A Hiker's Guide to the Geology 

of Old Rag Mountain, 

Shenandoah National Park,

Virginia

Old Rag Mountain seen from Skyline

Drive

Old Rag Mountain is one of the most 

popular hiking destinations in 

Shenandoah National Park and draws 

thousands of visitors each year from 

throughout the mid-Atlantic region. The 

scenic ridge trail along the mountain's 

rocky crest provides for an invigorating 

excursion enjoyed by outdoor 

enthusiasts of all ages, during all 

seasons of the year.

This presentation is designed as an 

earth science educational tool that you 

may use for a visit to Old Rag 

Mountain. The primary intent is to 

describe the geological features of the 

fascinating rock forming the mountain, 

a formation known as the Old Rag 

Granite. As you scramble and crawl 

over outcrops of Old Rag Granite on the 

mountain's craggy peak, you are 

treading on the evidence of an ancient 

mountain-building event that occurred 

over one billion years ago!

A map and walking tour of the ridge 

trail is included in this guide to help 

you appreciate the magnificent 

geological features visible in the rocks 

of Old Rag Mountain. In addition, you 

will learn about the physical forces and 

processes that have shaped the 

mountain, and a little about the plants 

and animals that inhabit its rugged 

slopes.

Preservation and conservation of 

Shenandoah National Park's natural 

resources are important things to keep 

in mind during your visit to Old Rag 

Mountain. The popularity of the Old 

Rag Mountain ridge trail has resulted in 

overuse and abuse and has necessitated 

an intensive effort by the National Park 

Service to educate visitors in 

environmental stewardship.

During a visit to this splendid natural 

monument that is your National Park, it 

is your civic duty to preserve, conserve, 

and to leave no trace of your visit, so 

that future guests in the Park may share 

its plentiful natural wealth. The beauty 

of the natural world is your reward for 

visiting Old Rag, a magnificent scenic 

resource to cherish and protect.

By Paul Hackley

U.S. Geological Survey

Open-File Report 00-263
gy g 

The geologic time scale - This diagram shows the various eons and eras of

geologic time and lists some significant events that have affected the Blue

Ridge. The notation "Ma" in the green column means "millions of years ago."

Notice that the earth is 4.6 billion years old! However, this great length of

time probably is too vast for our minds to fully comprehend. Just imagine; If

the age of the earth were only one year, then humans would have started to

walk about three hours ago, and Columbus would have "discovered" America

about three seconds ago!

Middle

Late

Paleozoic

Mesozoic

Early

Late

Middle

Early

Cenozoic

PHANEROZOIC

P

R

O

T

E

R

O

Z

OIC

A

R

C

H

E

A

N

EON ERA 

years ago

in millions

of years

P

R

E

C

A

M

B

RIA

N

700

900

500

400

355

300

179

66 66

245

545

900

1600

2500

3000

3400

4600

and in the Blue Ridge

~1060 Ma - Old Rag Granite

crystallizes during the

Grenville orogeny and the

formation of Rodinia.

~4600 Ma - The earth forms

~200 Ma - Pangea rifts and

the Atlantic begins to open.

~400 Ma - Assembly of

Pangea begins.

~570 Ma - The Iapetus Ocean

opens.

Today - you are hiking Old Rag

Mountain!

1935 - Shenandoah National

Park established.

1669 - First European

explorers reach the Blue

Ridge.

1492 - Columbus discovers

America.

~9500 years ago - Native

Americans camp and hunt in

the Blue Ridge.

~10000 years ago - The last

Ice Age ends. Mammoth and

mastadon roam the Blue

Ridge.

~1.5 Ma - Humans begin to

walk the earth

face is in a slowly

nuous state of motion.

n ranges are thrust up as

peaks and then slowly

, rolling hills. Ocean

w seas in rift basins and

e deep and wide. These

described by the theory

cs, which studies the

he earth's cool, rigid

hot and ductile rock of

rior. All of the earth's

ultimately have their

nic processes, whether it

rocks found in

where the earth's crust

r, or the basaltic rocks

basins where the

oves apart. The rocks

ountain tell us a

ry of plate tectonic

rth history that reaches

years into the past.

tain is underlain by a

Old Rag Granite. Old

rmed just a little over

rs ago during a

ding event known to

he Grenville orogeny.

orogeny formed a great

e that stretched from

ada and was perhaps as

cky Mountains of the

d States. These

mountains were formed when parts of

the present-day continents of North

and South America, India, Australia,

and Antarctica slowly collided over

tens of millions of years. This slow

assembly of the continents resulted in

formation of a supercontinent called

Rodinia that was surrounded by ocean

basins. As the individual continents

came together to form Rodinia,

intervening crust buckled, warped,

and became thicker and thicker as the

landmasses moved closer and closer

to one another. This same process is

creating the Himalaya Mountains

today, as India and Asia slowly collide

with one another and push up the crust

between them into great mountains.

Deep within the ancient Grenville

mountain range, the pressure and

temperature were great enough to melt

the rock in places, forming magma.

This molten rock slowly ascended

through fractures and conduits and

eventually crystallized into the rock

we now see today at the surface - Old

Rag Granite. Although Old Rag

Granite is now exposed at the surface

of the earth, the rock crystallized

several tens of miles below the land

surface. We know this because of the

minerals and textures found in the

granite. For example, as you examine

the rocks exposed on the crest of Old

Rag Mountain, you will notice the
large white crystals of a mineral called

alkali feldspar. The large size of these

crystals indicates that they had plenty

of time to form in a magma that cooled

slowly, deep below the earth's surface.

In contrast, fine-grained volcanic rocks,

exposed in narrow conduits called

dikes along the crest of Old Rag

Mountain, were erupted at or close to

the earth's surface, and thus did not

have time to form large crystals.

Millions of years after Old Rag Granite

crystallized from magma into rock,

uplift and erosion of overlying rocks of

the Grenville mountain range may have

exposed parts of the Old Rag Granite at

the surface. We know this because

cobbles and pebbles of rock similar to

Old Rag Granite were eroded from the

ancient land surface by running water

and then deposited as sediments on top

of the prehistoric landscape. These

sediments have since hardened into

rock and are known as the Swift Run

and Fauquier formations. Since Old

Rag Granite crystallized deep within

the earth's crust, it may have taken as

long as 300-500 million years for uplift

and erosion to eventually expose the

rock at the earth's surface.

Several hundred million years after the

Grenville orogeny the earth's crust

began to thin, stretch, and eventually

split as Rodinia began to pull apart due

to changing plate tectonic forces.

Basaltic magmas welled up from the

earth's mantle through conduits and

flowed onto the ancient land surface as

lava about 570-600 million years ago,

following deposition of the Swift Run

and Fauquier sediments. This lava

cooled quickly into rock known as the

Catoctin Formation or "greenstone"

exposed to the west of Old Rag

Mountain along Skyline Drive.

Repeated eruptions of lava eventually

covered the earth's surface and the Old

Rag Granite with thick layers of

volcanic rock. Continued splitting and

extension of Rodinia eventually

resulted in the formation of a deep

ocean basin to the east of the Grenville

mountain range called the Iapetus

Ocean. This ancient ocean was located

in the approximate area of the presentday Atlantic Ocean. Rivers draining

into the Iapetus Ocean deposited sandy

sediments eroded from the Grenville

Mountains on top of the Catoctin

Formation and underlying Old Rag

Granite. These sand particles were later

cemented into hard quartzite rock

known as the Weverton Formation. As

the Iapetus Ocean basin became deeper

and wider, finer-grained silty sediments

were deposited over the sands of the

Weverton Formation and eventually

hardened into a rock known as the

Harpers Formation, named for rocks

exposed at Harpers Ferry, West

Virginia. Sands deposited on top of the

silts of the Harpers Formation were

later turned into rocks of the Antietam

Formation, named for exposures of

sandstone along Antietam Creek in

Maryland. The Antietam Formation

contains fossils that are over 500

hundred million years old, the oldest

fossils known from Blue Ridge rocks

of the northern Virginia area.

Eventually, the Iapetus Ocean basin

widened to the extent that silt and sand

did not influence sedimentation, and

instead the tiny shells and skeletons of

planktonic marine organisms rained

down onto the ocean floor. Millions of

these minute creatures, called

Foraminifera, lived near the ocean's

surface and when they died, their hard

calcium carbonate shells fell to the

ocean floor to deposit a thick layer of

what would later become limestone.

Limestones created from the shells of

these creatures underlie much of the

Shenandoah Valley to the west of the

present-day Blue Ridge. During

deposition of the rocks of the Swift

Run, Fauquier, Catoctin, Weverton,

You may wonder why Old Rag Mountain is such a prominent feature in the

landscape. This diagram shows a portion of the geologic map of Virginia,

depicting the various rock types represented by different colors and patterns.

Old Rag Mountain is underlain by the hard and resistant Old Rag Granite,

represented by the oval pink shape in the small rectangle above. As you can

see, Old Rag Mountain is surrounded by different rock types, rocks which are

less resistant than Old Rag Granite. This is the reason Old Rag Mountain

stands high to the east of the main Blue Ridge, a topographic landform called

a monadnock.

20 miles

N Old Rag

Mountain

3
Harpers, Antietam, and overlying

limestone formations, Old Rag Granite

probably was buried beneath several

miles of rock.

At some point, the tectonic processes

that created the Iapetus Ocean reversed

and the ocean began to close. This

process may have started as long as

450-500 million years ago and lasted

several hundreds of millions of years.

The final closure of the Iapetus Ocean

resulted in formation of a mountain

range in much the same way as the

formation of the Grenville mountain

range about 700 million years earlier.

However, the mountains formed by

closure of the Iapetus Ocean have a

name that we know much better; the

Appalachians. The Appalachian

Mountains were created when all of

the continents again came together to

form a supercontinent called Pangea.

Tectonic stresses from the slow

assembly of Pangea generated the

structures of folds and faults that are

now evident in the rocks of the

Appalachians during a final mountainbuilding event known as the

Alleghenian orogeny. During the

Alleghenian orogeny, Blue Ridge

rocks were transported westward and

locally thrust over limestone rocks of

the Shenandoah Valley.

Pangea slowly began to break apart

into the earth's present-day continents

about 200 million years ago.

Geologists think that the breakup of

Pangea, and Rodinia earlier, was

caused by trapped heat that had risen

from the earth's interior and built up

underneath the large insulating

landmasses. During this time of crustal

extension and breakup of the Pangean

supercontinent, great rift basins

formed to the east of the present-day

Blue Ridge. Sediments eroded from

the Blue Ridge were deposited in these

basins and have since hardened into

the sedimentary rocks that underlie the

town of Culpeper to the east of Old

Rag Mountain. As had happened

during the formation of the Iapetus

Ocean, basaltic magma intruded the

crust and crystallized into rock called

diabase, or "traprock."

As Pangea split into the earth's

present-day continents, a new ocean

basin developed to the east of the Blue

Ridge. The name of this basin also is

one with which we are very familiar,

the Atlantic Ocean. The young Atlantic

probably was a very narrow rift,

similar to the modern Red Sea between

Africa and Saudi Arabia. Widening of

the Atlantic has continued to this day

and the rocks of the Appalachians,

including Old Rag Granite, have

passively ridden westward on the

eastern edge of this new ocean,

without undergoing any additional

tectonic processes except for erosion and

possibly gentle subsidence and uplift.

The rocks of the Appalachians probably

have been at the earth's surface for the

last 200 million years, since the breakup

of Pangea and the opening of the

Atlantic Ocean. Erosion has carved the

topography of the present-day Blue

Ridge province, carrying sediments east

to the Atlantic and south to the Gulf of

Mexico. Less resistant, soft rocks like

limestone are removed to form lowlands

like the Shenandoah Valley to the west

of the Blue Ridge. Hard, resistant rocks

like the Catoctin and Weverton

formations, and the Old Rag Granite, are

less susceptible to weathering and

erosion and stand tall as mountains.

Perhaps, hundreds of millions of years

from now, the highlands of the Blue

Ridge province will be gone, flattened

by erosion. And eventually, long after

we are gone, tectonic processes may

again create a tremendous mountain

range here.

A geologist trying to gain an

understanding of the earth's history is a

bit like a detective trying to solve a

mystery. To decipher geologic history, a

geologist must examine different rocks

at various places. To crack the case, a

detective must search far and wide for

different clues. Bits and pieces of the

last one billion years of earth history are

hidden in the rocks of Old Rag

Mountain but to know the whole story of

the Blue Ridge you will have to hike

many more trails over many other rocks!

For the reader who would like to know

more about the geologic history of

Shenandoah National Park and the Old

Rag Granite, a general list of suggested

reading material is provided at the end

of this presentation.

Dense summer foliage on Old Rag

Y our Visit to Old R ag Mountain

A visit to Old Rag Mountain is certain to

be a rewarding experience, no matter the

weather or time of year you choose to

visit. The different seasons offer

splendid vistas, each tinged with its own

distinct character. Summer brings a

warm, humid fullness to the mountain

views, and the enchanting sounds of

songbirds resonate throughout the

hollows. With the arrival of Fall comes

the rich color of the changing foliage

and a crisp edge to the mountain air. The

dead of Winter is fascinating in its own

right, as absolute quiet wreaths the

scenery. A light dusting of snow accents

the landforms and winter storms deck

the trees with ice. Spring brings the

invigorating smells of budding life and

the rich pink blossoms of mountain

laurel color the landscape.

The weather will play an important role

in your hike. A clear day will amaze you

with the vibrant colors transmitted from

the distant vistas of surrounding

mountains and valleys. A humid day will

remind you of how the Blue Ridge

acquired its name, as you gaze toward

mountain ridges framed in a misty haze.

Visits to Old Rag Mountain in rain, fog,

or snow offer a unique experience in the

outdoors, but you must be extremely

cautious. Exposure to the elements is of

4
Parking area along route 600

Ranger booth

A Walking T our of Old R ag Mountain

serious concern on the rugged trail, and

all precautions must be taken to ensure

your safety in poor weather. Electrical

storms occur in all seasons and

lightning strikes are frequent on the

upper part of the mountain. Snow and

ice are very slippery underfoot and a

bad fall can leave you suddenly

stranded in a hostile environment. Be

sure to dress appropriately for the

weather, bring water, and tell someone

where you are going if you plan to hike

alone.

Perhaps the best time to visit Old Rag is

in the winter, when you can gaze

through the bare woods and see the lay

of the land. Experiencing the stillness,

solitude, and rugged inhospitality of the

winter landscape is inspiring and instills

in one a great respect and reverence for

the mountain. A weekday in January

may offer complete seclusion for those

hikers who most enjoy the beauty of the

outdoors away from the crowds. On the

other hand, if you are visiting on a

weekend during the peak of October

foliage or on a beautiful summer day,

expect to wait in line on the outcrops

above. Nevertheless, the wait is well

worth the views and, in addition, offers

you the opportunity to converse with

like-minded nature-lovers.

Old Rag Mountain is easily accessed by

Virginia State Route 600, a two-lane

paved road that leads west to the

mountain from outside of the Park to

the east. The tremendous popularity of

the Old Rag ridge trail has necessitated

the establishment of an overflow

parking area about 0.8 miles before the

original trailhead at the end of Route

600. On a crowded weekend, a park

ranger will greet you here at the

entrance booth of the lower parking

area, where you will register for the

trail and pay a small fee. The National

Park Service has established an honor

system by which you can register and

pay the hiking fee on days when the

booth is not occupied. Your

responsibilities as a voluntary

environmental advocate for the Park

begin here. Hiking fees are used to

maintain the trails and Park facilities,

and for preservation of historic

structures. Once registered, you may

continue up Route 600 to the end of the

road and the beginning of the trail. The

road ends at the trailhead and from

there the ridge trail leads up the

mountain on an exhilarating journey of

discovery.

Camping is not allowed along the Old

Rag Mountain ridge trail or summit

area. You may camp below the Byrd's

Nest shelter on the far side of the

mountain but please be sure to follow

the rules for backcountry camping. Stay

out of sight of the trail and do not build

a fire. Use your common sense to make

Old Rag Mountain a safe and pleasant

place to visit overnight.

The remainder of this presentation

provides a walking tour of geological,

ecological, and cultural features present

along the Old Rag Mountain ridge trail.

As you hike, stops along the trail will

help you to discover the majesty and

splendor of the rocks that underlie the

mountain. The walking tour includes

landmarks to help guide you along your

way and stops are keyed to numbers on

the accompanying topographic map of

the mountain and trail. All of the

mileages in the walking tour begin from

the lower parking area. Blue blazes

painted on the trees and rocks help to

keep you on the trail. Double blazes

indicate you are approaching a

switchback or trail intersection. Your

roundtrip hike will be almost 9.7 miles

from the lower parking area and about

7.9 miles from the trailhead parking

area. You will climb up and down a

total of about 4800 vertical feet. Good

luck on your journey!

You begin your hike in dense hardwood

forest of oaks, hickories, and maples on

the lower slopes of the mountain. This

area, as is true for most of the hollows

in the Park, once was farmed, and corn

grew here instead of trees. The present

forest has grown only since

Shenandoah National Park was

established in 1935. Scattered Eastern

Hemlock and Eastern White Pine add

green to the forest canopy in the winter,

and cast the woods in dense shadow in

the summer. The forest in Shenandoah

National Park is slowly changing, as

disease, insects, and fierce winter

storms have killed many trees. As you

travel up and over the mountain you

will see many recently fallen limbs as

5
Old Rag trailhead

parking Old Rag

parking Stop 1

Stop 2

Stop 3

Stop 5 Stop 4

Stop 7 Stop 6

Stop 8

Stop 9

Stop 10

Stop 11

Stop 12

N

1 mile

Map of Old Rag Mountain. This diagram shows a portion of a topographic

map made by the U.S. Geological Survey. The brown lines are called contour

lines and delineate a constant elevation. In other words, if you were to walk

along a contour, you would never go up or down. The contour interval (the

difference in height above sea level of adjacent contours) is 40 feet on this

map. Notice how close together the contour lines are on the steep

sides of Old Rag Mountain! The green areas on the map are wooded;

the white areas are cleared fields or rocks. The boundary shown by a

tan line with a dashed black line is the Shenandoah National Park

boundary. Stay on the trail please!

Map Key

structure or parking area

stop along the trail

trail

dirt road

6
Stop 1. Boulder alongside the ridge trail

Please stay on the trail!

Following the blazes will help!

well as many standing dead trees.

Should you be backcountry camping

near Old Rag Mountain on a windy

night, be sure to sleep away

fromstanding dead timber as these

trees could easily topple in a strong

gust.

Stop 1 - (1.1 miles) Shortly beyond

the trailhead you will come upon a

tremendous rounded boulder to the

right of the trail. This boulder is in a

large fan of debris that came to rest

here following a tremendous landslide

thousands of years ago. The rock may

now be creeping downhill at an

imperceptibly slow rate because of a

process called frost heave. As water in

the soil below the boulder freezes into

ice and expands, the rock is forced

slightly upward. When the ice thaws

and melts, the boulder settles back to

earth but slightly downhill due to the

force of gravity. This process not only

moves boulders slowly downward, it

helps to break them off from the cliffs

above. As water filters into cracks in

the rock and then freezes, the

expanding ice forces the crack wider,

eventually splitting the rock and

sending it down slope. This process is

continually occurring on all of Old

Rag's slopes.

Be on the lookout for wildlife.

Shenandoah has an abundance of

mammals and here on the lower

slopes of the mountain you may see

black bears, white-tailed deer, red and

gray foxes, striped and spotted

skunks, bobcats, groundhogs,

chipmunks, and squirrels. Don't be

frightened by the possibility of seeing

bears - usually a bear will run as soon

as it smells you. Although aggressive

bears are rare, if one confronts you,

the best thing to do is face the bear

and slowly back away. Wildlife is best

treated by observing quietly from a

distance.

Stop 2 - (1.6 miles) Approximately

1.6 miles into your hike you will

reach the first switchback on the ridge

trail. This zigzag allows the trail to

maintain a gentle gradient instead of

going straight up the mountainside.

There are several reasons for building

the trail this way. Obviously, it makes

the hike easier on you. In addition, it

reduces the amount of erosion that

occurs on the trail surface during

storm events. The shallow ditches that

cross diagonal to the trail also were

built to reduce erosion by funneling

runoff of storm water off the trail.

Switchbacks present an enticing

opportunity to shortcut but please

resist the temptation. Shortcutting

damages the fragile plants, mosses,

and lichens that grow alongside the

trail and increases erosion of the

mountain. In addition, many hikers,

especially children, have become lost

in the woods by taking what they

thought was an easy switchback

shortcut. The Park spends thousands

of your tax dollars each year looking

for lost hikers, money which could be

spent on maintenance of the trail and

other park facilities. Please stay on the

trail and keep your children with you!

As you continue past another

switchback you will see many

tremendous boulders down slope to

your right. The reason for the

abundant boulders here is that you are

walking up the west side of a hollow

topped by rock outcrop on the

mountain above. As the boulders are

eroded from the mountain's summit

region they are channeled into this

hollow similar to water flowing down

a stream. Boulder streams such as this

are common in the Blue Ridge and

were probably initiated by the

voluminous snow and ice that covered

the landscape here at the end of the

last Ice Age about 10,000 years ago.

You may notice a slight change in the

forest vegetation as you continue your

ascent. The understory begins to

contain more mountain laurel, a thick,

bushy evergreen. In the spring and

7
early summer, these bushes have

beautiful pink blossoms that fill the air

with fragrance and color the landscape.

Farther upslope you will find the trail

weaving through mountain laurel so

dense it forms a sort of tunnel. In

addition, the forest canopy becomes

increasingly dominated by evergreens

and you see fewer and fewer oak,

hickory, and other broadleaf trees.

Stop 3 - (2.6 miles) A spring! As you

head up the north face of the ridge, the

trail crosses over water trickling from a

spring several tens of yards upslope

from the trail. By now you have passed

several switchbacks and are at an

elevation of over 2000 feet. Water from

this spring comes from rain that fell

upon the upper slopes of the mountain

where it passed through the thin soil

and followed joints and fractures in the

bedrock to come out at this spot.

Although this is a mountain spring it is

by no means safe to drink without

purification! Giardia bacteria in the

water can make you very ill. However,

the water is very cold and extremely

refreshing to splash on your face on a

hot summer's day. Be careful though,

as stinging nettles grow here in the wet

soil around the spring, identified by

their sharply toothed leaves.

Stop 4 - (3.3 miles) Not long after the

spring the trail becomes significantly

steeper and rockier. You are about to

come out of the woods and onto the

rocks! From this point on to the

summit, the trail is almost continuously

on rocks. At 3.3 miles into your hike

you reach a fantastic view to the

northwest. An Eastern White Pine

grows from beneath the outcrop here,

partially blocking your view of the

mountain scenery. A close look at this

tree and others in the area illustrates

the severity of the winds at this

elevation. The pine tree's branches

grow away from the prevailing wind

direction. This general area is a very

good place to take a break and have a

close look at the Old Rag Granite. Get

down on your hands and knees in a

place where hiker's boots have not

stained or worn the rock smooth and

look very closely at the rock's coarsegrained minerals. Be careful not to get

too close to any of the precipitous cliffs

on all sides. You will notice two kinds

of minerals immediately - a white

mineral that is frequently rectangular

or square called alkali feldspar, and a

blue or gray mineral called quartz. The

rectangular or square shape of alkali

feldspar is called euhedral crystal form.

The large size and perfect crystal form

of this mineral indicates that conditions

were ideal for its growth in the slowly

cooling Old Rag magma. The blue

color of the quartz is only found in

rocks of similar age to Old Rag Granite

and is thought to result from inclusion

of minute crystals of titanium-rich

minerals called ilmenite and rutile. You

may notice that the quartz appears to

be strung out into thin bands or

"stringers." This structure is called

foliation and its orientation is used to

determine the direction of applied

tectonic stresses that deformed Old

Rag Granite in the ancient past.

Continued close examination of the

rock's surface will reveal the presence

of a red mineral called garnet, a semiprecious gemstone. Once you have

spotted garnet the first time, you will

begin to notice it everywhere, as it

makes up about one percent of Old Rag

Granite all over the mountain.

Stop 5 - (3.4 miles) After crossing over

several hundred feet of rock the trail

descends into a narrow crevice about 8

feet deep. This crevice, which goes

right through the mountain, results

from preferential weathering and

erosion of a rock called diabase that is

less resistant relative to the adjacent,

harder and more resistant Old Rag

Granite. The narrow fissure is called a

dike and was a vertical conduit for the

diabase magma. When you are down

inside the crevice, notice that the rock

on the floor appears very different than

the Old Rag Granite. If you take a

close look at the diabase or "traprock"

you will notice that, unlike Old Rag

Granite, it is impossible to identify

individual mineral grains. This is

because the diabase dike intruded Old

Rag Granite very close to the surface

of the earth, cooled very quickly, and

did not have time to grow large mineral

crystals.

As you proceed through the fissure

toward the north face of the mountain,

keep your eyes on the right-hand wall.

At the northern end of the crevice there

is another dike cutting the Old Rag

Granite at about eye-level. This dike

also is very fine-grained compared to

Old Rag Granite and is about 2 feet

thick. It is called biotite granite. You

may be able to identify alkali feldspar

and quartz in this rock just as you can

Stop 4. Windblown Eastern White Pine

Stop 4. Euhedral alkali feldspar

Stop 3. Stinging nettles

8
g Granite, but the minerals 

maller in size. The magma 

d this rock probably cut 

d Rag Granite soon after 

g magma had cooled and 

nto rock. Geologists think 

a for this dike was "left 

m the same magma that 

d into Old Rag Granite. If 

y y g

shiny black mineral called biotite 

mica. This mineral is significantly 

different from other minerals found in 

the rocks of Old Rag Mountain in that 

it contains water in its atomic 

structure.




Stop 6 - (3.5 miles) Here the trail 

goes through a short "cave." As you 

can tell, this is not really a cave, 

rather a tremendous crack in the Old 

Rag Granite. The crack occurred due 

to a physical process called unloading 

or exfoliation. As erosion of surface 

materials slowly exposes Old Rag 

Granite, the confining pressure is 

reduced and eventually the rock 

splits, resulting in cracks oriented 

perpendicular to the direction of 

pressure release and parallel to the 

exposed rock surface. This process is 

identical to that which produces 

deadly rock bursts in newly excavated 

mine shafts and tunnels.




Keep your eyes glued to the trail and 

your hands out of your pockets as you 

hike. There are two very good reasons 

for this. First of all, with your eyes on 

the trail you are less apt to stumble 

and, with your hands free, you are 

better able to arrest your fall when 

you do. Secondly, as most of the trail 

at this elevation is over rock, you will 

g , , 

able to pick them up for a closer 

glance (But no collecting please!). 

Most of the rock is, of course, Old 

Rag Granite, but you will also see 

boulders, chips, and pebbles of 

diabase. If you are very observant you 

may occasionally notice a rock that 

has coarse-grained minerals like Old 

Rag Granite but is much darker in 

color, almost a dusky-blue compared 

to the rather whitish color of Old Rag 

Granite. This rock is called 

charnockite and is found in several 

dikes similar in size and shape to the 

diabase and biotite granite dikes you 

have seen. The rock to the left of the 

hiker in the photograph to the right is 

charnockite. Geologists think that this 

rock is related to Old Rag Granite by 

a process that occurs in magmas 

called fractionation. Fractionation 

describes how crystallization of 

minerals from magma changes the 

composition of leftover magma by 

removing certain elements which 

form the crystallizing minerals. 

Ultimately, this process results in the 

formation of very different rocks - 

literally, a succession of rocks with 

different minerals that have formed 

from a common "parent" magma.

Stop 7 - (3.6 miles) Here you will 

find a natural staircase developed on 

the surface of a weathered diabase 

dike. The individual steps in the 

staircase are called columnar joints. 

These joints developed in the diabase 

as it cooled and contracted. The

ts in Old Rag Granite

Stop 5. Biotite granite dike cutting Old 

Rag Granite

Stop 6. "Cave" entrance. Charnockite

is to the left of the hiker.

base dike crevice 
rosion. This location is 

pot to observe the 

een the coarse-grained 

ite and the fine-grained 

As explained earlier, the 

the size of the minerals 

ks results from slow 

d Rag Granite at great 

ick cooling of diabase 

rface of the earth. The 

olor results from the 

ifferent chemical 

of the two rocks. Old 

ontains much more 

ure form commonly 

rtz, and potassium, an 

Old Rag Granite, contained in 

minerals called plagioclase feldspar 

and pyroxene. These differences in 

mineralogical and chemical 

composition can be attributed to the 

sources of the different rocks. Old Rag 

Granite was derived by melting of 

crustal rocks: low-density continental 

rocks rich in silica that "float" upon 

the earth's mantle. The diabase was 

derived by direct intrusion of mantlederived magma that welled up from 

the earth's molten interior.




Stop 8 - (4.0 miles) At this point you 

will find several tremendous rounded 

boulders along the trail. These 

boulders have weathered in place by 

the forces of wind and water at the 

mountain's summit. As mentioned 

earlier, the change in confining 

pressure causes planar fractures or 

joints to develop in the rock when it is 

exposed at the surface. Weathering by 

wind and water rounded the corners 

and edges of planar fractures to 

eventually produce the spherical shape 

of the boulders you see here.




Take a close look at the abundant 

lichens growing on the rocks 

alongside the trail. These curious 

"plants" are actually algae living 

inside a fungus. Lichens grow very 

slowly and may live as long as 4500 

years! The tiny plants secrete 

soil on Old Rag Mountain.




Stop 9 - (4.3 miles) At the mountain's 

summit (approximately 3300 feet 

above sea level) you will be treated to 

wonderful views of the Blue Ridge to 

the north and west if the weather is 

clear. Due west is the rounded peak of 

Hawksbill, 5 miles away. At 4049 feet, 

Hawksbill Mountain is the highest 

point in Shenandoah National Park. To 

the northwest, across Weakley Hollow, 

is Robertson Mountain. The summit 

elevation of Robertson Mountain is 

3296 feet, about the elevation of the 

Old Rag Mountain summit. 

Examination of the topographic trail 

map included in this presentation 

indicates an elevation of 3268 feet 

labeled near the summit of Old Rag 

Mountain. This is the top of the large 

outcrop of Old Rag Granite just to the 

west and a little below the mountain's 

summit.

al staircase of columnar 

se

Stop 8. Spheroidally weathered boulders

This series of diagrams illustrates how 

the processes of weathering gradually 

create the rounded boulders found on 

Old Rag Mountain's summit. The top 

diagram depicts freshly exposed rock 

cut by planar fractures. As water, wind, 

and organisms such as lichens wear at 

the rock, edges and corners are worn

down to eventually produce spheroidal 

boulders and a thin soil cover. 
Stop 9. Blue quartz chips in the soil Stop 9. Opferkessel

On a clear day you will be able to see

the sun glint off cars traveling along

Skyline Drive to the west, and in the

winter you will be able to see the

Weakley Hollow fire road in the

valley below. In the summer, the thick

foliage on the surrounding mountains

looks almost like a soft shaggy rug,

inviting one to imagine that maybe it

would be possible to just step off and

leap from mountain to mountain like a

giant. On the other hand, you may find

that your vista over the surrounding

mountains and countryside is severely

limited by a thick haze. One of the

worst environmental problems facing

Shenandoah National Park is that of

atmospheric pollution. Airborne

particulates and sulfur dioxide, the

cause of acid rain, may dramatically

decrease the visibility from the

mountain's summit.

While you are at the summit area, take

a close look at the thin soil that has

developed on the Old Rag Granite.

Get down on your hands and knees to

get a look at the chips of blue quartz

that are readily visible. The abundance

of quartz chips illustrates the relative

resistance of this mineral to chemical

and physical weathering in the surface

environment. In other words, quartz

does not easily react with water and

also is very hard and thus resistant to

breakdown by frost action and other

physical processes. Rocks of the Swift

Run and Fauquier Formations are

partially composed of quartz chips and

pebbles eroded from the Old Rag

Granite over 600 million years ago!

Small circular pits or depressions in

the Old Rag Granite are called

opferkessels and are created by

weathering and solution of the rock by

standing water. Geologists think that

solution of the very resistant Old Rag

Granite is initiated by organic acids

derived from the decay of evergreen

trees that populate the mountain's

summit.

The Old Rag saddle trail leads down

from the summit area into Weakley

Hollow. The short pines growing on

this side of the mountain are called

Table Mountain pines or "Hickory

pines." The spiny cones of this pine

grow in clusters and remain attached

to the tree for many years. Table

Mountain pines are unique to the

Appalachian Mountains and only

grow at higher elevations from

Georgia to New Jersey.

Stop 10 - (4.9 miles) The Byrd's Nest

shelter was built by the Civilian

Conservation Corps (CCC) in 1934

using rocks found on Old Rag

Mountain including Old Rag Granite,

charnockite, and diabase. Flagstones

used in the front of the shelter are a

type of rock called quartzite and were

hauled here from elsewhere. The

Byrd's Nest is meant to be used as a

place to take a break or shelter in bad

weather and no camping is allowed

here.

Stop 11 - (5.2 miles) These steps also

were built by the CCC, using rocks

found scattered about the

mountainside. The saddle trail used to

be an old, narrow logging trail and

these steps are testament to the hard,

back-breaking work the Corps

accomplished in establishing over 500

miles of trails within the Park. The

Potomac Appalachian Trail Club

(PATC) now is responsible for

maintenance of some trails in the Park

and all work done here by this

organization is, amazingly, on a

volunteer basis. If you should happen

upon a PATC volunteer during your

hike, make sure to give them a hearty

thanks and a pat on the back for doing

such a good job.

Table mountain pine

Stop 10. The Byrd's Nest

11
Stop 11. Stone staircase on the

saddle trail

Stop 12. Information board at trail

intersection

Stop 12 - (6.3 miles) Eventually your

hike brings you to the intersection of

the Old Rag saddle trail, Old Rag road,

and the Berry Hollow and Weakley

Hollow roads. Be careful here - this is

where many hikers make the wrong

turn and add many more miles to their

Old Rag Mountain hike. Turn right

onto the Old Rag road, go about 75

feet and bear right onto the Weakley

Hollow road. The Weakley Hollow

road leads back to the Old Rag parking

areas.

The village of Old Rag, named after

Old Rag Mountain (named so because

of the irregular outline and unusually

rocky nature of the ridge), used to lie

here at this cross roads on the western

flank of the mountain. This small

village was an industrious

establishment in its time, and the site

of two stores, two churches, a school,

a post office, and a number of houses.

The road that ran through the hollow

initially was established around 1750.

The town, and its inhabitants and

structures, were removed when

Shenandoah National Park was

established in 1935. As you meander

down the fire road you will see many

small piles of rocks to either side,

remnants of chimneys, foundations,

and fences - all that remains of the

many structures that once dotted

Weakley Hollow. In the spring you

may see a few daffodils growing in the

ruins - scarce vestiges of the hardy

mountain people who once lived in

these hollows.

The gentle walk through Weakley

Hollow is a pleasant way to wind

down after the vigorous hike over the

rocky summit of Old Rag Mountain. It

is by now late in the afternoon and

shadows are growing a little longer. If

your hike has been in hot humid

weather, a cooling late-day summer

shower may pleasantly surprise you. If

it's cold and the trees are bare, your

walk back to civilization will include

many wonderful views of the

mountain you have just clambered up

and over. The walk through Weakley

Hollow will take only another hour or

so more, depending on your pace, so

enjoy the little time you have left in

Shenandoah National Park. Sit down

and be very quiet for a while - you

may be startled by all the sounds that

suddenly seem much louder than

before. Keep your eyes and ears

peeled for wildlife. You may be

surprised by the sudden explosion of a

grouse taking flight or the staccato

knock of a busy woodpecker. Many

slithering creatures are found on the

ground, including poisonous

rattlesnakes and copperheads. Don't be

surprised to see a wild turkey strutting

boldly down the road - after all, these

woods are full of wild creatures.

Thanks for visiting Old Rag Mountain.

The Park hopes that your hike has

been safe, exciting, a learning

experience, and above all, a journey

that you will make time and time

again.

12
o continues hiking the mountain with me today.

L., 1999, Geology along Skyline Drive, Shenandoah National Park, 	

: Falcon Publishing, Helena, Montana, 231 p.

, and Engle, R., 1997, Shenandoah: The story behind the scenery, 	

lications, Las Vegas, Nevada, 48 p.

W., 1990, Circuit hikes in Shenandoah National Park: Potomac 	

chian Trail Club, Vienna, Virginia, 106 p. 

. M., II, 1976, Geology of the Shenandoah National Park, Virginia: 	

 Division of Mineral Resources Bulletin 86, 93 p.

H., 1988, Guide to Shenandoah National Park and Skyline Drive: 	

doah Natural History Association Bulletin 9: Luray, Virginia, 		

ision of Mineral Resources, 1993, Geologic Map of Virginia: 	

 Division of Mineral Resources, scale 1:500,000.

aefer, K., 1994, Hiking Virginia's National Forests: Globe Pequot 	

Old Saybrook, Connecticut, 191 p.

Suggested Reading

By Paul Hackley, U.S. Geological Survey, Reston, VA 20192

U.S. Geological Survey Open-File Report 00-263