 kilometers (km) thick that were erupted from at least 

15 calderas (Steven and Lipman, 1976). These volcanic rocks 

unconformably overlie metamorphosed sedimentary and volcanic rocks and intrusive rocks of Precambrian age, as well as 

sedimentary rocks of Paleozoic, Mesozoic, and early Cenozoic 

age (Steven and others, 1974; Casadevall and Ohmoto, 1977). 

Vegetation Zones in the San Juan Mountains

Because of their elevation and relief, the San Juan Mountains contain a number of vegetation zones, as discussed in 

Maher (1961). Below about 2,285 m are what Maher (1961) 

describes as the “transition and Upper Sonoran zones.” In 

these zones, pinyon pine (Pinus edulis) and various species of 

juniper (Juniperus scopulorum, J. monosperma, and J. osteosperma) are abundant, as is scrub oak (chiefly Quercus gambelii). Ponderosa pine (Pinus ponderosa) is common down 

to about 1,825 m, and Douglas fir (Pseudotsuga menziesii) is 

restricted to north-facing slopes (Maher, 1961). 

In the montane zone, between about 2,285 and 2,900 m, 

Douglas fir, scrub oak, and aspen (Populus tremuloides) are 

abundant (Maher, 1961). Blue spruce (Picea pungens) and 

white fir (Abies concolor) are also abundant above about 

2,400 m. Dense stands of ponderosa pine can be found 

up to about 2,600 m, whereas Engelmann spruce (Picea 

engelmannii) and subalpine fir (Abies lasiocarpa) grow above 

this elevation (Maher, 1961). Limber pine (Pinus flexilis) is a 

minor component of the vegetation in this zone (Maher, 1961).

The subalpine forest zone, extending from about 2,900 m 

to timberline, is dominated by Engelmann spruce and subalpine fir (Maher, 1961). Above timberline, krummholz are 

dominated by Engelmann spruce and minor amounts of 

subalpine fir that reach to treeline. In the San Juan Mountains, 

timberline generally lies between 3,535 and 3,600 m, whereas 

treeline generally lies at about 3,660 m. Although bristlecone 

pine (Pinus aristata) and limber pine, two species commonly found at treeline in many areas in Colorado, have been 

reported in the San Juan Mountains (Maher, 1961; Arno and 

Hammerly, 1984), none were noted in the study area.

In the alpine zone, above about 3,660 m, vegetation 

consists mainly of herbs, such as sagebrush (Artemisia spp.), 

sedges (Cyperaceae), and grasses (Poaceae) that can survive 

the short growing season. Occasional patches of dwarf willow 

(Salix spp.) can be found up to an elevation of about 4,000 m.

The Timberline-Treeline Ecotone and

Controlling Factors

The upper limits of alpine tree growth in the Northern Hemisphere generally correlate with the 10°C isotherm 

of the warmest month, usually July (Arno and Hammerly, 

1984). This correlation is only approximate, because trees in 

most upper timberlines gradually become smaller and more 

deformed upslope. Thus, the positions of timberline and 

treeline are not precise (Arno and Hammerly, 1984, p. 13), 

and estimates of their positions can be subjective. In this 

study timberline was estimated as the upper altitudinal limit of 

upright trees generally taller than about 5 m. 

Treeline was estimated as the upper altitudinal limit of 

krummholz, although in rare cases an individual grew above 

this limit. An example of a unique individual that grew above 

the general treeline limit is a dead krummholz found on a 

ridge above California Gulch (fig. 2). Although the tree had 

grown at an elevation of 3,870 m, about 200 m above the general treeline, it had grown in a sheltered niche at the base of a 

small south-facing cliff, where it received a sizeable amount 

of solar radiation. Wood collected from this individual yielded 

a radiocarbon age of 280±80 years before present (yr B.P.) 

(laboratory number W–6237; age is outside calibration range). 

In addition to temperature, other factors such as aspect, 

cold-air drainage, snow, wind, and snow avalanches can influence local timberline and treeline. In the northern hemisphere 

they are generally higher on west- and south-facing slopes 

because these slopes receive more insolation.

Owing to cold-air drainage, many cirque and valley floors 

in the northern San Juan Mountains are devoid of conifers, 

whereas surrounding slopes or ridges may support patches 

of krummholz. Cold air commonly settles along valley floors 

such that frost may occur on most nights, while nearby slopes 

and ridges remain frost free (Arno and Hammerly, 1984).
4 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 2. Study sites near headwater region of Animas River. Elevations in meters.

Treasure Mountain

 (3,915)

Eureka Mountain

 (3,942)

(4,046)

(4,100)

(4,057)

(3,949)

Emery Peak

(4,058)

Lake Emma Site

Eureka Gulch Bog

(4,051)

Placer Gulch Bog

Picayne Gulch

Bog

California Gulch Bog

(4,102)

California Mtn

 (4,020)

Eureka

Gulch

Animas

River

Sunnyside

Mine

Evening

Star Mine

California Gulch

Placer Gulch

0 

0 1 MILE 

1 KILOMETER

Animas

River

EXPLANATION

Prominent ridge 

Forested area

Krummholz area

107o35' 

37o55'
Introduction 5

Snow can affect the position of timberline and treeline 

in several ways. Deep snow, melting late into the growing 

season, restricts the amount of time trees have to respond to 

the already short growing season (Jamieson and others, 1996). 

Conversely, snow may act as a protective blanket that shields 

young trees from the desiccating effects of winter winds. In 

some dry, windy areas, such as the Front Range in Colorado, 

windward slopes are swept free of snow, and the resulting lack 

of snow protection prevents trees from becoming established, 

thus lowering treeline (Arno and Hammerly, 1984).

Wind can also exert an important influence on the timberline and treeline ecotones. Strong winds can break branches 

and treetops as well as desiccate exposed trees. Wind-blown 

ice crystals can severely damage unprotected trees. Because 

the hardness of ice increases with decreasing temperature, 

during the winter trees are “sand blasted” by high winds and 

blowing snow (at 0°C ice has a hardness of 1.5 on the Mohs 

scale; at –70°C the hardness is 6 (Nesje and Dahl, 2000, 

p. 50)). The effects of desiccation and mechanical damage by 

wind are commonly seen in krummholz patches near treeline. 

The trees commonly have a “flagged” appearance—limbs are 

absent or dead on the windward side, whereas branches on the 

leeward side have living foliage (Jamieson and others, 1996; 

Brunstein, 2006). 

Extreme examples of wind-blasted trees have been noted 

at various locations in the Colorado Front Range. At these 

locations bristlecone pines at their upper elevation limit have 

been stripped of bark on the windward (western) side, and the 

only living tissue is a strip of bark on the leeward side that 

supports several branches containing living foliage. In extreme 

examples the erosion caused by wind blasting has removed not 

only the bark on the windward side but also so much of the 

wood that the pith of the tree is no longer present (Brunstein, 

2006).

In many areas, including the San Juan Mountains, timberline and treeline have been locally suppressed by frequent 

snow avalanches. Snow avalanches can scar or strip branches 

off trees and, if the avalanches are large enough, they can snap 

tree trunks (Carrara, 1979) and locally lower timberline on 

steep slopes and in high basins. In many cases the avalanche is 

confined by steep gullies along valley walls until it reaches the 

valley floor, where it spreads out and may represent a hazard 

to manmade structures. However, in intervening areas between 

avalanche gullies, timberline and treeline can reach their 

regional climatically controlled limits.

Climate

The San Juan Mountains are affected by air masses originating in several regions (fig. 3). Between October and May, 

air masses from the Pacific Ocean reach the San Juan Mountains. These air masses can originate in the Gulf of Alaska 

or off the coast of Baja California. Storms from this latter 

source bring the large amounts of snow for which the San Juan 

Mountains are noted.

Between July and September the San Juan Mountains 

are affected by the North American monsoon that brings 

frequent afternoon thunderstorms. At this time, communities in the San Juan Mountains receive about 30–35 percent 

of their annual precipitation (although higher elevations may 

receive proportionally more precipitation in the winter). During the monsoon, low-level moisture is drawn from the eastern 

tropical Pacific and Gulf of California by a thermal low in the 

desert southwest (Hales, 1972, 1974; Brenner, 1974; Adams 

and Comrie, 1997). In addition, air masses from the Gulf 

of Mexico may at times flow west across the highlands of 

Mexico and north along the Gulf of California, thereby contributing upper-level moisture and reinforcing the monsoonal 

circulation. Precipitation from the monsoon may continue into 

the late fall, usually through much of October (T.B. McKee, 

written commun., 1991). 

Climatological records exist for several stations in the 

northern San Juan Mountains (table 1). However, because no 

permanent weather stations exist at or above timberline in this 

mountain range, values given for annual precipitation and for 

mean January and July temperatures were estimated. Precipitation values for timberline, treeline, and the Lake Emma site 

(table 1) were extrapolated from a lapse rate (79 centimeters 

per kilometer (cm/km)) derived between a station in the town 

of Silverton and one on Red Mountain Pass (fig. 1).

Mean January and July temperatures were estimated for 

timberline, treeline, and the Lake Emma site by extrapolation 

from the Red Mountain Pass station (table 1) and by using a 

winter lapse rate of 4.1 degrees Celsius per kilometer (°C/km) 

and a summer lapse rate of 6.2°C/km from the Colorado Front 

Range (Barry and Chorley, 1976). Regardless of the accuracy 

of these temperature estimates, harsh periglacial conditions 

in the higher regions of the San Juan Mountains are indicated 

by numerous active rock glaciers (Howe, 1909; Atwood and 

Mather, 1932; Carrara and Andrews, 1975; White, 1979), such 

as one on the northern flank of Emery Peak about 1 km south 

of the Lake Emma site (figs. 1, 4). 

Acknowledgments

I thank the Sunnyside Gold Corporation, Silverton, Colo., 

for allowing access to the Lake Emma site. D.M. Cheney 

(U.S. Geological Survey, Denver, deceased) provided grainsize data. D.J. Christensen, J.T. Quirk, and A.C. Wiedenhoeft 

(Center for Wood Anatomy Research, U.S. Forest Service, 

Madison, Wisc.) identified many of the wood fragments 

collected in this study. E.A. Bikis (Wright Water Engineers, 

Denver) kindly provided wood fragments from the Telluride 

water well. Radiocarbon ages over the years were provided 

J.P. McGeehin, Meyer Rubin, and D.A. Trimble of the U.S. 

Geological Survey. J.A. Janssens (Limnological Research 

Center, University of Minnesota, Minneapolis) identified the 

mosses in the basal sediment of Lake Emma. I also thank W.N. 

Mode (University of Wisconsin at Oshkosh), K.L. Pierce (U.S. 

Geological Survey, Bozeman, Mont.) and S.A. Elias (Royal 
6 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

UNITED STATES

CANADA

ALASKA

MEXICO

Gulf of

Alaska

Gulf of

California Gulf of

Mexico

PACIFIC OCEAN ATLANTIC OCEAN

Study area

HIGHLANDS

EXPLANATION

October to May path

July to September path

Table 1. Climate data from stations in the northern San Juan Mountains, Colo.

[°C, degree Celsius; °C/km, degree Celsius per kilometer; cm, centimeter; cm/km, centimeter per kilometer; m, meter]

Station location Altitude

(m)

Mean

Annual precipitation

(cm)

January temperature

(°C)

July temperature

(°C)

Ouray1

Telluride1

Silverton1

Red Mountain Pass2

Timberline3

Treeline3

Lake Emma3

2,390

2,680

2,825

3,415

3,5704

3,660

3,740

58.5

58.6

62.4

109.2

121.5

128.6

134.9

–3.4

–6.0

–8.9

–8.6

–9.2

–9.6

–9.9

18.2

15.1

13.1

10.8

9.8

9.3

8.8

1

Unpublished data accessed August 26, 2009, at http://www.wrcc.dri.edu/summary/Climsmco.html

2

Data from Keen, 1996, p. 125.

3

Precipitation values are estimates based on extrapolation of lapse rate between Silverton and Red Mountain Pass (79 cm/km). Temperature 

values are based on extrapolation from Red Mountain Pass using a winter lapse rate of 4.1°C/km, and a summer lapse rate of 6.2°C/km (Barry 

and Chorley, 1976). Both the precipitation and temperature estimates for timberline, treeline and Lake Emma and similar elevations are thought 

to be representative of the northern San Juan Mountains.

4

Average of range of values given in text.

Figure 3. Trajectories of major air masses that cross the San Juan Mountains, Colo. 

(after Hales, 1972, 1974; Brenner, 1974; Siemer, 1977; Callahan. 1986; Keen, 1987; and 

Adams and Comrie, 1997). 
Glacial Geology of the San Juan Mountains 7

Holloway College, University of London), who visited the 

Lake Emma site with the author in September 1979, September 1981, and August 1987, respectively, and S.P. Schilling 

(U.S. Geological Survey, Vancouver, Wash.) who visited most 

of the sites in this report in August 1985 and 1986. Their ideas 

and suggestions were greatly appreciated. Preliminary drafts 

of this report benefi ted substantially from reviews by P.L. Fall 

(Arizona State University, Tempe), R.W. Mathewes (Simon 

Fraser University, Burnaby, Canada), T.B. McKee (Colorado 

State University, Fort Collins) and L.E. Strickland, and R.S. 

Thompson (U.S. Geological Survey, Denver, Colo.), and M.M. 

Coates (ATA Services, Denver, Colo.). 

Glacial Geology of the San Juan 

Mountains

Extent of Pinedale Glaciers 

The San Juan Mountains were extensively glaciated during the Pleistocene as indicated by numerous cirques, broad 

U-shaped valleys, and moraines that emerge from canyons at 

the mountains’ base (Howe, 1909; Atwood and Mather, 1932; 

Richmond, 1965; Gillam and others, 1984). Tills of three 

different ages were recognized in the San Juan Mountains by 

Atwood and Mather (1932). From oldest to youngest, these 

tills were termed the Cerro, Durango, and Wisconsin. However, after concluding that the type locality of the Cerro “till” 

is of landslide origin, Dickinson (1965) suggested that the 

term be abandoned. 

From the limits of late Wisconsin (Pinedale) age deposits, 

as determined by Atwood and Mather (1932), the San Juan 

Mountains were covered by about 5,000 km2

 of glacial ice 

(fi g. 5). Although Atwood and Mather (1932) inferred that 

most of the ice in the San Juan Mountains was in the form 

of valley glaciers, analysis of later large-scale topographic 

maps and air photos suggests that broad regional ice fi elds 

and transection glaciers existed as well (Porter and others, 

1983). One of the largest of these ice bodies (1,300 km2

), 

the Animas-Uncompahgre transection glacier, was centered 

over the headwaters of the Animas River (fi g. 1). The Animas 

glacier fl owed south 65 km down the Animas River valley and 

formed a series of moraines near the present town of Durango 

Figure 4. Site of former Lake Emma in 1979 after accidental drainage of the lake. Note an active rock glacier flowing from the 

cirque on Emery Peak about 1 km south of the former Lake Emma and the lack of coniferous trees, even krummholz forms.

Active rock glacier

Emery Peak

Site of former

Lake Emma
8 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

(Atwood and Mather, 1932; Richmond, 1965; Steven and 

others, 1974; Gillam and others, 1984). The Uncompahgre 

glacier flowed north 35 km along the valley of the Uncompahgre River (figs. 1, 5). Moraines from this glacier were 

formed about 3 km north of the town of Ridgway (Atwood 

and Mather, 1932; Steven and Hail, 1989) (fig. 5). The formation of Pinedale glaciers appears to have been initiated prior 

to 30,000 yr B.P. (about 35,000 cal yr) and were near their 

maximum extent about 22,000 yr B.P. (about 26,000 cal yr) 

(Benson et al., 2004). 

Note: radiocarbon ages in this report are given in radiocarbon years before present (yr B.P.), and “present” is defined 

as 1950 A.D.; calibrated radiocarbon ages given in calendar 

years (cal yr) are from Fairbanks and others, 2005, and also set 

to 1950 A.D.; cosmogenic ages are labeled as 10Be or 36Cl ka 

and are set from the year they were dated.

Timing of Deglaciation

Radiocarbon Ages

Some of the oldest dates of Pinedale deglaciation from 

the Rocky Mountains have been reported from the San 

Juan Mountains. Molas Lake is located on a broad glaciated divide west of the Animas River valley at an elevation 

of 3,205 m (figs. 1, 6). During maximum Pinedale glaciation, this area was covered by several hundred meters of 

glacial ice. Radiocarbon ages of 15,450±220 yr B.P. (Y-1147) 

and 13,360±120 yr B.P. (Y-1437) were obtained from a 

220-cm-long sediment core collected from this lake (Maher, 

1961, 1972). The older radiocarbon age was obtained from 

organic sediment at a depth of 170–175 cm from a transition 

zone between underlying inorganic, laminated clay and overlying organic-rich sediment. The younger radiocarbon age was 

obtained from material at a depth of 142–147 cm at the base of 

the organic-rich sediment (Maher, 1972). The relation of the 

Molas Lake site to the Animas-Uncompahgre transection glacier indicates that when the Molas Lake site was deglaciated, 

at least half the glacier had melted (Porter and others, 1983). 

Hence, the radiocarbon age from Molas Lake could indicate 

extensive deglaciation within the San Juan Mountains before 

15,500 yr B.P. (about 18,700 cal yr). However, because Molas 

Lake is underlain by Paleozoic limestone the radiocarbon ages 

from this lake may have been subject to a “hard-water effect,” 

and hence may be older than the true age of the sediment analyzed (L.J. Maher Jr., oral commun., 1983). 

Lake Emma, at an elevation of 3,740 m, is located above 

present-day treeline in a south-facing cirque, near the headwaters of the Animas River (figs. 1, 4, 5). It was accidentally 

drained in 1978 when the roof of an underlying mine stope 

collapsed. Radiocarbon ages of 14,900±250 yr B.P. (W-4209), 

14,130±150 yr B.P. (W-4289), and 14,940±140 yr B.P. 

(W-4525) (fig. 7) were obtained from moss fragments from 

the basal organic sediment of Lake Emma (Carrara and others, 

1984). Because Lake Emma is underlain by a noncarbonatebearing volcanic rock and the moss fragments were determined to be from terrestrial moss (J.A. Janssens, written commun., 1982), the radiocarbon ages were initially thought to be 

accurate. Because the Lake Emma site lies near the center of 

the former Animas-Uncompahgre transection glacier, these 

ages were originally interpreted as indicating that the glacier, 

one of the largest in the southern Rocky Mountains, no longer 

existed by 15,000 yr B.P. (about 18,100 cal yr) (Carrara and 

others, 1984). However, subsequent work by Elias and others 

(1991) indicated that the radiocarbon ages of mosses from the 

Lake Emma site must also be viewed with suspicion. Radiocarbon ages (obtained mostly by accelerator mass spectrometry (AMS)) from wood fragments and insect fossils from the 

same horizons as the mosses yielded radiocarbon ages 3,500 

to 6,000 years younger than the moss ages (Elias and others, 

1991) (fig. 7). 

Disregarding the moss ages, the oldest radiocarbon age 

from the high regions of the San Juan Mountains is from a bog 

in a cirque near Hurricane Basin (figs. 1, 5). Here a radiocarbon age of 9,620±440 yr B.P. (St-3909) (about 11,000 cal yr) 

(fig. 8) was obtained from organic material concentrated 

from relatively inorganic clay (Andrews and others, 1975). 

Other similar ages have been obtained from the Lake Emma 

site (fig. 7). A willow fragment from Lake Emma sediments yielded an AMS radiocarbon age of 9,400±70 yr B.P. 

(AA-3888) (about 10,600 cal yr). The oldest reliable radiocarbon age of a conifer fragment from the Lake Emma site is 

now believed to be the 9,220±120 yr B.P. age (W-4524) (about 

10,400 cal yr) (table 2) obtained from a spruce fragment in 

a section sampled for pollen analysis (Carrara and others, 

1984). The radiocarbon age of this spruce fragment represents 

an interval of higher than present-day treeline and therefore 

may postdate deglaciation by a considerable period, possibly 

several of thousands of years.

Cosmogenic Ages

Cosmogenic dating of glacially polished bedrock surfaces in the Animas River valley indicates that glacial retreat 

from the Pinedale terminal position, several kilometers north 

of the present-day town of Durango (fig. 5), began about 

19.4±1.5 10Be ka (about 16,300 yr B.P.) (Guido and others, 2007). This study investigated various sites from near 

the terminal position upvalley to near the town of Silverton 

(figs. 1, 5) and concluded that retreat proceeded at an average rate of 15.4 meters per year (m/yr). A cosmogenic age of 

12.3±1.0 10Be ka (about 10,450 yr B.P.), which was obtained 

from a glacially polished bedrock surface near Highland Mary 

Lakes (fig. 9) at an elevation of 3,704 m near the Continental 

Divide, indicates that by this time deglaciation of the San Juan 

Mountains was essentially complete (Guido and others, 2007). 

Although the locality of the dated sample is about 500 m 

higher in elevation than Molas Lake, the age from this sample 

is considered a more reliable estimate of the timing of regional 
Glacial Geology of the San Juan Mountains 9

Ouray

Telluride

Silverton

Lake Emma

Hurricane Basin

Molas Lake

Durango

25 KILOMETERS

15 MILES

0

0

108o00' 107o30'

 37o30'

 38o00'

ANIMAS GLACIER

RIO GRANDE GLACIER

UNCOMPAHGRE GLACIER

SAN MIGUEL GLACIER

Black Mountain Lake

C O N T I N T E N T A L D I V I D

E

Lake City

Ridgway

SAN CRISTOBAL GLACIER

La Plata Mountains

VALLECITO GLACIER

Highland Mary Lakes

COLORADO

Study area

Little Molas Lake

EXPLANATION

Ice flow direction

Site of published pollen diagram

Figure 5. Limits of Wisconsin (Pinedale) glaciation in western San Juan Mountains (after Atwood and Mather, 

1932), including the Animas-Uncompahgre transection glacier. This glacier stretched from near the present-day 

town of Ridgway south for about 100 km to near the present-day town of Durango. 
10 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 6. Molas Lake area, located on a broad glaciated divide west of the Animas River valley. Elevation 3,205 meters.

deglaciation than the radiocarbon ages from Molas Lake or the 

radiocarbon ages of moss fragments from Lake Emma. 

Similar ages of initial deglaciation were obtained 

from sites in the Rio Grande drainage (Benson and others, 

2005). The Rio Grande glacier (fi g. 5), which covered about 

1,000 km2

 and was about 60 km long, also emerged from the 

ice mass that covered the San Juan Mountains. Cosmogenic 

dating of boulders from the terminal moraine deposited by the 

Rio Grande glacier and the nearby Red Mountain glacier to 

the south (Atwood and Mather, 1932) yielded a mean age of 

18.9 36Cl ka (about 15,700 yr B.P.) (Benson and others, 2005). 

Cirque Moraines and Present-Day Snowfields

Two sets of late Pleistocene cirque moraines were found 

in the San Juan Mountains (Andrews and others, 1975; Carrara 

and Andrews, 1975). The older set (designated as the Yankee 

Boy moraines (fi g. 10) because of the excellent examples 

in Yankee Boy Basin (fi g. 1)) is widespread throughout the 

study area. These moraines range in elevation from 3,350 

to 3,780 m (average 3,635 m) and are commonly found in 

cirques oriented N. 85° W. to N. 100° E. The distance between 

these moraines and the cirque headwall ranges from 0.4 to 

2 km; distances of 0.8–1 km are the most common. The crests 

of these moraines are commonly 15 m wide but may be as 

little as 5 m, and are commonly overlain by 10 cm of loess. 

Depth of stream dissection through the terminal arc is usually 

about 10 m. These moraines are commonly situated in cirques 

underlain by Tertiary andesitic fl ows and ashes that are very 

susceptible to weathering.

The younger set of moraines is designated as the Grenadier moraines (fi g. 11) because these moraines have been 

found only at the northern base of the Grenadier Range 

(fi g. 1). The moraines range in elevation from 3,640 to 

3,780 m (average 3,730 m) and are commonly found in 

cirques oriented from north to N. 40° E. Moraines of this 
Glacial Geology of the San Juan Mountains 11

Deformed bedding

Massive silty clay

(5Y 6/2)

Laminated silty clay

(5Y 6/2)

Massive silty clay

(2.5Y 6/2)

Laminated sand, silt,

and clay (5Y 4/1)

Organic-rich laminated sand,

silt, and clay (5Y 4/1)

Pebbly sand

(2.5Y 7/4)

 Sandy till

(10YR 7/1)

W

W

W

W W

X

X X

Insect Moss

(X)

Wood

(W)

5,440±100

6,670±70

8,625±90

9,220±120

9,400±70

8,200±160

8,940±85

9,020±80

12,900±200

13,580±200

14,130±150

14,900±250

14,940±140

Radiocarbon ages (yr B.P.) of wood, fossil insects,

and moss fragments from the stratigraphic

sections sampled at Lake Emma.

50

100

150

200

250

DEPTH, IN CENTIMETERS

Figure 7. Composite stratigraphic section at the Lake Emma site (after Carrara and others, 1984; and Elias 

and others, 1991) showing radiocarbon ages (yr. B.P.) of wood, insect fossils, and detrital moss. 
12 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

9,620±440 

C 

W 

Eureka

Gulch Bog

California

Gulch Bog

Placer

Gulch Bog

Picayne

Gulch Bog

Bog near

Hurricane Basin

0 

50 

100 

150 

200 

250 

DEPTH, IN CENTIMETERS 

9,260±60 

8,320±70 

6,180±160 

3,905±40 

C 

C 

C 

C 

C 

W 

W 

W 

W 

W 

C 

3,600±50 

4,330±35 

6,850±40 

7,860±40 

W 

C 

C 

C 

C 

C 

C 

C 

C 

C 

C 

C 

C 

C 

6,890±200 

7,020±200 

8,790±260 

C 

C 

C 

C 

C 

C 

C 

C 

C 

W 

W 

W 

6,000±300 

8,350±250 W C 

C W 

W C 

W C 

C 

W W 

W W 

3,520±100 

5,630±250 

8,330±75 

8,420±250 

C C 

W W 

280±80 

1,295±70 

W 

Bedrock 

Inorganic clay 

Sandy clay 

Conifer fragments 

Willow fragments 

EXPLANATION

Woody peat

(conifer fragments) 

Woody peat

(willow fragments) 

Organic silty clay 

Interval from which

radiocarbon sample

was taken 

Peat 

Figure 8. Stratigraphic sections and radiocarbon ages (yr B.P.) from bogs in the northern San Juan Mountains. The 

section for the bog near Hurricane Basin represents a composite stratigraphy from Andrews and others (1975) and 

Carrara and others (1991).
Glacial Geology of the San Juan Mountains 13

Table 2. Radiocarbon ages of coniferous wood fragments from the Lake Emma site, northern San Juan Mountains, Colo.

[Elevation of Lake Emma site is 3,740 m. The 66 radiocarbon ages from 53 coniferous wood fragments here presented include 25 previously unpublished 

ages. Ages of samples shown in the same color were obtained from the same wood fragment. A, genus Abies (fir); P, genus Picea (spruce); Lk, Lake; AA, 

National Science Foundation–Arizona AMS Facility, University of Arizona, Tucson; DIC, Dicarb Radioisotope Laboratory, Chagrin Falls, Ohio; USGS, U.S. 

Geological Survey, Menlo Park, Calif.; W, U.S. Geological Survey, Reston, Va.; WW, U.S. Geological Survey, Reston, Va.; ≈, approximately equal to]

No. 14C age 

(yr B.P.) Laboratory ID Calendar yr (cal yr)a Field ID Genus Number of 

rings

Weight

(gm)

01 3075±35 USGS–2617 3304±46 Stand Metals 01–78 P 60 ≈1,500

02 3120±80 W–4242 3339±86 Stand Metals 01–78 P 60 ≈1,500

03 **5370±110 USGS–3005 6158±130 Lk Emma 04–86 P 34 69

04 5420±200 W–5616 6201±215 Lk Emma 09–83 P ≈75 2,530

05 5440±100 USGS–2135 6235±103 LE Pollen (81)–79 A? 10 2

06 5570±70 W–4680 6349±64 Lk Emma 04–79 P 67 ≈100

07 5755±45 USGS–1340 6546±64 Lk Emma 11–79 A 31 40

08 **5839±40 *WW–6118 6652±44 Lk Emma 04–79 P 67 ≈100

09 **5858±40 *WW–6115 6671±42 Lk Emma 04–86 P 34 69

10 **5870±30 *WW–6593 6683±31 Lk Emma 09–83 P ≈75 2,530

11 5950±100 W–4682 6776±122 Lk Emma 05–79 A 29 ≈100

12 **6020±70 USGS–3003 6860±92 Lk Emma 02–86 P 70 164

13 6170±70 W–4676 7064±103 Lk Emma 01–79 P 44 ≈50

14 6300±280 W–5613 7186±302 Lk Emma 06–83 P ≈100 439

15 6330±110 W–4686 7252±119 Lk Emma 08–79 P 67 ≈80

16 6670±70 W–4523 7545±55 LE Pollen (96)–79 P 23 ≈20

17 6690±60 USGS–2472 7561±46 Lk Emma 08–85 P 46 103

18 6870±65 DIC–3163 7695±59 Lk Emma 02–83 P 24 77

19 6995±40 USGS–2312 7828±50 Lk Emma 07-85 P 43 327

20 7010±40 USGS–2471 7845±49 Lk Emma 06–85 P ≈40 170

21 7400±40 USGS–2313 8218±52 Lk Emma 10–85 P ≈60 214

22 **7430±70 USGS–3006 8255±80 Lk Emma 05–86 P ≈40 225

23 **7550±70 USGS–3004 8366±53 Lk Emma 03–86 P 29 41

24 7620±100 USGS–2616 8417±81 Lk Emma 09–85 P ≈35 57

25 7670±75 DIC–2895 8454±65 Lk Emma 10–83 P ? 430

26 **7730±40 USGS–3183 8507±46 Lk Emma 03–87 P 25 242

27 7780±100 W–4678 8562±114 Lk Emma 03–79 P 63 ≈100

28 7780±320 W–5608 8620±374 Lk Emma 01–83 P ≈30 112

29 7810±120 W–4243 8604±153 Stand Metals 02–78 A 33 ≈500

30 **7820±30 *WW–6592 8591±26 Lk Emma 01–87 P 21 375

31 **7830±50 USGS–2827 8602±52 Lk Emma 02–85 P ≈25 32

32 7850±90 W–5674 8643±129 Lk Emma 07–79 P 48 ≈1,000

33 7870±90 W–4684 8671±139 Lk Emma 07–79 P 48 ≈1,000

34 7870±110 USGS–1339 8679±164 Lk Emma 10–79 P 23 73

35 **7900±80 USGS–3007 8712±141 Lk Emma 06–86 P 18 20

36 **7920±45 USGS–2828 8730±106 Lk Emma 03–85 P ≈30 50

37 7970±90 USGS–1344 8834±161 Lk Emma 15–79 P 15 16

38 8010±80 USGS–1346 8904±139 Lk Emma 18–81 P 150 ≈5,000

39 **8040±50 *WW–962 8967±74 Lk Emma 04–85B P 28 46
14 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Table 2. Radiocarbon ages of coniferous wood fragments from the Lake Emma site, northern San Juan Mountains, Colo.

[Elevation of Lake Emma site is 3,740 m. The 66 radiocarbon ages from 53 coniferous wood fragments here presented include 25 previously unpublished 

ages. Ages of samples shown in the same color were obtained from the same wood fragment. A, genus Abies (fir); P, genus Picea (spruce); Lk, Lake; AA, 

National Science Foundation–Arizona AMS Facility, University of Arizona, Tucson; DIC, Dicarb Radioisotope Laboratory, Chagrin Falls, Ohio; USGS, U.S. 

Geological Survey, Menlo Park, Calif.; W, U.S. Geological Survey, Reston, Va.; WW, U.S. Geological Survey, Reston, Va.; ≈, approximately equal to]

No. 14C age 

(yr B.P.) Laboratory ID Calendar yr (cal yr)a Field ID Genus Number of 

rings

Weight

(gm)

40 8040±120 W–4677 8937±183 Lk Emma 02–79 P 56 ≈100

41 **8070±50 *WW–963 8998±54 Lk Emma 04–85C P 28 ≈100

42 **8095±45 USGS–3182 9019±41 Lk Emma 02–87 P 65 247

43 8100±70 DIC-2893 9026±85 Lk Emma 04-83 P ≈30 180

44 **8190±50 *WW-728 9131±81 Lk Emma 04-85 P 28 0.57

45 8260±40 USGS-2314 9246±74 Lk Emma 11-85 P 57 465

46 8310±75 DIC-2894 9319±112 Lk Emma 07-83 A ≈50 410

47 8370±135 DIC-2197 9371±153 Lk Emma 17-80 P 17 9

48 8520±80 USGS-1342 9510±48 Lk Emma 13-79 P 51 61

49 8520±80 DIC-3164 9510±48 Lk Emma 03-83 P ≈15 32

50 **8590±60 USGS-3002 9546±36 Lk Emma 01-86 P ≈60 42

51 8625±90 *AA-3889 9581±88 LE Fos Insect 02-87 P 4 0.6

52 **8640±50 USGS-2826 9575±45 Lk Emma 01-85 P ≈30 47

53 **8656±50 *WW-5548 9589±52 Lk Emma 16-80 P 20 28

54 8730±90 W-4685 9711±156 Lk Emma 06-79 P 35 ≈100

55 **8772±47 *WW-6119 9765±108 Lk Emma 06-79 P 35 ≈100

56 **8796±57 *WW-6117 9822±138 Lk Emma 14-79 P 55 25

57 8810±80 USGS-1343 9857±176 Lk Emma 14-79 P 55 25

58 **8820±50 USGS-2830 9877±133 Lk Emma 05-85 P ≈45 71

59 **8855±51 *WW-5547 9959±134 Lk Emma 05-83 P 25 181

60 **8872±47 *WW-6116 9998±122 Lk Emma 05-85 P ≈45 71

61 8910±100 USGS-1338 10,041±177 Lk Emma 09-79 A 47 43

62 **8950±45 USGS-3181 10,139±81 Lk Emma 01-87 P 21 375

63 9000±300 W-5611 10,123±399 Lk Emma 05-83 P 25 181

64 9220±120 W-4524 10,391±154 LE Pollen (161)-79 P 83 ≈20

65 9520±90 USGS-1345 10,817±174 Lk Emma 16-80 P 20 28

66 9580±130 USGS-1341 10,921±222 Lk Emma 12-79 P 19 32

a

Data accessed March 20, 2009 at http://www.radiocarbon.LDEO.columbia.edu. See Fairbanks, R.G. and others, 2005, Marine radiocarbon calibration curve 

spanning 0 to 50,000 years B.P., based on paired 230Th/234U/238U and 14C dates on pristine corals: Quaternary Science Reviews, v. 24, p. 1781–1796.

*

Accelerator mass spectrometry radiocarbon age (n=13).

**Previously unpublished radiocarbon ages (n=25).

—Continued
Figure 9. Highland Mary Lakes area from which a cosmogenic age of deglaciation of 12.3±1.0 10Be ka was obtained (Guido and others, 2007).
16 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 10. Yankee Boy moraine in Yankee Boy Basin. 

Figure 11. Grenadier moraine at the northern base of the Grenadier Range.
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 17

group are usually found within 450 m of the cirque headwall. 

Moraine crests are usually no more than 5 m wide, and only 

one moraine of this group has been dissected by a stream. 

These moraines are in cirques underlain by the Precambrian 

Uncompahgre Formation, which consists of interlayered 

quartzite and slate (Barker, 1969), which are very resistant 

to weathering. 

The exact ages of these two sets of moraines are not 

known. No datable organic material has been found in association with either set of moraines. Furthermore, differences in 

lithology make problematic the use of relative weathering 

criteria to differentiate the two moraine sets. 

The two sets of moraines are distinguished on the basis of 

two criteria: the downvalley distance between the cirque headwall and moraine (S), and the ratio of the distance between the 

cirque headwall and moraine to the relief between the moraine 

crest and top of headwall (R). As mentioned above, Yankee 

Boy moraines are found 0.4–2 km from their cirque headwalls; distances of 0.8 to 1 km are most common. Grenadier 

moraines are usually found about 450 m from their cirque 

headwall. The difference between the two sets of moraines is 

even more striking when their S/R ratios are compared. This 

S/R ratio is a function, at least in part, of the severity of the 

climate that produced the former glaciers. The average S/R 

value for Yankee Boy moraines is 3.57:1, whereas the average value for Grenadier moraines is 1.46:1. In the case of 

Grenadier moraines a given amount of shading by the cirque 

headwall produced significantly smaller glaciers than was the 

case with Yankee Boy moraines. This relation suggests that the 

two sets of moraines formed under different climatic conditions and hence at different times. 

Both the Yankee Boy and Grenadier moraines, as well 

as other correlative deposits (rock glaciers and protalus 

ramparts) found in the San Juan Mountains are thought to 

be late Pleistocene in age (Carrara and others, 1984). As will 

be discussed later in this report, radiocarbon ages of coniferous wood from Lake Emma clearly indicate that, by at least 

9,200 yr B.P. (about 10,400 cal yr), summer temperatures were 

higher than at present and thus both sets of moraines must 

have been deposited before that time. Although both moraine 

sets are found mostly in north-facing cirques (Carrara and 

Andrews, 1975), and Lake Emma is in a south-facing cirque, 

it is climatically incompatible that small glaciers lingered in 

north-facing cirques during a period of warmer than presentday summer temperatures. Cosmogenic dating of rock surfaces 

indicatesthat deglaciation in the Animas River drainage was 

essentially complete by 12.3±1.0 10Be ka (about 10,450 yr 

B.P.) (Guido and others, 2007) and suggests that at least Yankee Boy moraines were probably deposited by that time.

Yankee Boy moraines represent a regional period of 

moraine formation throughout the northern San Juan Mountains. These moraines are thought to correlate with the Satanta 

Peak moraines of the Colorado Front Range (Benedict, 1973, 

1985), and the Temple Lake moraine of the Wind River Range, 

Wyoming (Davis, 1988) and are probably Younger Dryas in 

age (11,000–10,000 yr B.P. (about 12,800–11,500 cal yr)). 

Grenadier moraines probably formed a short time after the 

Yankee Boy moraines and represent the product of remnant 

glacier ice lying in well-insulated niches in a mountain range 

undergoing rapid deglaciation. 

Although the San Juan Mountains are generally higher 

than the Colorado Front Range, which contains small glaciers and snowfields fronted by neoglacial moraines (Outcalt 

and MacPhail, 1965), the San Juan Mountains contain few 

permanent snowfields and no present-day glaciers. Only two 

snowfields of any consequence have been noted by the author; 

one, at the northern base of the Trinity Peaks in the Grenadier 

Range (figs. 1, 12), appears to be fronted by a Little Ice Age 

moraine. Another snowfield is present in a northwest-facing 

cirque about 1 km northeast of Mount Wilson (fig. 1). Thus it 

appears that the higher snowfalls of the San Juan Mountains 

are offset by a longer melt season or more southerly latitude 

compared with the Front Range. The San Juan Mountains 

are also less windy than the Front Range, where wind drifting is thought to be responsible for the presence of many of 

the small glaciers (Outcalt and MacPhail, 1965; Hoffman and 

others, 2007). 

Postglacial Forestation and 

Holocene Treeline in the Northern 

San Juan Mountains

Introduction 

Ten sites in the northern San Juan Mountains have 

yielded information about postglacial forestation and Holocene treeline fluctuations. Many of these sites are the subject 

of reports: Maher (1961, 1963, 1972), Andrews and others 

(1975), Elias (1982), Carrara and others (1984, 1991), Elias 

and others (1991), Reasoner and Jodry (2000), and Toney and 

Anderson (2006). This information consists primarily of pollen analyses of radiocarbon-dated sediment cores, but it also 

includes sites where coniferous wood fragments and fossil 

insects were recovered and radiocarbon dated. Because the 

coniferous wood fragments recovered from these sites originated at higher elevations and were carried downslope to be 

deposited, the elevation of these sites represent the minimum 

elevation at which these recovered coniferous wood fragments 

originally grew.

The following section synthesizes 

1. Previously published information, 

2. Unpublished radiocarbon ages of coniferous wood fragments from a water well drilled near the town of Telluride 

in 1991, and 

3. Unpublished radiocarbon ages of coniferous wood fragments, including AMS radiocarbon ages, from the Lake 

Emma site (these ages require a moderate revision of 

previous interpretations) (Carrara and others, 1984, 1991). 
18 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 12. Snowfield at the northern base of Trinity Peaks in the Grenadier Range. Snowfield appears to be fronted 

by a Little Ice Age moraine; an active rock glacier is in the foreground.
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 19

The sites are discussed in order of increasing elevation. In 

addition to those sites in the San Juan Mountains, approximately 20 other sites throughout western North America that 

have yielded conifer remains above present-day limits demonstrating treelines higher than at present throughout much 

of the Holocene are presented in the appendix at the end of 

this report.

Telluride Water Well (2,665 m asl) 

A well that was drilled immediately southwest of the 

town of Telluride (fig. 1) provides evidence of the forestation of the San Juan Mountains after deglaciation. The well, 

at an elevation of 2,665 m, was drilled though the floodplain 

of the San Miguel River to bedrock at a depth of 33.5 m 

(fig. 13). The interval between depths of 18.3 and 22.9 m 

contained abundant coniferous wood fragments identified as 

spruce (D.J. Christensen, Center for Wood Anatomy Research, 

written commun., 1992). Conventional radiocarbon dating 

of these wood fragments yielded ages of 10,870±110 yr B.P. 

(W–6405) from wood fragments between depths of 18.3 and 

19.8 m; 11,000±110 yr B.P. (W–6407) from wood fragments 

between depths of 19.8 and 21.3 m; and 11,840±120 yr B.P. 

(W–6409) from wood fragments between depths of 21.3 and 

22.9 m (table 3). A subsequent AMS radiocarbon date obtained 

from wood fragments in the lower interval yielded an age of 

10,906±54 yr B.P. (WW–6114). This date suggests that the 

age obtained for sample W–6409 may be in error by approximately 800 radiocarbon years and that the entire interval 

between depths of 18.3 and 22.9 m may have been rapidly 

deposited about 11,000 yr B.P. (about 12,900 cal yr).

Two additional AMS radiocarbon ages were obtained at 

greater depth in the core; their interpretation is problematic. 

In the interval between 25.9 and 27.4 m, a spruce fragment 

(D.J. Christensen, Center for Wood Anatomy Research, written commun., 1992) was recovered. This fragment yielded an 

AMS radiocarbon age of 10,972±127 yr B.P. (AA–9533) and, 

because this age is no older than the radiocarbon ages between 

18.3 and 22.9 m, it may represent downhole contamination. 

Finally, in the interval between 32 and 33.5 m, another small 

spruce fragment (D.J. Christensen, Center for Wood Anatomy 

Research, written commun., 1992) was recovered. This fragment yielded an AMS radiocarbon age of 11,219±100 yr B.P. 

(AA–9532) and again may represent downhole contamination.

Conversely, if the lower two radiocarbon ages do not 

represent downhole contamination, then because 5 of the 

6 radiocarbon ages obtained from the Telluride water well site 

overlap at 2s (standard deviations) (W–6409 is the exception), 

then the entire sediment column from 18.3 m down to the 

bedrock base at 33.5 m depth may have been rapidly deposited 

about 12,900 cal yr.

The Telluride water well is located about 10 km upvalley 

from the Pinedale-age terminal moraines of the San Miguel 

glacier (Atwood and Mather, 1932) (fig. 5). Hence, the radiocarbon ages of coniferous wood fragments from the well site 

clearly indicate substantial deglaciation before about 11,000 yr 

B.P. (about 12,900 cal yr) and the establishment of forest in a 

formerly glaciated valley. The exact position of timberline during this time is not known, because these wood fragments may 

have been washed down from higher elevations and deposited 

at the site of the water well. All that can be concluded is that 

timberline was no more than about 900 m below its presentday limit and possibly much less. 

Molas Lake (3,205 m asl)

Molas Lake, at an elevation of 3,205 m, is about 400 m 

below present-day timberline. The lake is about 7 km south 

of the town of Silverton (fig. 1) on a broad divide (fig. 6) 

between the Animas River and Lime Creek. The lake has a 

surface area of about 0.07 km2

 and is probably no more than 

several meters deep. As previously mentioned, a core 220 cm 

in length was collected from Molas Lake for pollen analysis 

(Maher, 1961, 1972). 

The stratigraphy of the core is such that it can be divided 

into three sections. The lowest section (below 175 cm) consisted of greenish-gray, inorganic, laminated silts and clays. 

Between 175 and 145 cm the core consisted of light brown, 

calcareous gyttja that represents a transition zone between the 

underlying inorganic silts and clays and the overlying (above 

145 cm depth) dark brown gyttja (Maher, 1961). 

The pollen spectra from the inorganic sediment (below 

175 cm) at Molas Lake are dominated by high values of Artemisia spp. and low values or absence of tree pollen (Maher, 

1961). Percentages of Artemisia spp. pollen ranges from about 

45 to 65 percent, whereas total nonarboreal pollen ranges from 

about 60 to 75 percent. Picea spp. and Pinus spp. pollen abundances range from about 1 to 10 percent and 18 to 35 percent, 

Table 3. Radiocarbon ages from spruce fragments recovered 

from Telluride, Colo., water well.

[cal yr, calendar year; m, meter; yr B.P., year before present]

Depth

(m)

Laboratory

number

14C Age

(yr B.P.)

Calendar age

(cal yr)a

18.3–19.8 W–6405 10,870±110 12,771±93

19.8–21.3 W–6407 11,000±110 12,875±95

21.3–22.9 W–6409 11,840±120 13,682±109

WW–6114 10,906±54b 12,798±54

25.9–27.4 AA–9533 10,972±127b 12,853±107

32.0–33.5 AA–9532 11,219±100b 13,062±99

a

Data accessed on the World Wide Web at http://www.radiocarbon.LDEO.

columbia.edu on March 20, 2009. See Fairbanks, R.G and others 2005, 

Marine radiocarbon calibration curve spanning 0 to 50,000 years B.P., based 

on paired 230Th/234U/238U and 14C dates on pristine corals: Quaternary Science 

Reviews, v. 24, p. 1781–1796.

b

Radiocarbon age determined by accelerator mass spectrometry.
20 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

10 

15 

20 

25 

30 

35 

Gravelly sand (7.5YR 5/2) 

Sandy gravel (10YR 5/2) 

Sandy gravel (10YR 6/2) 

Sand (10YR 5/1) 

Gravelly sand (7.5YR 5/2) 

Sand (5YR 6/1) 

Gravelly sand (5YR 6/2) 

Coarse sand (10R 6/2) 

11,000± 110 yr B.P. (W–6407) 

10,870± 110 yr B.P. (W–6405) 

11,840±120 yr B.P. (W–6409) 

10,972± 127 yr B.P. (AA–9533) 

11,219± 100 yr B.P. (AA–9532) 

Intervals from which radiocarbon ages

of spruce fragments were determined 

Bedrock 

10,906±54 yr B.P. (WW–6114) 

DEPTH, IN METERS

EXPLANATION

Figure 13. Stratigraphic cross section and 

radiocarbon ages (yr B.P.) from the Telluride water well.
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 21

respectively. Because these inorganic sediments consist of 

finely laminated silty layers, they were probably deposited 

soon after deglaciation and before vegetation was established 

around the site (Maher, 1961). Because of the possible hardwater contamination of the radiocarbon ages obtained from 

this core, which yielded ages older than the actual age of the 

sediment, the cosmogenic age of 12.3±1.0 10Be ka (about 

10,450 yr B.P obtained from a glacially polished bedrock 

surface near Highland Mary Lakes (Guido and others, 2007), 

approximately 500 m higher in elevation than Molas Lake, is 

thought to represent a closer minimum date of deglaciation for 

the Molas Lake area. 

The pollen spectra in the interval between 147 and 

175 cm depth, represent a transition zone between the underlying postglacial inorganic sediments and the overlying organicrich sediments. This interval has lower values of nonarboreal 

pollen and higher values of tree pollen than the underlying 

sediments. Pollen data including spruce/pine pollen ratios 

indicate that, when Molas Lake first began accumulating 

organic sediment, the average July temperature was about 

5°C cooler and timberline about 650 m lower than at present 

(Maher, 1961). This finding places timberline at an elevation 

of about 2,885 m at that time. Unfortunately, as previously 

mentioned, the radiocarbon age (13,360±120 yr B.P., Y–1437) 

obtained from material at the base of the organic-rich sediment 

in Maher’s (1961) core must be viewed with suspicion (L.J. 

Maher, Jr., oral commun., 1983). 

The pollen spectra from the organic-rich sediment above 

147 cm resemble the modern pollen spectra from the site. 

Artemisia spp. pollen is about 10 percent throughout this section, whereas total nonarboreal pollen ranges from about 10 

to 35 percent. Picea spp. and Pinus spp. pollen ranges from 

about 15 to 45 percent and 15 to 65 percent, respectively. Also 

in this interval, at about 125 cm, nonarboreal pollen reaches its 

lowest value (about 10 percent), whereas Picea spp. and Pinus

spp. are quite abundant (about 30 and 55 percent respectively). 

Maher (1961) concluded that some time during this period the 

maximum postglacial July temperature was about 2°C warmer 

than at present. Because the modern pollen spectra appear to 

have been established by about 9,000 yr B.P. at the Hurricane 

Basin and Little Molas Lake sites (figs. 1, 5), and probably 

about that same time at the Lake Emma site (discussed below), 

it is suggested here that the base of the organic-rich sediment 

at Molas Lake (147 cm depth) also dates to about 9,000 yr B.P. 

(about 10,150 cal yr). 

Little Molas Lake (3,370 m asl)

Little Molas Lake, about 2 km west of Molas Lake 

(fig. 1), lies at an elevation of 3,370 m, about 230 m below 

present-day timberline. Two sediment cores 4.5 m long and 

a short core less than 1 m long, which sampled the upper 

unconsolidated sediments at the sediment-water interface, 

were collected from Little Molas Lake for pollen analysis 

(Toney and Anderson, 2006). Although this lake, like Molas 

Lake, is underlain by Paleozoic limestone, radiocarbon ages 

obtained from discrete macrofossils such as spruce needles, 

bark, and seeds and insect remains provide a reliable chronology. Seven radiocarbon ages were obtained from one of 

the long cores. A radiocarbon age of about 11,200 cal yr was 

obtained from remains of aquatic insects near the base of the 

core at a depth of 409–412 cm (Toney and Anderson, 2006). 

The pollen and macrofossil data were interpreted as indicating that the replacement of tundra vegetation by spruce 

forest corresponded with the end of the Younger Dryas, about 

10,000 yr B.P. (about 11,500 cal yr) (Toney and Anderson, 

2006). At this time an increase in Diploxylon pollen (compare 

with P. ponderosa) was interpreted as indicating the influence of the southwestern monsoon. In addition, the pollen and 

macrofossil data indicated that spruce and other conifers have 

grown near Little Molas Lake throughout the Holocene (Toney 

and Anderson, 2006). 

Black Mountain Lake (3,413 m asl)

Black Mountain Lake, in the Rio Grande drainage east 

of the Continental Divide (figs. 1, 5), lies at an elevation 

of 3,413 m about 100–150 m below present-day timberline 

(Reasoner and Jodry, 2000). Although the Black Mountain 

Lake site was not glaciated during the Pinedale glaciation, it is 

located on a hillside above Pinedale-age deposits (M.A. Reasoner, oral commun., 2007). A sediment core 7.8 m long was 

collected from this site and the upper 3.2 m analyzed for plant 

macrofossils and pollen content (Reasoner and Jodry, 2000). 

A conifer twig yielded a radiocarbon age of 11,430±70 yr B.P. 

(CAMS-38704) (about 13,300 cal yr) that was interpreted 

as indicating that timberline had migrated upslope to near its 

present elevation during the Bølling-Allerod warming (about 

14,600 to 13,000 cal yr). Subsequent declines in arboreal pollen during the Younger Dryas interval (about 12,800 to 11,500 

cal yr) suggested a timberline retreat of 60–120 m with a 

corresponding cooling of summer temperatures by 0.4°–0.9°C 

(Reasoner and Jodry, 2000). Timberline readvanced to elevations at or above Black Mountain Lake by about 10,000 yr 

B.P. (about 11,500 cal yr) as indicated by a radiocarbon age 

of 9,930±70 yr B.P. (CAMS-38702) from an Abies needle 

(Reasoner and Jodry, 2000). 

Placer Gulch Bog (3,600 m asl)

The Placer Gulch bog (fig. 2), at an elevation of 3,600 m, 

is about 2 km downvalley from the head of Placer Gulch. 

Although no conifers are present near the site, possibly owing 

to cold-air drainage, krummholz are present on a bedrock 

ridge about 0.3–0.8 km upvalley from the bog at an elevation 

of 3,700 m. In addition, several unusually high krummholz 

live about 0.6 km to the northwest at an elevation of 3,810 m. 

Timberline is about 1.5 km downvalley at an elevation of 

3,560 m; hence, the Placer Gulch bog can be considered to be 

between present-day timberline and treeline. 
22 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

An exposure of very dark brown (10YR 2/2 wet) woody 

peat, 85 cm thick (fig. 14), was found in 1987 next to the 

tailings pile of the Evening Star mine (fig. 2) (Carrara and 

others, 1991). The surface at the site and surrounding area had 

been eroded owing to the mining activity of the late 1800s 

and early 1900s. Conifer fragments more than 30 cm long and 

3 cm in diameter were present throughout the entire section. 

In addition, several tree stumps about 20–30 cm in diameter appeared to be in growth position. Radiocarbon ages of 

6,890±200 yr B.P. (W-6006), 7,020±200 yr B.P. (W-6008), 

and 8,790±260 yr B.P. (W-6009) were obtained from spruce 

fragments at depths of 20 cm, 55 cm, and 85 cm, respectively 

(fig. 8, table 4). The size of the tree stumps suggests that 

full-sized trees grew at the Placer Gulch site between 8,800 to 

6,900 yr B.P. (about 9,800 to 7,700 cal yr), indicating a timberline (and probably a treeline) approximately 40 m higher 

than at present.

California Gulch Bog (3,615 m asl)

The California Gulch bog, at an elevation of 3,615 m, 

is about 2 km downvalley from the head of California Gulch 

(figs. 2, 15). No conifers presently grow near the bog and, in 

general, the bog is considered to be at present-day treeline 

although several patches of krummholz about 0.8 km east of 

the bog on the valley side reach an elevation of about 3,675 m. 

Timberline is about 1.5 km downvalley near the confluence of 

California and Placer Gulches at an elevation of 3,585 m.

A pit dug in this bog in 1987 exposed a 155-cm section of 

very dark brown (10YR 2/2 wet) peat containing four woody 

peat layers overlying a bedrock surface (fig. 8) (Carrara and 

others, 1991). Radiocarbon ages were obtained from each of 

the woody peat layers (fig. 8, table 4). A radiocarbon age of 

3,600±50 yr B.P. (USGS–2600) was obtained from willow 

fragments recovered from the woody peat layer at a depth of 

40–50 cm. Radiocarbon ages of 4,330±35 yr B.P. (USGS–

2601), 6,850±40 yr B.P. (USGS–2602), and 7,860±40 yr B.P. 

(USGS–2603) were obtained from spruce fragments recovered 

from the woody peat layers at 65–75 cm, 105–115 cm, and 

140–155 cm depths, respectively. 

Spruce fragments within these woody peat layers were 

usually less than 20 cm long and 3 cm in diameter. However, one fragment from the woody peat layer between 65 

and 75 cm was 60 cm long and 10 cm in diameter, and it 

weighed about 800 g. Because contorted annual growth 

rings and reaction wood were common in these fragments, 

they probably were from krummholz growing on the nearby 

slopes and deposited in the bog by snow avalanches or other 

mass-wasting processes. 

Numerous small coniferous wood fragments, usually less 

than 3 cm long and 0.5 cm in diameter, were noted in the peat 

layers between depths of 50 and 65 cm, 75 and 105 cm, and 

115 and 140 cm (fig. 8). If one takes into account the overlying 

radiocarbon age of 3,600±50 yr B.P. (USGS–2600), then the 

uppermost coniferous wood fragments in the underlying peat 

are estimated to date from about 3,700 yr B.P. In addition, a 

small conifer fragment was recovered at a depth of 30 cm. This 

fragment was too small (0.04 g dry weight) for a conventional 

radiocarbon age determination, but a sedimentation rate based 

on the underlying radiocarbon age of 3,600±50 yr B.P. (USGS–

2600) suggests an age of about 2,400 yr B.P. for this fragment. 

Because the California Gulch bog is considered to be at 

the present-day treeline, and the wood fragments recovered 

from the bog were undoubtedly washed into the bog from surrounding higher slopes, the wood fragments indicate a treeline 

higher than that of present-day from at least as early as 7,900 

to 3,700 yr B.P. (about 8,700 to 4,050 cal yr) and possibly for a 

short time about 2,400 yr B.P. (about 2,450 cal yr). 

Bog Near Hurricane Basin (3,660 m asl)

A bog about 1 km from a cirque headwall in the headwaters region of Henson Creek, a tributary of the Gunnison River, 

is at an elevation of 3,660 m (figs. 16, 17). Although previously 

referred to as the “Hurricane Basin bog” (Andrews and others, 

1975), the bog is actually in a northeast-facing cirque about 

1 km northwest of Hurricane Basin. The bog is near presentday treeline, although some krummholz reach an elevation of 

about 3,700 m on a bedrock ridge 0.6 km northeast of the bog. 

Only several patches of krummholz are present in the cirque, 

these being about 100 m north of the bog. Timberline is below 

the cirque lip about 0.5 km downvalley at an elevation of about 

3,600 m. 

Much of the evidence that first documented Holocene 

treeline fluctuations in the northern San Juan Mountains came 

from this bog. Initial investigations of this bog in 1971 yielded 

a radiocarbon age of 9,620±440 yr B.P. (St–3909) (about 

11,000 cal yr) from organic material concentrated from the 

basal clay at a depth of 215 cm (Andrews and others, 1975). 

In 1973 a 210-cm-long core was collected from this bog for 

radiocarbon dating and pollen and plant macrofossil analysis 

(Andrews and others, 1975). The core consisted mainly of 

dark-brown peat that contained three woody peat layers at 

depths of 70–95 cm, 115–140 cm, and 185–210 cm (Andrews 

and others, 1975, their table 3) (fig. 8, table 4). A radiocarbon 

age of 3,520±100 yr B.P. (GaK–3858) was obtained from 

woody peat at the base of the upper woody layer that contained spruce cone scales and needles. A radiocarbon age of 

8,330±75 yr B.P. (SI-1230) was obtained from woody peat 

near the top of the lower woody layer that also contained 

spruce macrofossils. Additional radiocarbon ages that provided age control for the pollen diagram were 280±80 yr B.P. 

(GaK–3859), obtained from a wood fragment at a depth of 

10 cm, and 1,295±70 yr B.P. (SI-1231), obtained from peat 

at a depth of 55 cm (Andrews and others, 1975) (fig. 8). No 

spruce macrofossils were noted above the horizon dated at 

3,520±100 yr B.P., whereas spruce macrofossils were noted 

below the horizon dated at 8,330±75 yr B.P. (Andrews and 

others, 1975) (fig. 18). The base of the core was thought to date 

from about 9,000 yr B.P.
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 23

Figure 14. Placer Gulch bog study site; exposure is near Evening Star Mine.

Analysis of the Hurricane Basin core indicated that before 

about 8,500 yr B.P., percentages of spruce, pine, and total arboreal pollen were relatively low, whereas percentages of sagebrush pollen were relatively high (Andrews and others, 1975, 

their fig. 7). In addition, spruce/pine pollen ratios were low 

(Andrews and others, 1975) (fig. 18). These data seemingly 

indicate a cooler climate with a lower than present-day timberline; however, the presence of spruce macrofossils indicates 

that spruce was growing near this bog and suggests that even at 

this time timberline was probably higher than at present.

Between about 8,500 and 3,500 yr B.P. (about 9,500 and 

3,775 cal yr) spruce pollen is relatively abundant (>12 percent), as is total arboreal pollen, and spruce/pine pollen ratios 

are generally higher than at present. In addition, during this 

period spruce macrofossils are common in the core (Andrews 

and others, 1975) (fig. 18). A drop in spruce/pine ratios about 

7,600 yr B.P. (about 8,400 cal yr) seems to have been caused 

by an abnormally large amount of pine pollen in the section 

at this time and is not thought to represent a climatic event, 

because the percentage of spruce pollen remained fairly high. 

These data indicate that spruce was closer to the bog (hence 

the climate was warmer than at present) during this period. 

After about 3,500 yr B.P., spruce pollen and spruce/pine pollen ratios decrease, and in addition, no spruce macrofossils 

were identified after this time. These data suggest a lower 

timberline and hence cooler climate after 3,500 yr B.P. (about 

3,775 cal yr). 

This bog was revisited in 1986 and a 215-cm-deep pit 

was dug to the underlying bedrock surface (Carrara and others, 

1991). The pit exposed dark yellowish-brown (10YR 4/6 wet) 

peat that contained three woody peat layers at depths of 

70–90 cm, 140–150 cm, and 190–205 cm (fig. 8). The woody 

peat layer between 140 and 150 cm contained numerous 

spruce fragments. Conifer fragments in this layer were generally small and platy, usually less than 3–5 cm in length and 

width and 1 cm thick. These fragments yielded a radiocarbon 

age of 5,630±250 yr B.P. (W–5867) (fig. 8, table 4). In addition, Carrara and others (1991) obtained a radiocarbon age of 

8,420±250 yr B.P. (W–5866) from willow fragments from the 

lower woody peat layer from which spruce macrofossils were 

identified (fig. 8, table 4). 

Together the Hurricane Basin data may be interpreted 

as indicating that treeline was at least slightly higher than at 

present from about 9,000 to 3,500 yr B.P. (about 10,150 to 

3,770 cal yr). After 3,500 yr B.P. (about 3,770 cal yr) treeline 

was lower than at present and rose to its present elevation only 

in the last several hundred years (Andrews and others, 1975) 

(fig. 18).
24 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Table 4. Radiocarbon ages from bogs in the northern San Juan Mountains, Colo.

[cal yr, calendar years before present; m, meters; yr B.P., radiocarbon years before present]

Bog

(altitude)

Material

dated

Depth

(m)

Laboratory

number

14C age

(yr B.P.)

Calendar year

(cal yr)a

Placer Gulch

(3,600 m)

Spruce fragments 20 W–6006 6890±200 7731±179

Spruce fragments 55 W–6008 7020±200 7849±188

Spruce fragments 85 W–6009 8790±260 9848±345

California Gulch 

(3,615 m)

Willow fragments 40–50 USGS–2600 3600±50 3903±66

Spruce fragments 65–75 USGS–2601 4330±35 4872±35

Spruce fragments 105–115 USGS–2602 6850±40 7675±31

Spruce fragments 140–155 USGS–2603 7860±40 8629±50

Near Hurricane Basin 

(3,660 m)

Willow fragment 10 GaK–3859 280±80b Out of calibration range

Peat 55 SI–1231 1295±70b 1229±72

Woody peatc 90–95 GaK–3858 3520±100b 3799±132

Spruce fragments 140–150 W–5867 5630±250 6424±273

Woody peat 190–195 SI–1230 8330±75b 9347±105

Willow fragmentsd 190–205 W–5866 8420±250 9404±293

Organic material in basal clay 210 SI–1232 8455±85b 9468±66

Organic material in basal clay 215 St–3909 9620±440b 10,986±647

Eureka Gulch

(3,665 m)

Organic silty claye 105–130 USGS–2466 3905±40 4353±67

Conifer fragments 170–180 USGS–2716 6180±160 7069±199

Spruce fragments 200–205 USGS–2467 8320±70 9335±103

Willow fragments 235–240 USGS–2468 9260±60 10,440±97

Picayne Gulch

(3,750 m)

Willow fragmentsf 95–105 W–5863 6000±300 6846±349

Willow fragments 120–130 W–6013 8350±250 9323±300

a

Data accessed on the World Wide Web at http://www.radiocarbon.LDEO.columbia.edu on March 20, 2009. See Fairbanks, R.G. and others, 2005, Marine 

radiocarbon calibration curve spanning 0 to 50,000 years B.P., based on paired 230Th/234U/238U and 14C dates on pristine corals: Quaternary Science Reviews, 

v. 24, p. 1781–1796.

b

Radiocarbon age from Andrews and others (1975); depth represents a combined stratigraphy of Andrews and others (1975) and Carrara and others (1991). 

c

Woody peat containing spruce macrofossils (Andrews and others, 1975).

d

From same woody peat layer in which Andrews and others (1975) identified spruce macrofossils.

e

Contained two conifer fragments too small for a conventional radiocarbon age determination.

f

Woody layer contained mainly willow fragments, but two small spruce fragments were also recovered from this layer.
Figure 15. California Gulch looking upvalley; bog study site is in foreground. 
26 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 16. Hurricane Basin area. Elevations are in meters.

EXPLANATION

(4,173) 

(4,179) 

(3,798) 

Seigal Mountain 

 (4,047) 

Gravel Mountain

 (4,139)

Bog Site 

 

0 1 MILE

0 1 KILOMETER

Prominent ridge 

Forested area

Krummholz area

37°57’30” 

Hurricane Basin

Schafer

Gulch

Henson Creek

107o32’30” 
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 27

Figure 17. Bog near Hurricane Basin; bog was originally visited in 1971 and again in 1973 and 1986. S.P. Schilling augering 

through the sediments to determine sediment thickness and presence of woody peat layers. 

Eureka Gulch Bog (3,665 m asl)

The Eureka Gulch bog, at an elevation of 3,665 m, is 

near the head of Eureka Gulch, about 0.8 km south of the 

Lake Emma site (figs. 2, 19). The bog lies near treeline, and 

the nearest krummholz are a small patch of subalpine fir at the 

base of a south-facing cliff about 200 m west of the bog. Timberline is about 1 km downvalley at an elevation of 3,600 m.

A pit was dug in the Eureka Gulch bog in 1972 and 

radiocarbon ages of 740±70 (SI–1238), 3,150±135 (SI–1239), 

and 5,665±60 (SI–1240), were obtained from peat and organic 

silt layers at depths of 45 cm, 115 cm, and 145 cm respectively (Andrews and others, 1975, their table 2). In addition, a 

radiocarbon age of 8,785±45 yr B.P. (SI–1241) was obtained 

from a woody peat at a depth of 215–220 cm at the base of the 

pit. Unfortunately, it is not known if this wood was coniferous, 

and hence its relation to treeline fluctuations is not clear. 

The Eureka Gulch bog was revisited in 1986 and 

1987, and a new pit was dug near the site of the 1972 pit 

that exposed a 245-cm section overlying a bedrock surface 

(Carrara and others, 1991). This section contained layers of 

dark yellowish-brown (10YR 3/4 wet) peat and dark-gray 

(10YR 4/1 wet) to very dark grayish-brown (10YR 3/2 wet) 

organic silty clays that overlay a greenish-gray (5G 6/1 wet), 

inorganic sandy clay (fig. 8). 

Four radiocarbon ages were obtained from the Eureka 

Gulch bog (fig. 8, table 4) (Carrara and others, 1991). The 

uppermost coniferous macrofossils consisted of two small 

wood fragments at depths of 110 and 120 cm. Because 

these conifer fragments were too small (0.2 g dry weight) 

for conventional radiocarbon dating, a radiocarbon age of 

3,905±40 yr B.P. (USGS–2466) was obtained from the organic 

silty clay between depths of 105 and 130 cm from which these 

conifer fragments were recovered. In addition, a radiocarbon 

age of 6,180±160 yr B.P. (USGS–2716) was obtained from 

several small (1.6 g dry weight) conifer wood fragments at a 

depth of 170–180 cm at the base of a peat (fig. 8). 

Numerous small wood fragments were found in an 

organic silty clay unit between depths of 200 and 240 cm (Carrara and others, 1991). Most of these wood fragments were 

less than 2 cm long. A radiocarbon age of 8,320±70 yr B.P. 

(USGS–2467) was obtained from spruce fragments collected 

between depths of 200 and 205 cm. All wood fragments recovered below this depth were identified as willow. A radiocarbon 
28 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 18. Changing apparent elevation of the Hurricane Basin core based on spruce/pine 

ratios; radiocarbon ages (yr. B.P.) and location of macrofossils also shown (after Andrews 

and others, 1975).

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

O X 

O 

O 

O 

O 

O 

O 

O 

O 

3,200 3,600 4,000 

200 

150 

100 

50 

0 

DEPTH, IN CENTIMETRS

3,200 3,600 4,000 

APPARENT ELEVATION, IN METERS

X Spruce macrofossil 

EXPLANATION

O Willow macrofossil

Elevation of bog 

280±80 yr B.P. 

1295±70 yr B.P

3520±100 yr B.P

8330±75 yr B.P
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 29

Figure 19. Eureka Gulch bog, about 0.8 km south of Lake Emma; study site in foreground.

age of 9,260±60 yr B.P. (USGS–2468) was obtained from 

willow fragments collected between depths of 235 and 240 cm 

(fig. 8, table 4).

The radiocarbon ages of the conifer fragments from the 

Eureka Gulch bog indicate a treeline at least as high as at present about 8,300 yr B.P. (about 9,300 cal yr) and 6,200 yr B.P. 

(about 7,100 cal yr), and possibly about 3,900 yr B.P. (about 

4,350 cal yr). From the sparse evidence at this site, nothing 

definitive can be determined about the position of treeline 

between these three dates. 

Lake Emma Site (3,740 m asl)

The Lake Emma site, at an elevation of 3,740 m in a 

south-facing cirque, is at the head of Eureka Gulch near the 

headwaters of the Animas River (figs. 1, 2). The cirque has 

been the site of extensive gold and silver mining since 1874, 

when the San Juan Mountains were opened to settlement 

(Bird, 1986) (fig. 20). Lake Emma had a surface area of about 

0.05 km2

 and a maximum depth of about 5 m; its outlet was 

across a bedrock lip at its southern end. On 4 June 1978, 

it was suddenly and completely drained by the collapse of 

underground mine workings (fig. 4) (Carrara and Mode, 1979; 

Marcus and Marcus, 1983; Carrara and others, 1984, 1991). 

This unfortunate event had beneficial consequences for paleoclimate studies because the lake’s sediments were incised by 

the escaping waters, which exposed a well-preserved archive 

of subfossil wood above present-day treeline.

Radiocarbon-Dated Conifer Fragments 

Numerous spruce and fir wood fragments were found 

scattered among the deposits of Lake Emma (Marcus and 

Marcus, 1983; Carrara and others, 1984, 1991). Most of these 

fragments were small, usually less than 20 cm long, 5 cm in 

diameter, and 500 g (dry weight). That these wood fragments 

are probably from krummholz is indicated by the 

1. Small size of the fragments, 

2. Contorted patterns of the annual rings (fig. 21), and 

3. Presence of reaction wood commonly found in trees 

above timberline (Tranquillini, 1979). 

However, one spruce fragment recovered from the sediment of 

Lake Emma, which was 70 cm long, 30 cm in diameter, and 

more than 5 kilograms (kg) (dry weight), yielded a radiocarbon age of 8,010±80 yr B.P. (Lk Emma 18–81) (table 2). This 

fragment contained about 150 annual rings that displayed a 

complacent growth record and, hence, appeared to have come 

from a large, upright tree.

Krummholz do not presently grow in the Lake Emma 

cirque, which is occupied solely by alpine tundra vegetation 
30 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Figure 20. Lake Emma and the adjacent Sunnyside Mine, about 1900 AD. Photograph courtesy of the San Juan County, Colo., Historical 

Society. 
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 31

Figure 21. Cross section of a spruce fragment from the Lake Emma site that yielded radiocarbon ages of 3,120±80 yr B.P. (W−4242) and 

3,075±35 yr B.P. (USGS−2617) (see also table 2). Contorted pattern of annual rings and the presence of reaction wood suggest that the 

fragment was from krummholz.
32 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

(fig. 4). Treeline in Eureka Gulch is about 0.9 km downvalley 

from the Lake Emma site at an elevation of 3,660 m, although 

a few krummholz attain an elevation of about 3,700 m on the 

northeast shoulder of Emery Peak about 1.25 km to the southsoutheast (fig. 2). Timberline is about 1.5 km downvalley at an 

elevation of 3,600 m. 

No evidence was found to suggest that the krummholz 

limit (treeline) was altered by mining activity at Lake Emma 

during the 19th or 20th century. It is doubtful that early miners 

would have removed krummholz from the Lake Emma cirque, 

because it is not suitable for mine timbers and does not appear 

to have been removed elsewhere in the San Juan Mountains. 

Krummholz is not present on photographs of Lake Emma and 

the Sunnyside Mine taken about 1900 AD (fig. 20). A miner 

employed at the Sunnyside Mine during the 1920s does not 

recall any trees in the Lake Emma cirque (Eric Hoffman, 

oral commun., 1979). At lower elevations where full-sized 

trees were logged for mining operations, stumps are numerous and well preserved; no stumps are present in the Lake 

Emma cirque. Furthermore, allowing for differences caused by 

aspect, snow avalanches, and cold-air drainage, the elevation 

of treeline is similar in valleys in the northern San Juan Mountains regardless of the degree of past mining activity.

It is unlikely that these conifer fragments were blown 

upvalley to the Lake Emma site. Oven-dry weights of these 

fragments were generally more than 25 g and as much as 

5 kg. These fragments were from trees, probably krummholz (except for the fragment discussed above, Lk Emma 

18–81; table 2) growing on the surrounding slopes and 

deposited in Lake Emma by snow avalanches or other 

mass-wasting processes. 

Coniferous wood fragments were collected during visits 

to the Lake Emma site between 1978 and 1987. Each year, 

further erosion of the lake deposits revealed additional wood 

fragments that were collected for identification and radiocarbon dating. In any given year fragments were collected several 

meters from one another to reduce the probability of dating 

more than one fragment from the same tree. All wood fragments were collected from surface exposures of Lake Emma 

deposits, with 5 exceptions: 3 conifer fragments (LE Pollen 

(81)−79, (96) −79, and (161)−79; table 2) that were collected 

from a section sampled for pollen analysis in 1979 (Carrara 

and others, 1984); and 1 conifer fragment (LE Fos Insect 

02–87) (table 2), and 1 willow fragment (9,400±70 yr B.P. 

(AA–3888)) that was collected from a nearby section in 1987 

for fossil insect analysis (Elias and others, 1991) (fig. 7).

Sixty-six radiocarbon ages were obtained from 53 

coniferous wood fragments from the Lake Emma site (table 

2). Carrara and others (1991) presented 41 of these radiocarbon ages, 39 of which formed an almost continuous suite of 

radiocarbon ages from about 9,600 to 5,400 yr B.P. (about 

10,950 to 6,200 cal yr). In addition, two radiocarbon ages 

from the same spruce fragment (Stand Metals 01–78, table 2) 

gave ages of about 3,100 yr B.P. (about 3,300 cal yr) (fig. 21).

The number of annual rings in 52 of the 53 dated conifer 

fragments ranged from 4 to 100 (average about 40). The pith 

was present in most of the samples submitted for radiocarbon 

dating. Hence, the age range of the wood in each fragment was 

in most cases smaller than the error limit of the radiocarbon 

age. Because one sample (Lk Emma 18–81; table 2) contained 

about 150 annual rings, the wood submitted for dating from 

this sample was taken from the pith region. 

Of the 53 coniferous fragments submitted for radiocarbon dating, 47 (89 percent) were spruce and 6 (11 percent) 

were fir (table 2). Of the 35 krummholz trees inspected on 

the northeast flank of Emery Peak (fig. 2), 32 (91 percent) 

were spruce and 3 (9 percent) were fir. Hence, the present-day 

dominance of Engelmann spruce as compared with subalpine 

fir among krummholz is mirrored by the radiocarbon-dated 

fossil samples. 

Many of the coniferous wood samples from Lake 

Emma no longer exist, because they were completely consumed by either radiocarbon dating (Carrara and others 

1984, 1991; Elias and others, 1991) or deuterium analyses 

(Friedman and others, 1988; Epstein and others, 1999). 

These consumed samples include two of the oldest samples 

(9580±130 yr B.P. (USGS–1341), and 9220±120 yr B.P. 

(W–4524)). To confirm the reliability of earlier age determinations, some older remaining wood fragments originally 

dated by conventional radiocarbon methods were subsequently dated by AMS. One sample (Lk Emma 16–80) 

originally dated at 9,520±90 yr B.P. (USGS–1345) yielded 

an AMS radiocarbon age of 8,656±50 yr B.P. (WW–5548) 

(table 2). Another sample (Lk Emma 05–83), originally 

dated at 9,000±300 yr B.P. (W–5611) yielded an AMS age 

of 8,855±51 yr B.P. (WW–5547). Although the error limit of 

these two radiocarbon ages overlap, indicating that they may 

be the same, the AMS age produced a substantially younger 

age with a much smaller error limit and is thus considered the 

more accurate of the two. A third sample (Lk Emma 01–87) 

originally dated at 8,950±45 yr B.P. (USGS–3181) yielded an 

AMS radiocarbon age of 7820±30 (WW–6592). Three additional samples (Lk Emma 06–79, Lk Emma 14–79, and Lk 

Emma 05–85), which initially yielded conventional radiocarbon ages between about 8,820 and 8,730 yr B.P., subsequently 

yielded similar AMS ages (table 2). Hence, subsequent AMS 

radiocarbon dating cannot confirm that any of the coniferous 

wood fragments recovered from the Lake Emma site are older 

than 9,000 yr B.P. (about 10,150 cal yr).

However, the radiocarbon age from the spruce fragment 

(LE Pollen (161)–79), which yielded a conventional radiocarbon age of 9,220±120 yr B.P. (W–4524), appears to be accurate. As mentioned above, this sample was consumed in prior 

radiocarbon dating and deuterium analyses, so it cannot be 

dated again. However, the sample was recovered from a stratigraphic section sampled for pollen analysis in 1979 (Carrara 

and others, 1984). In 1987 two small wood fragments were 

recovered from a nearby section (about 2 m away) sampled for 

insect analysis (Elias and others, 1991). An AMS radiocarbon 

age of 9,400±70 yr B.P. (AA–3888) was obtained from a willow fragment at the same the horizon as this spruce fragment 

(fig. 7) (Elias and others, 1991, their table 2). In addition, 
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 33

another spruce fragment recovered approximately 15 cm 

above these two wood fragments yielded an AMS radiocarbon 

age of 8,625±90 yr B.P. (AA–3889) (Elias and others, 1991, 

their table 2). Hence, it appears that spruce was indeed growing around the Lake Emma site (above present-day treeline) 

by 9,200 yr B.P. (about 10,400 cal yr). 

Again, to confirm the reliability of earlier age determinations, some younger remaining wood fragments 

from the Lake Emma site originally dated by conventional radiocarbon methods were subsequently dated by 

AMS. One sample (Lk Emma 04–86), originally dated at 

5,370±110 yr B.P. (USGS–3005), yielded an AMS radiocarbon age of 5,858±40 yr B.P. (WW–6115) (table 2). 

A second sample (Lk Emma 09–83), originally dated at 

5,420±200 yr B.P. (W–5616), yielded an AMS radiocarbon age of 5,870±30 yr B.P. (WW–6593). Finally, a third 

sample (Lk Emma 04–79), originally dated at 5,570±70 yr 

B.P. (W–4680), yielded an AMS age of 5,839±40 yr B.P. 

(WW–6118). An additional small wood fragment (LE Pollen 

(81)–79) originally collected in 1979 yielded a conventional 

radiocarbon age of 5,440±100 yr B.P. (USGS–2135) (fig. 7). 

Although this age fits reasonably well with the other radiocarbon ages from wood fragments in the stratigraphic section 

(fig. 7), because this fragment was completely expended in 

the process of radiocarbon dating, subsequent AMS dating 

cannot verify its age. Hence, except for the spruce fragment 

(Stand Metals 01–78) dated by conventional radiocarbon 

methods at about 3,100 yr B.P. (about 3,300 cal yr) by two 

different labs (table 2), subsequent AMS radiocarbon dating 

cannot confirm that any of the coniferous wood fragments 

recovered from the Lake Emma site are younger than about 

5,800 yr B.P. (about 6,600 cal yr). 

From the above discussion and the radiocarbon ages 

listed in table 2, it can be concluded that treeline at the Lake 

Emma site was at least 80 m higher than at present from 

about 9,200 to 5,800 yr B.P. (about 10,400 to 6,600 cal yr). 

Treeline may have remained at this higher elevation until 

about 5,400 yr B.P. (about 6,200 cal yr), and possibly returned 

for a brief period about 3,100 yr B.P. (about 3,300 cal yr). In 

addition, the radiocarbon age of 8,010±80 yr B.P. (USGS–

1346) of a large conifer fragment (Lk Emma 18–81) from a 

full-sized tree indicates timberline was at least 140 m higher 

than at present about 8,000 yr B.P. (about 8,900 cal yr). 

Pollen Analysis

In addition to the radiocarbon age determinations of the 

coniferous fragments at Lake Emma, a section of exposed 

sediment was sampled for pollen analysis in 1979 (Carrara 

and others, 1984; their fig. 3) and a nearby section (2 m away) 

was sampled in 1987 for fossil insect analysis (Elias and 

others, 1991); the combined stratigraphic section is presented 

in figure 7. Because the upper 29 cm of the section was 

disrupted by the sudden draining of the lake, no samples were 

taken from that part of the section. In addition, because the 

uppermost sediments were eroded during the drainage of the 

lake, a surface sample was collected from a nearby locality 

where sediments appeared undisturbed. 

Radiocarbon ages were originally determined from 

organic material taken from three horizons (Carrara and 

others, 1984, their fig. 3). Spruce fragments at depths of 96 

and 161 cm yielded radiocarbon ages of 6,670±70 yr B.P. 

(W–4523) and 9,220±120 yr B.P. (W–4524), respectively 

(fig. 7). Detrital terrestrial moss in the basal 10 cm of the 

organic-rich laminated sediments yielded radiocarbon 

ages of 14,900±250 yr B.P. (W–4209), 14,130±150 yr B.P. 

(W–4289), and 14,940±140 yr B.P. (W–4525) (fig. 7). These 

radiocarbon ages are now thought to be too old; see the 

following discussion. 

An age of the uppermost sediment sampled at Lake 

Emma for pollen analysis was originally estimated on the 

basis of sedimentation rates (Carrara and others, 1984). 

By using the radiocarbon ages of the two spruce fragments 

at depths of 96 and 161 cm (see above), a sedimentation 

rate of 0.025 centimeters per year (cm/yr) was calculated. 

Extrapolation of this rate yielded an estimated age of about 

4,000 yr B.P. for the uppermost sediment sampled (at 31 cm 

depth). A subsequent radiocarbon age of 5,440±100 yr B.P. 

(USGS–2135) was obtained from a conifer fragment (LE Pollen (81)–79) at a depth of 81 cm (fig. 7). A new sedimentation 

rate (0.012 cm/yr) determined between the radiocarbon ages 

of 6,670±70 (age of the spruce fragment at 96 cm depth) and 

5,440±100 yr B.P. (LE Pollen (81)-79) was extrapolated and 

yielded an estimated age of about 1,300 yr B.P. for the upper 

pollen sample at a depth of 31 cm. However, because of the 

problems of extrapolation, this estimated age of 1,300 yr B.P. 

must be viewed with caution, and it is best to interpret the 

pollen data only to the 5,440 yr B.P. date. 

The age of the basal sediment in Lake Emma has also 

been revised from the original ages given by Carrara and 

others (1984) (Elias and others, 1991). The radiocarbon ages 

of the detrital moss fragments are now thought to be as much 

as 6,000 radiocarbon years too old (fig. 7) (Elias and others, 

1991). A radiocarbon age of 12,900±200 yr B.P. (W–5972) 

was obtained from detrital moss fragments at a depth of 

170–180 cm. Willow fragments from this same depth yielded 

a radiocarbon age of 9,400±70 yr B.P. (AA–3888) (Elias and 

others, 1991, their table 2). Furthermore, the spruce fragment originally collected in 1979 that yielded a radiocarbon 

age of 9,220±120 yr B.P. (W–4524) is also from this same 

horizon (fig. 7). A radiocarbon age of 13,580±200 yr B.P. 

(W–5974) was obtained from detrital moss fragments at 

a depth of 195–205 cm, whereas insect remains from this 

same depth yielded a radiocarbon age of 8,200±160 yr B.P. 

(AA–4826). Finally, as previously mentioned, radiocarbon 

ages of 14,000–15,000 yr B.P. were obtained from detrital 

moss fragments from the lowermost 10 cm of sediments 

from Lake Emma that also contained several spruce needles. 

Insect remains from this same sediment yielded radiocarbon 

ages of 8,940±85 yr B.P. (AA–4824) and 9,020±80 yr B.P. 

(AA–4825) (fig. 7) (Elias and others, 1991, their table 2). 
34 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Hence at Lake Emma, radiocarbon ages of wood fragments are about 3,600 years younger than radiocarbon ages 

of detrital moss fragments from the same horizon (fig. 7). 

Radiocarbon ages of insect remains range from about 5,000 

to 6,000 years younger than radiocarbon ages of detrital moss 

fragments from the same horizon. Although radiocarbon ages 

were not obtained from wood fragments and insect remains 

from the same horizon, inspection of figure 7 shows that 

radiocarbon ages of insect remains are at least 1,000 years 

younger than those of the wood fragments.

Reasons for the differences in radiocarbon ages from 

different material from the Lake Emma site are not known 

(Elias and others, 1991). As previously mentioned, the moss 

fragments were determined to be from a terrestrial moss (J.A. 

Janssens, written commun., 1982). Hence, the radiocarbon 

ages obtained from the moss fragments from Lake Emma were 

thought to be free of any hard-water effect. Terrestrial mosses 

assimilate only atmospheric carbon, whereas aquatic mosses 

are known to assimilate carbon from water and hence may 

be subject to a “hard water” effect (MacDonald and others, 

1987). Even if the moss fragments were from aquatic mosses, 

it is doubtful that Lake Emma was a hard-water lake. Although 

minor amounts of carbonate minerals (calcite and rhodochrosite) are associated with the local mineralization, the bedrock 

in the Lake Emma cirque is volcanic and contains no carbonate. Discussions with several experts in the field of radiocarbon dating and bryophyte biology have failed to provide 

any insights into this problem (Minze Stuiver, oral commun., 

1980; Meyer Rubin, oral commun., 1989; T.W. Stafford, oral 

commun., 1990; P.H. Glaser, oral commun., 1988). Hence, 

the reason for the spurious radiocarbon ages of the mosses 

remains unexplained. Nevertheless, it is apparent that the age 

of deglaciation of the San Juan Mountains must be revised.

The age of the base of the exposed sediment section 

sampled at Lake Emma is problematic. Clearly, the moss ages 

are too old. However, radiocarbon ages of the wood fragments 

suggest an age of about 10,000 yr B.P. (about 11,500 cal yr) 

or greater, whereas the radiocarbon ages of the insect remains 

suggest an age of about 9,000 yr B.P. (about 10,150 cal yr). 

Reasons for the differences in radiocarbon ages between the 

wood fragments and the insect remains are not known (Elias 

and others, 1991).

The pollen-percentage diagram from Lake Emma is dominated by Picea, Pinus, Artemisia, and Poaceae (Carrara and 

others, 1984) (fig. 22). The Picea curve is relatively uniform 

throughout the entire section; values range between 4 percent 

(at 46 cm depth) and 12 percent (at 91 cm depth) and average 

about 8 percent. The surface sample contains 6 percent Picea. 

The Pinus curve has two sections, a lower one (below 131 cm 

depth, about 8,000 yr B.P.) that averages about 40 percent and 

an upper section that averages about 25 percent. The surface 

sample contains 25 percent Pinus. The Artemisia curve can 

also be divided into two sections, a lower section (below 

171 cm depth, about 9,450 yr B.P.) that averages about 27 percent and an upper section that averages about 20 percent. In 

the surface sample Artemisia is 23 percent. The Poaceae curve 

indicates an overall increase from the bottom to the top of the 

section. Below 151 cm depth (about 8,850 yr B.P.), Poaceae

percentages are low and average only 10 percent; above this 

depth percentages increase and average about 19 percent; 

they reach 30 percent in the upper pollen sample. The surface 

sample contains 15 percent Poacea. 

The pollen-concentration summary diagram (Carrara and 

others, 1984) (fig. 23) shows high and increasing concentrations of most pollen types between the base of the section and 

138 cm depth (about 8,300 yr B.P.). Above 138 cm, pollen 

concentration drops sharply. This drop in pollen concentration 

is especially marked for Pinus and Artemisia.

The decline in pollen concentration above 138 cm may 

have been caused by nonclimatic factors. Although sediment 

size data revealed no significant changes, an increase in sedimentation rate could account for a decrease in pollen concentration. Additionally, Lake Emma may not have existed 

immediately upon deglaciation. The bedrock that underlies 

the Lake Emma basin is cut by numerous faults and fractures, 

and miners working in the large stope under the lake had long 

complained of leakage (Eric Hoffman, oral commun., 1979). 

After deglaciation, leakage may have delayed the establishment of Lake Emma. This interpretation is supported by the 

fact that the basal sediment consisted of terrestrial moss fragments rather than fine-grained lacustrine sediment (Carrara 

and others, 1984). Hence, the decrease in pollen concentration 

may simply reflect the sealing of the basin by sediment and 

establishment of Lake Emma. An outlet stream of the newly 

formed lake may have flushed some of the pollen rain from 

the basin. 

The section of exposed sediments sampled for pollen 

analysis at Lake Emma contained three spruce fragments 

that were radiocarbon dated (LE Pollen (81)–79, (96)–79, 

and (161)–79) (table 2). In addition, because the percentage 

of spruce pollen in the pollen-percentage diagram (fig. 22) 

is relatively uniform throughout the section and, as previously mentioned, the lowermost 10 cm of sediments contained several spruce needles, it can be argued that spruce 

was growing near Lake Emma continuously during the time 

represented by the section—from about 10,000 to 5,800 yr 

B.P. (about 11,500 to 6,600 cal yr) and maybe as late as 

5,400 yr B.P. (about 6,200 cal yr). As discussed above, the 

radiocarbon ages of coniferous wood fragments from this 

site clearly indicate the presence of conifers (and a higher 

than present-day treeline) about 9,200–5,800 yr B.P. (about 

10,400–6,600 cal yr).

Fossil Insects 

A higher than present-day treeline during the early 

Holocene is also supported by analysis of fossil insects from 

the basal sediments of Lake Emma (Elias, 1982; Elias and 

others, 1991). Sampling for fossil insects at the Lake Emma 

site began in 1979 (Elias, 1982) when about 20 kg of organicrich sediment was collected from the lower 10 cm of the basal 

organic unit. At that time, about 8 kg of modern lake sediment 
Postglacial Forestation and Holocene Treeline in the Northern San Juan Mountains 35

5 10 40 5 5 30 10 5 20 5 

5,440±100

years B.P. 

6,770±70

years B.P. 

9,220±120

years B.P. 

200 

150 

100 

50 

0 

DEPTH, IN CENTIMETERS

PERCENTAGE

Abies

Picea

Pinus

Quercus

Ambrosia

Artemisia

Chenopodiineae

Compositae-Tubuliflorae

Poaceae

Indeterminable

TreesShrubs

Herbs

Figure 22. Pollen percentage summary diagram from Lake Emma. Solid line for Abies and Quercus

represents 5× exaggeration (after Carrara and others, 1984). B.P., before present.
36 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

25,000 75,000 12,500 

200 

150 

100 

50 

0 

DEPTH, IN CENTIMETERS

87 

5,000 12,500 10,000 12,500 50,000 12,500 25,000 

NUMBER PER GRAM (OVEN DRY WEIGHT) 

6,770±70

years B.P.

9,220±120

years B.P.

5,440±100

years B.P.

164 

91 

141 

109 

117 

92 

130 

123 

103 

217 

95 

199 

136 

159 

123 

210 

174 

168 

149 

190 

119 

202 

166 

199 

127 

117 

104 98 

98 

151 99 

141 

110 105 

103 93 

158 110 

105 

256 58 

90 

132 

111 

131 

120 

635 256 

166 

120 

108 

87 

Abies

Picea

Pinus

Quercus

Ambrosia

Artemisia

Chenopodiineae

Compositae-Tubuliflorae

Poaceae

Indeterminable

Eucalyptus Count

Figure 23. Pollen concentration summary diagram from Lake Emma (after Carrara and others, 1984). 

B.P., before present.
Discussion 37

was also collected for comparison with present-day insects. 

In 1987, samples were taken from three horizons in the basal 

organic unit (fig. 7); each sample weighed about 15 kg (Elias 

and others, 1991). In addition, modern insects were collected 

in the vicinity of the former Lake Emma. 

The sediment samples from the three horizons in the basal 

organic unit yielded remains of a minimum of 185 insects that 

represented 44 taxa, in 18 families, from 5 insect orders (Elias 

and others, 1991). The samples show a surprising diversity 

of species (19 were identified), given the small number of 

individuals recovered. Because no differences in the environmental conditions are implied by the fossil insects from the 

three horizons, information from the horizons was combined 

(Elias and others, 1991). The fossil insect assemblages contain 

a high proportion (31 percent) of bark beetles. This assemblage 

contrasts sharply with the insect remains in the modern lake 

sediment and with those insects collected live; only one bark 

beetle was recovered. In addition, the fossil assemblages contained a number of flightless insect species not subject to wind 

transport and indicative of coniferous forests, whereas tundrarelated fossil insects were absent (Elias and others, 1991). The 

insect fossils from the basal organic sediments of Lake Emma 

were interpreted as indicating a higher than present-day treeline 

during the early Holocene (Elias and others, 1991).

Picayne Gulch Bog (3,750 m asl)

The Picayne Gulch bog, at an elevation of 3,750 m, is 

about 0.7 km downvalley from the head of Picayne Gulch 

(fig. 2). No conifers presently grow near the bog, which is 

about 100 m above present-day treeline. The nearest krummholz are about 0.4 km east of the bog at an elevation of 

3,650 m, and timberline is about 1.2 km downvalley at an 

elevation of 3,565 m.

The bog was originally visited in 1972 and a hole was 

augered to bedrock at a depth of about 200 cm (Andrews and 

others, 1975, their fig. 3). The stratigraphy consists of alternating layers of peat and clay that contain some organic material. 

Wood fragments (unidentified) were found at a depth of about 

100 cm. A radiocarbon age of 8,430±85 yr B.P. (SI–1558) was 

obtained from organic material concentrated from the basal 

clay at 200 cm depth (Andrews and others, 1975). 

The Picayne Gulch bog was revisited in 1986 and a pit 

was hand-dug at a site near the 1972 location; this new pit 

exposed a section of a very dark grayish-brown (10YR 3/2 wet) 

peat, 130 cm thick, overlying a greenish-gray (5G 6/1 wet) 

inorganic clay (fig. 8) (Carrara and others, 1991). Three layers 

of woody peat were noted at depths of 60–65 cm, 95–105 cm, 

and 120–130 cm. Wood fragments in these layers were small, 

usually less than 5 cm long and 1 cm in diameter. 

Radiocarbon ages were obtained from two of the 

woody peat layers (fig. 8, table 4). A radiocarbon age of 

6,000±300 yr B.P. (W–5863) was obtained from willow fragments from the woody peat layer at a depth of 95–105 cm. 

Although this layer contained mainly willow fragments, 

two small spruce fragments were also recovered. Hence, the 

radiocarbon age indicates the presence of spruce (probably 

krummholz) near the Picayne Gulch bog about 6,000 yr B.P. 

(about 6,850 cal yr), approximately 100 m above present-day 

treeline. Only willow fragments were found in the woody peat 

layers at 60–65 cm and 120–130 cm depths; a radiocarbon age 

of 8,350±250 (W–6013) was obtained from the lower layer. 

Discussion

Holocene Treeline and Climatic Fluctuations in 

the San Juan Mountains 

From about 9,200 to 5,800 yr B.P. (about 10,400 to 

6,600 cal yr) and possibly as late as 5,400 yr B.P. (about 

6,200 cal yr), treeline in the northern San Juan Mountains 

was at least 80 m higher than at present (fig. 24). This higher 

treeline is indicated by 66 radiocarbon ages of 53 coniferous wood fragments from the Lake Emma site that form an 

almost continuous suite of ages during this interval (table 

2). Because these wood fragments originated from trees that 

grew on the surrounding slopes, 80 m is a minimum estimate 

of the height of early to middle Holocene treeline above its 

present-day elevation. The highest evidence of treeline above 

present-day limits came from the Picayne Gulch bog, where, 

at about 6,000 yr B.P. (about 6,850 cal yr), treeline was at least 

100 m higher. 

 During the period 9,200–5,800 yr B.P. (about 10,400–

6,600 cal yr), timberline was also correspondingly higher than 

present-day, and at 8,000 yr B.P. both treeline and timberline 

may have been as much as 140 m higher. This higher elevation is indicated by the large spruce fragment from a large, 

upright tree (Lk Emma 18–81) that yielded a radiocarbon age 

of 8,010±80 yr B.P. (USGS–1346) (table 2). Because presentday timberline in Eureka Gulch is at an elevation of about 

3,600 m, then timberline, and probably treeline as well, was 

at least 140 m above present-day limits about 8,000 yr B.P. 

(about 8,900 cal yr). 

With the exception of a single spruce fragment radiocarbon-dated at about 3,100 yr B.P. (about 3,300 cal yr), all other 

coniferous wood fragments (n=52) yielded radiocarbon ages 

greater than 5,400 yr B.P. (about 6,200 cal yr). Hence, it is 

unlikely that trees (including krummholz) grew at or above the 

Lake Emma site during the last 5,400 yr B.P. After this period 

treeline elevation was probably below Lake Emma, hence, less 

than 80 m above its present-day limit. 

Between 5,400 and 3,500 yr B.P. (about 6,200 and 

3,770 cal yr), sparse macrofossil evidence suggests that treeline 

probably was near its present-day limits. At California Gulch 

bog (fig. 2), which is only slightly above present-day treeline 

(fig. 24), radiocarbon ages indicate the continuous presence 

of spruce from about 7,900 to 3,700 yr B.P. (about 8,700 to 

4,050 cal yr). At Eureka Gulch bog, near present-day treeline, 

a radiocarbon age of 3,905±40 yr B.P. (USGS–2466) (about 

4,350 cal yr) (table 4) from organic silty clay containing two 
38 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

0 2 4 6 8 10 

CALENDAR YEARS AGO (× 1,000) 

Timberline 

Treeline 

Eureka Gulch bog 

California Gulch bog 

Bog near Hurricane Basin 

Placer Gulch bog 

Lake Emma site 

Picayne Gulch bog 

RELATIVE ELEVATION, IN METERS 

0 2 4 6 8 

RADIOCARBON YEARS BEFORE PRESENT (× 1,000) 

+100 

+80 

+60 

+40 

+20 

0 

−20 

−40 

−60 

? ? 

? 

Conventional carbon ages

Older conventional radiocarbon age, now viewed with suspicion

Newer accelerator mass spectrometry radiocarbon ages

Continuous presence of coniferous wood

Estimated age of coniferous wood

EXPLANATION

Figure 24. Radiocarbon ages of coniferous wood fragments (squares) from the Lake Emma and other sites in the northern San 

Juan Mountains and their elevational relation to treeline and timberline. Age of coniferous wood estimated by interpolation 

or extrapolation of radiocarbon ages. Data for the bog near Hurricane Basin from Andrews and others (1975) and Carrara and 

others (1991). 
Discussion 39

small conifer fragments is the only evidence of conifers at 

this site after 5,400 yr B.P. At the bog near Hurricane Basin 

(figs. 1, 16), at present-day treeline, a woody peat containing 

spruce wood, needles, and cone scales yielded a radiocarbon 

age of about 3,500 yr B.P. (about 3,770 cal yr) (Andrews and 

others, 1975) (fig. 18), and the age of an underlying peat that 

contains coniferous macrofossils is estimated to be about 

5,600-4,500 yr B.P. (about 6,400-5,150 cal yr) (fig. 24). 

Evidence of treeline position after 3,500 yr B.P. (about 

3,770 cal yr) is very sparse at all of the bogs discussed earlier, 

suggesting that treeline lay at a lower than present-day elevation. Two radiocarbon ages of about 3,100 yr B.P. (about 

3,300 cal yr) were obtained from a spruce fragment (Stand 

Metals 01–78) from the Lake Emma site (fig. 21; table 2). 

The significance of this wood fragment is problematic. The 

fragment was about 30 cm long and 12 cm in diameter, and 

it weighed about 1.5 kg (dry weight). It contained about 

60 annual rings that had a contorted growth pattern and 

several lenses of reaction wood that suggest that it was from 

krummholz. Because this fragment was the only one recovered 

from the Lake Emma site that yielded a radiocarbon age less 

than 5,400 yr B.P. (about 6,200 cal yr), it is not clear if the 

age represents a short-term warm period or if it was simply 

from a unique krummholz individual that grew in a sheltered 

niche above the general treeline limit about 3,100 radiocarbon 

years ago.

As previously mentioned, a unique krummholz individual, now dead, was found on a ridge above California Gulch. 

Although the tree had grown at an elevation of 3,870 m, about 

200 m above the general treeline, it had grown in a sheltered 

niche at the base of a small south-facing cliff and as such 

received a sizeable amount of solar radiation. Wood from 

this individual yielded a radiocarbon age of 280±80 yr B.P. 

(W–6237), which is outside the calibration range. 

A single small conifer fragment found at a depth 

of 30 cm in the California Gulch bog was estimated by 

sedimentation rate to date to about 2,400 yr B.P. (about 

2,450 cal yr). Again, the interpretation of this single small 

fragment is also problematic. Pollen data from the bog near 

Hurricane Basin suggest that treeline after about 3,500 yr 

B.P. (about 3,770 cal yr) was lower than present-day treeline 

(Andrews and others, 1975).

The amount of climatic warming responsible for a 

treeline and timberline rise of at least 80 m and perhaps at 

least 140 m above present-day limits can be estimated from 

present-day lapse rates. Because growing-season temperatures 

are thought to control treeline and timberline (Fritts, 1976; 

Tranquilli, 1979), an estimate of the amount of warming can 

be obtained by using a summer lapse rate. Assuming a lapse 

rate of 6.2°C/km (Barry and Chorley, 1976) a warming of at 

least 0.5°-0.9°C is estimated for rises of the treeline and timberline ecotones of at least 80 m and 140 m respectively. Additionally, Maher (1961) estimated that the highest postglacial 

July temperatures in the San Juan Mountains were about 2°C 

warmer than at present. Combining Maher’s (1961) estimate 

of postglacial temperature rise with the lapse rate of 6.2°C/km 

(Barry and Chorley, 1976) suggests that, during the early to 

middle Holocene, the treeline and timberline ecotones may 

have been more than 300 m higher than at present. 

Early Holocene Intensification of the 

Summer Monsoon?

About 9,000 calendar years ago (about 8,100 radiocarbon yr B.P.), July solar radiation in the Northern Hemisphere 

is estimated to have been 8 percent greater than at present 

(Kutzbach, 1987). At that time, summer temperatures are generally thought to have been higher in North America except 

over and near remaining ice sheets (Kutzbach and Guetter, 

1986; Kutzbach, 1987). In places this increase is thought to 

have exceeded 2°C (Kutzbach, 1987)—an amount similar to 

Maher’s (1961) estimate of postglacial July temperature rise. 

Because 9,000 calendar years ago summer temperatures were 

warmer than at present, treeline should also have been higher 

at that time. Furthermore, because the summer (Arizona) monsoon is driven by a thermal low caused by a heating of desert 

areas in the American southwest, an increase in July radiation 

suggests that the summer monsoon circulation was probably 

more intense during the early Holocene than it is today (Friedman and others, 1988).

Air masses that originate over the Gulf of California and 

move to the northeast (fig. 3) arrive over the San Juan Mountains without passing over high mountains and thus without 

accompanying isotopic fractionation of hydrogen. Hence, 

precipitation from these air masses will have a higher deuterium concentration than precipitation from air masses that 

originate in the Gulf of Alaska and pass over several ranges 

before arriving over the San Juan Mountains. Furthermore, a 

decrease in July radiation from 9,000 calendar years ago to the 

present should be accompanied by a corresponding decrease in 

monsoonal activity and, hence, a decrease through time in the 

concentration of deuterium in water available to trees. 

Concentrations of deuterium in the celluose of 18 

radiocarbon-dated conifer fragments from the Lake Emma 

site and in samples of wood from present-day krummholz 

spruce and fir downvalley from the Lake Emma site were 

determined (Friedman and others, 1988). The dD was found 

to have decreased by about 45 per mil from about 9,000 

to 3,100 yr B.P. (about 10,150 to 3,300 cal yr) and by an 

additional 25 per mil from 3,100 yr B.P. to the present. This 

decrease in deuterium concentration was interpreted as 

indicating a major change during the Holocene in the sources 

of moisture or in the seasonality of precipitation (or both), 

and suggested that the summer monsoon was more intense 

(that is, wetter) during the early Holocene (Friedman and 

others, 1988). 

However, all sources of moisture available to Lake 

Emma, including the northern Pacific and the Gulfs of Alaska, 

Mexico, and California, would have been affected by early to 

middle Holocene warming (Epstein and others, 1999). Hence, 

moisture derived from any of these sources during this period 
40 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

would have had a higher dD value. Therefore, the decrease in 

deuterium concentration, although suggesting the possibility 

of more intense monsoonal flow during the early to middle 

Holocene, cannot be distinguished from the effect of warmer 

global temperatures.

However, support for an early Holocene intensification of 

the summer monsoon was found in central Colorado (Markgraf and Scott, 1981). The climate changed from cool and 

moist before 10,000 yr B.P. (about 11,500 cal yr) to warm and 

moist about 10,000-4,000 yr B.P. (about 11,500-4,450 cal yr). 

Evidence for this climatic shift is based on pollen changes in 

an erosional cut near the town of Crested Butte, about 120 km 

north of the Lake Emma site and suggests that a northward 

shift of the summer monsoon pattern about 10,000 yr B.P. 

increased summer precipitation and summer temperatures 

(Markgraf and Scott, 1981). A shift to a warm and dry climate 

with fewer monsoon events after about 4,000 yr B.P. resulted 

in the present-day climatic regime in which precipitation in 

the area is partially monsoonal and partially derived from 

Pacific storms originating in the Gulf of Alaska (Markgraf and 

Scott, 1981).

Similar conclusions, based on pollen and plant macrofossil analysis, were also reached by Fall (1997), who found 

that between about 9,000 and 4,000 yr B.P. (about 10,150 and 

4,450 cal yr) the climate of central Colorado was both warmer 

and wetter (owing to intensification of the summer monsoon) 

than at present. At that time spruce-fir forest covered a broader 

elevation range than today. Upper timberline was higher than 

at present suggesting that summer temperatures were about 

2°C warmer than at present (Fall, 1997). Conversely, the lower 

limits of the montane and subalpine forests were lower than at 

present suggesting an annual precipitation about 80–100 mm 

greater than at present.

Because lake levels in the southwest, thought to be controlled by winter precipitation, were lower during the early and 

middle Holocene than at present, Shuman and others (2009) 

concluded that summer precipitation was a greater proportion 

of the total precipitation during this period.

Finally, estimates of even larger Holocene temperature 

change were obtained by Epstein and others (1999). Concentrations of deuterium in the celluose of 39 radiocarbon-dated 

conifer fragments from the Lake Emma site were determined 

(Epstein and others (1999, their fig 1). For any given period 

a range of values of dD was obtained. The highest dD values 

were thought to represent those trees growing at Lake Emma, 

whereas the lowest dD values represent those trees growing 

at treeline (at the highest elevation and lowest temperature) 

above Lake Emma. For instance, for those trees radiocarbon 

dated at about 9,000 yr B.P. (about 10,150 cal yr), dD values 

ranged from a high of about −30 per mil (trees growing at 

Lake Emma) to a low of about −63 per mil (treeline above 

Lake Emma) (Epstein and others, 1999; their fig. 1)). The dD 

range and the maximum dD value, decrease as the ages of the 

samples decrease. Hence, the temperature (and treeline) in the 

Lake Emma cirque was highest about 9,000 yr B.P. and thereafter progressively decreased with time. If we assume that the 

coefficient relating temperature and dD values is 8 per mil/°C 

(Yapp and Epstein, 1982), then the maximum average climatic 

temperature change from about 9,000 to 5,400 yr B.P. (about 

10,150 to 6,200 cal yr), to when treeline dropped below Lake 

Emma, was about 4°C (Epstein and others, 1999). 

Conclusions 

Shortly after deglaciation of the Molas Lake site, organic 

sediments began to accumulate from which pollen data, including spruce/pine ratios, were developed. These data suggest that 

the average July temperature was about 5°C cooler and timberline was about 650 m lower than at present (Maher, (1961), 

or at an elevation of about 2,885 m (fig. 25). Unfortunately, 

the radiocarbon ages from the Molas Lake core associated 

with this period may have been subject to a hard-water effect 

(L.J. Maher Jr., oral commun., 1983), yielding radiocarbon 

ages older than the true age of the sediment. On the basis of 

information discussed in this report, the minimum age of this 

initial organic sediment may be about 10,450 yr B.P. (about 

12,300 cal yr).

At Black Mountain Lake, an unglaciated site about 

100–150 m below present-day timberline, evidence suggests 

that timberline was at or above the site by about 11,400 yr B.P. 

(about 13,250 cal yr). It then declined about 60–120 m during 

the Younger Dryas interval, but by about 10,000 yr B.P. (about 

11,500 cal yr) had readvanced to elevations at or above the site 

(Reasoner and Jodry, 2000) (figs. 1, 5). These data suggest that 

trees may have been present near the Black Mountain Lake site 

during the Pinedale glaciation, much as forest surrounds the 

tongues of glaciers in present-day Alaska. 

Forestation of formerly glaciated valleys in the San Juan 

Mountains was underway by about 11,000 yr B.P. (about 

12,900 cal yr) (fig. 25). This forestation is indicated by the 

radiocarbon ages of coniferous wood fragments from the well 

site near the town of Telluride, about 10 km upvalley from the 

Pinedale terminal moraines of the San Miguel glacier. 

Between about 9,200 and 5,400 yr B.P. (about 10,400 and 

6,200 cal yr) treeline was at least 80 m higher than at present (fig. 25). The highest evidence of conifer establishment, a 

spruce fragment, was found at the Picayne Gulch bog (fig. 2) 

and yielded a radiocarbon age of about 6,000 yr B.P. (about 

6,850 cal yr) indicating the presence of spruce at least 100 m 

above present-day treeline at that time. In addition, a radiocarbon age from a large conifer fragment with a complacent 

annual ring record suggests that timberline may have been at 

least 140 m above its present-day elevation about 8,000 yr B.P. 

(about 8,900 cal yr) at Lake Emma. A warming of at least 

0.5°-0.9°C is estimated from a rise in treeline and timberline of 

80 and 140 m, respectively. Because these rises are minimum 

values, estimates of the amount of warming are also minimum 

values. Indeed, Epstein and others (1999) suggested that the 

average temperature change from about 9,000 to 5,400 yr B.P. 

(about 10,150 to 6,200 cal yr), when treeline dropped below 

Lake Emma, was about 4°C.
References Cited 41

Between about 5,400 and 3,500 yr B.P. (about 6,200 and 

3,770 cal yr) evidence of treeline elevation is sparse, but it 

appears that during this period treeline was near its present-day 

limit. This conclusion is based on the lack of coniferous macrofossils at the two highest sites (Lake Emma and Picayne Gulch) 

and the presence of macrofossils at several of the sites at or 

near present-day treeline (Eureka Gulch, California Gulch, and 

Hurricane Basin) (figs. 1, 2).

After 3,500 yr B.P. (about 3,770 cal yr) evidence of 

treeline position is very sparse, suggesting that treeline lay at 

a lower than present-day elevation. However, two radiocarbon 

ages of about 3,100 yr B.P. (about 3,300 cal yr) were obtained 

from a spruce krummholz fragment from the Lake Emma 

site. Because these radiocarbon ages are the only ones from 

this period at the Lake Emma site, it is not clear if this wood 

fragment represents a short-lived climatic amelioration or if 

it came from a unique krummholz individual that grew above 

the general treeline limit at that time. In addition, a small 

conifer fragment at a depth of 30 cm in the California Gulch 

bog was estimated by sedimentation rate to date from about 

2,400 yr B.P. (about 2,450 cal yr). Again, the interpretation of 

this fragment is difficult. 

The summer monsoon circulation that currently brings a 

large part of the annual precipitation to the San Juan Mountains probably was more intense during the early and middle 

Holocene. This intensification may be indicated by the higher 

than present-day deuterium concentrations in the early to 

middle Holocene-age wood fragments from the Lake Emma 

site (Friedman and others, 1988) and pollen evidence from 

surrounding regions. 

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20 18 16 14 12 10 8 6 4 2 0 

2,000 

1,600 

2,400 

2,800 

3,200 

3,600 

4,000 

Retreat from Pinedale limit in Animas Valley

 (Guido and others, 2007) 

Retreat from Pinedale limit 

in Rio Grande Valley

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Telluride water well 

Lake Emma 

Lake Emma 

Hurricane Basin 

Eureka Gulch 

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California Gulch 

Present-day treeline 

Treeline reconstruction in formerly glaciated areas 

EXPLANATION

Treeline reconstruction in unglaciated 

 areas (Black Mtn Lake) 

Figure 25. Timeline of events in the San Juan Mountains showing initial timing of Pinedale glacial retreat, deglaciation 

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Introduction

In addition to those sites in the San Juan Mountains discussed above, other sites throughout western North America 

(fig. A−1) have yielded conifer remains above present-day 

limits demonstrating treelines higher than at present throughout much of the Holocene. This evidence consists of conifer 

fragments, needles, and cones recovered from lakes and bogs; 

remnant conifer fragments lying on the surface or emerging 

from shrinking snowbanks; and standing dead snags above 

present-day limits. These remains were commonly radiocarbon dated and in some cases cross-dated by dendrochronology, 

yielding very precise dates. The results of many of these studies are similar to those obtained from the San Juan Mountains, 

and they indicate a higher than present-day treeline during 

the early to middle Holocene. Moreover, several studies also 

indicate a higher treeline extending into the late Holocene 

(Lamarche and Mooney, 1972; LaMarche, 1973; and K.L. 

Pierce, oral commun., 1989). In the discussion below, approximately 20 of these sites are documented.

Appendix A

Evidence of Higher Treelines During the Holocene in Western North America, 

as Indicated by Radiocarbon Ages or Cross-Dated Conifer Remains

Appendix A 45

125o 115o 105o 

50o 

45o 

40o 

35o 

0 500 KILOMETERS

Front Range 

Sawatch Range 

San Juan Mountains

Sangre de Cristo

Mountains 

Wind River Range 

Yellowstone National Park 

La Sal Mountains 

Beartooth Mountains

West Mountains

Snake Range 

White Mountains

Sierra Nevada 

Jasper National Park 

Coast Mountains

ARIZONA 

NEW MEXICO 

NEVADA UTAH 

OREGON 

IDAHO 

WYOMING 

CALIFORNIA 

WASHINGTON 

MONTANA 

COLORADO 

BRITISH COLUMBIA 

ALBERTA SASKATCHEWAN 

CANADA

UNITED STATES

NORTH DAKOTA 

SOUTH DAKOTA 

MANITOBA 

NEBRASKA 

MEXICO

UNITED STATES

PACIFIC

OCEAN

Figure A–1 Sites of Holocene conifer fragments above present-day treeline in western North America.
46 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Front Range, Colorado

Several sites in the Colorado Front Range (fig. A–1) 

provide evidence of a timberline similar to or higher than 

at present during the early Holocene. In the Caribou Lake 

valley west of the Continental Divide, timberline is presently at an elevation of about 3,350–3,400 m, and treeline 

is about 3,450 m (Benedict, 1985). A log recovered from a 

bog at an elevation of 3,400 m yielded a radiocarbon age of 

9,200±135 yr B.P. (I–6520) (Benedict, 1973, 1985; locality 12). The age was interpreted to indicate that timberline 

had risen either to, or above, its present elevation by about 

9,200 yr B.P. (about 10,400 cal yr) (Benedict, 1973, 1985). 

Radiocarbon ages of 9,915±165 yr B.P. (laboratory number 

I–6335) and 9,700±215 yr B.P. (I–6336) from a basal peat 

containing spruce needles from this same bog suggests that 

treeline may have risen to about its present-day limit by about 

9,900 yr B.P. (about 11,300 cal yr) (Benedict, 1985). 

At another site in the Caribou valley evidence also indicates an early Holocene timberline elevation similar to that at 

present. Radiocarbon ages of 9,720±220 yr B.P. (I–12198A) 

and 9,520±160 yr B.P. (I–12198B) (weighted average 

9,590±130 yr B.P.) were obtained from basal samples of 

humus-rich silt, spruce cones, and wood fragments (Benedict, 

1985, locality H). The samples were recovered from a small 

bog at the forest-tundra ecotone at an elevation of 3,395 m and 

were interpreted as indicating that mature spruce trees grew 

near the bog and, hence, near present-day timberline at this 

time (about 10,950 cal yr) (Benedict, 1985). 

In the Mummy Pass area of Rocky Mountain National 

Park evidence of a higher than present-day timberline during 

the middle Holocene was also found. Here, trunks of fullsized trees (Picea engelmanni) were found emerging from the 

melting edge of a snowbank in a northeast-facing recess at an 

elevation of 3,475 m, just below the modern tree limit (Benedict and others, 2008). Radiocarbon ages of the outer growth 

rings of four of these trees ranged between 3,860±15 yr B.P. 

(CURL−8892) and 3,780±20 yr B.P. (CURL−8884) and indicated that their date of death was about 4,200 calendar years 

ago (Benedict and others, 2008). The trunks were interpreted 

as indicating that forest vegetation grew on the present-day 

tundra during this time and that regional timberline was 

at least 100–150 m higher than at present (Benedict and 

others, 2008).

Sawatch Range, Colorado

In the Cottonwood Pass area of the Sawatch Range 

(fig. A–1) in central Colorado, a sediment core was taken 

from a small pond at an elevation of 3,670 m (Fall, 1997). On 

the basis of pollen and plant macrofossil analysis of this core 

and accompanying radiocarbon ages, Fall (1997) concluded 

that between about 9,000 and 4,000 yr B.P. (about 10,150 and 

4,450 cal yr) the climate of central Colorado was both warmer 

and wetter than at present. Fall (1997) also concluded that 

upper timberline was 270 m higher and that mean annual and 

mean July temperatures were 1°-2°C warmer than at present 

during this period. However, during a visit to Cottonwood 

Pass Pond in July 2009, the author noted full-sized spruce 

encroaching upon the pond, such that the pond is no more than 

30 m above present-day timberline and scattered patches of 

krummholz extend at least 50 m above this site. Hence, Fall’s 

estimate of an early Holocene timberline 270 m above that of 

the present appears to be too high. 

Sangre de Cristo Mountains, Colorado 

Lake Como, at an elevation of 3,580 m, is in a glacial 

valley on the west side of the Sangre de Cristo Mountains 

(fig. A–1), about 110 m below present-day timberline (Jodry 

and others, 1989; Shafer, 1989). Here, a 300-cm sediment 

core was collected and analyzed for pollen and plant macrofossils. High percentages of Artemisia pollen (>60 percent) in late Pleistocene assemblages were interpreted as 

evidence of widespread sagebrush (Artemisia tridentata) in 

the adjacent San Luis Valley to the west. A radiocarbon age 

of 10,260±230 yr B. P. (A-04895) was obtained from spruce 

macrofossils at a depth of 240–250 cm and indicate that by 

this time trees (full-sized or krummholz?) were growing by the 

lake (Jodry and others, 1989; Shafer, 1989). The abundance of 

macrofossils of Engelmann spruce, bristlecone pine, and subalpine fir suggests that upper treeline reached its highest elevation about 10,000–9,000 yr B.P. (about 11,525–10,150 cal yr). 

In addition, comparison of spruce/pine ratios in the pollen of 

the Lake Como core with modern spruce/pine ratios suggests 

that timberline around Lake Como was about 175–203 m 

higher than today in the earliest Holocene (Jodry and others, 

1989; Shafer, 1989).

Wind River Range, Wyoming

 In the Temple Lake area in the southern Wind River 

Range (fig. A–1) a radiocarbon age of 3,520±155 yr B.P. (GX–

2748) was obtained from a bog at an elevation of 3,260 m, 

0.1 km downvalley of Temple Lake (Currey, 1974). The bog, 

on the inner moraine of the type Temple Lake moraine, contained well-preserved plant debris such as wood fragments and 

conifer needles (Currey, 1974). In this valley the upper limit of 

continuous forest lies at an elevation of about 3,135 m, scattered stands of trees grow up to about 3,200 m (Fall and others, 

1995), and patches of dwarf (krummholz) whitebark pine and 

Engelmann spruce extend to about 3,230 m. The nearest trees 

to the bog site are about 0.5 km downvalley. The radiocarbon 

age (about 3,800 cal yr) was thought to “represent a time when 

the site was more suitable for conifer growth, perhaps warmer 

and drier than at present” (Currey, 1974). 

An additional radiocarbon age of 3,825±230 yr B.P. 

(GX–11700) was obtained from unidentified conifer needles 

and wood fragments between 78 and 81 cm depths in a core 
from Miller Lake (Zielinski, 1989; Fall and others, 1995), 

approximately 0.5 km downvalley from Currey’s (1974) bog 

site. This radiocarbon age (about 4,200 cal yr), along with pollen analyses, led Fall and others (1995) to conclude that during 

much of the Holocene, treeline was probably at least 100 m 

higher than today. 

Finally, large standing snags were observed in 1938 above 

timberline (3,200 m elevation) on the northern side of Shale 

Mountain between Ross and Simpson Lakes in the northern 

Wind River Range (G.M. Richmond, oral commun., 1989). 

The site has not been revisited for collection of samples to be 

radiocarbon dated.

Yellowstone National Park, Wyoming

On the northeast side of Mount Washburn in Yellowstone National Park (fig. A–1), wood (probably from 

krummholz whitebark pine) was found lying on alpine tundra 

(K.L. Pierce, oral commun., 1989). The area is at an elevation of about 3,015 m, approximately 25 m above present-day 

treeline. Radiocarbon ages of three fragments yielded ages of 

830±70 yr B.P. (W–4239), 1,060±200 yr B.P. (W–4154), and 

1,190±60 yr B.P. (W–4240) (K.L. Pierce, oral commun., 1989), 

suggesting a warmer than present-day climate corresponding 

with the Medieval Warm Period (about 800 to 1,300 AD). 

La Sal Mountains, Utah

Although no precise location is given, Richmond (1962, 

p. 15) mentions that a possible higher timberline in the past 

is suggested by local stands of dead, gnarled, and twisted 

relatively large trees in the La Sal Mountains (fig. A–1) that 

extend farther upslope from the present-day forest than the 

upper limit of new growth. 

West Mountains, Idaho

In a cirque in the West Mountains of west-central Idaho 

(fig. A–1), a radiocarbon age of 11,540±70 yr B.P. (WW–443) 

was obtained from a small spruce cone overlying a Glacier 

Peak ash in a bog at an elevation of 2,255 m (Doerner and 

Carrara, 1999). In this mountain range, timberline is at an 

elevation of about 2,285 m and is dominated by subalpine fir 

and whitebark pine, while treeline is at an elevation of about 

2,380–2,500 m. However, the present-day limit of Engelmann 

spruce is at an elevation of about 2,250 m. It is not clear 

whether the small spruce cone recovered from the bog came 

from a full-sized tree or a krummholz form; nonetheless, by 

about 11,540 yr B.P. (about 13,400 cal yr) spruce was already 

near its present-day elevation limits.

Beartooth Mountains, Montana 

In the Grass Mountain area of the Beartooth Mountains (fig. A–1), radiocarbon ages of 7,935±15 yr B.P. 

(CURL−8843) and 7,955±15 yr B.P. (CURL−8845) were 

obtained from two rooted stumps of Engelmann spruce 

emerging from the melting edge of a snowbank (Lee, 2006). 

Because this snowbank is presently above treeline, the radiocarbon ages indicate a time (about 8,800 cal yr) of higher than 

present-day treeline and warmer climate. Additional discoveries of subfossil trees emerging from retreating snowbanks 

were also made on the Silver Run and Hell Roaring Plateaus 

(Lee, 2006). 

Snake Range, Nevada

On Mount Washington in the Snake Range of east-central 

Nevada (fig. A–1), 14 radiocarbon ages were obtained from 

13 standing dead snags and remnants of bristlecone pine 

lying on the ground above present-day limits (LaMarche and 

Mooney, 1972). Living bristlecone pines at this site show a 

progressive gradation from tall, erect trees in the upper forest 

zone to dwarfed, prostrate krummholz at higher elevations. 

LaMarche and Mooney (1972) determined that a similar 

gradation existed from 4,000 yr B.P. to at least 2,000 yr B.P. 

(about 4,450 to at least 1,950 cal yr), but that the boundaries 

between the tree forms were at least 100 m higher than today. 

Because an indeterminate amount of wood had been eroded 

from the original outside and pith parts of many of these 

remnants, this time span represents a minimum estimate of the 

time during which the treeline was higher than at present.

White Mountains, California 

Two locations in the White Mountains of California 

(fig. A–1) were investigated by LaMarche (1973). On the basis 

of both radiocarbon ages and cross-dating of bristlecone pine 

remnants above present-day treeline he concluded that on 

Campito Mountain treeline was 70 to 110 m above the modern 

treeline from about 7,400 yr B.P., the beginning of his record, 

to about 800 yr B.P. (LaMarche, 1973, his fig. 17). On nearby 

Sheep Mountain, where the oldest remnant dated back to about 

5,600 yr B.P., treeline was determined to be about 50–150 m 

above the modern limit from 5,600 to 800 yr B.P. (LaMarche, 

1973, fig. 17). Lamarche (1973) cautioned that his estimates 

of higher treeline positions should be considered minimum 

values in that remnants higher than those collected may have 

existed at one time but may have disintegrated with time.

Appendix A 47
48 Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado

Sierra Nevada, California

At Cirque Peak in the southern Sierra Nevada of California (fig. A–1), Scuderi (1987) used a combination of radiocarbon ages and cross-dating of foxtail pine (Pinus balfouriana) 

remnants to determine timberline fluctuations since 6,300 yr 

B.P. Scuderi (1987) found that timberline descended from a 

high at 6,300 yr B.P. to a low at 900 yr B.P. Between 6,300 

and 3,300 yr B.P. (about 7,200 and 3,500 cal yr) timberline 

was about 70 m higher than the present-day limit. From 3,300 

to 2,400 yr B.P. (about 3,500 to 2,450 cal yr) it was about 

35 m above the present limit; from 2,400 to 1,300 yr B.P. 

(about 2,450 to 1,250 cal yr) it was 12 m above the present 

limit; from 1,300 to 900 yr B.P. (about 1,250 to 800 cal yr) it 

was below the present limit; and since 900 yr B.P. timberline 

has been at its present limits (Scuderi, 1987). 

Jasper National Park, Alberta, Canada

In the southern Canadian Cordillera, several sites at or 

above present-day treeline have yielded coniferous wood 

fragments. Radiocarbon ages of these fragments are in 

general agreement with those from the Lake Emma site. In 

the Maligne Range, Jasper National Park, Alberta (fig. A–1), 

timberlines were at least 100 m, and possibly as much as 

200 m, higher than at present between about 8,800 and 

5,200 yr B.P. (Kearney and Luckman, 1983; Luckman and 

Kearney, 1986). This conclusion was based in part on five 

radiocarbon ages from spruce and fir fragments recovered 

from the Watchtower Basin and the Maligne Pass area, both 

of which are about 50–100 m above present-day timberline. 

The wood fragments from the Watchtower Basin displayed 

a complacent growth record. On the basis of the radiocarbon 

ages and pollen data from Watchtower Basin and the nearby 

Excelsior Basin, Kearney and Luckman (1983) concluded that 

during the early and middle Holocene there were two periods 

of higher timberline: the first from about 8,800 to 7,500 yr B.P. 

(about 9,800 to 8,300 cal yr) and the second from about 7,200 

to 5,200 yr B.P. (about 8,000 to 5,950 cal yr). These periods of 

higher timberline were separated by a period when timberline 

was near its present elevation. After about 4,500 yr B.P. (about 

5,150 cal yr) timberline has been similar to or lower than at 

present, and it reached its lowest limit within the last 500 yr 

(Kearney and Luckman, 1983; Luckman and Kearney, 1986).

Coast Mountains, British Columbia

In the southeastern Coast Mountains of British Columbia (fig. A–1), timberline was at least 60 m and perhaps more 

than 130 m higher than at present between about 9,100 and 

8,200 yr B.P. (about 10,250 and 9,150 cal yr) (Clague and 

Mathewes, 1989). This higher timberline is indicated by five 

radiocarbon ages from fir and pine fragments found above 

present-day timberline in a cirque in the Castle Peak area. 

Using present-day lapse rates, Clague and Mathewes (1989) 

estimated that the mean summer temperature during this 

period was 0.4°-0.8°C warmer than at present.

Clague and Mathewes (1989, their table 2) also presented 

29 radiocarbon ages from conifer and charcoal fragments collected from various sites at or above present-day timberline in 

southern British Columbia and the adjacent Jasper National 

Park area of Alberta. These radiocarbon ages ranged from 

about 9,100 to 4,400 yr B.P. (about 10,250 to 5,000 cal yr). 

If only the 16 radiocarbon ages from above timberline are 

considered, the ages range from about 9,100 to 5,300 yr B.P. 

(about 10,250 to 6,075 cal yr), similar to the ages from the 

Lake Emma site. 

One group of radiocarbon ages in the Clague and 

Mathewes (1989, their table 2) study fell between about 9,100 

and 7,600 yr B.P. (about 10,250 and 8,400 cal yr), whereas a 

second group of radiocarbon ages fell between about 6,600 

and 5,100 yr B.P. (about 7,500 and 5,850 cal yr). Hence, 

the two groups of radiocarbon ages are separated by about a 

thousand-year interval. Timberline may have been at a lower 

elevation during this period; however, Clague and Mathewes 

(1989) thought it equally possible that this long interval could 

simply be a consequence of the small size of their data set. 

The first 22 radiocarbon ages from coniferous wood fragments from the Lake Emma site also contained a gap (Carrara and others, 1984). One group of 15 ages fell between 

about 9,600 and 7,800 yr B.P., and a second group of 6 ages 

fell between about 6,700 and 5,400 yr B.P. Hence, these two 

groups of radiocarbon ages were separated by an interval of 

about 1,100 years. Of the 44 subsequent radiocarbon ages, 11 

fall within this 1,100-yr interval (table 2). Therefore, in the 

case of the Lake Emma site, the interval in which no radiocarbon ages were initially obtained (Carrara and others, 1984) 

was a consequence of the small size of the original data set 

and not of a lower treeline.
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Carrara, Paul E.—Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado—Professional Paper 1782