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Prepared in cooperation with the National Park Service 

Geological Survey Water-Supply Paper 1766
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY

Thomas B. Nolan, Director

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1965

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 25 cents (paper cover)
CONTENTS

Acknowledgments ........

The pool by Inscription Rock ....

Where the water is stored: the geology of

El Morro

El Morro's water supp

For more information about

y

Page

V

VI

10

26

30

in
FIGURES

Page

1. Inscription Rock, El Morro National Monument, New Mexico ........... 2

2. The pool at El Morro as it is today .... 2

3. Facsimile of Simpson-Kern inscription ... 3

4. Prehistoric Indian village on top of Inscription Rock ............... 5

5. Index map of northwestern New Mexico . . 6

6. The process of mountain building and

erosion ................ 12

7. Tilted layers of rock at El Morro . ..... 15

8. Division of geologic time ......... 16

9. Volcanic cinder cone 14 miles east of El

Morro ................ 24
Acknowledgments

This report was prepared in response to a request from 

the National Park Service for assistance from the U.S. 

Geological Survey in appraising possible sources of water 

supply for El Morro National Monument. The fieldwork was done intermittently from July 1959 to April 1, 

1961. Mr. John Kell, Regional Chief of Lands, 01 Mr. 

Bill Fields, engineer, National Park Service, accompanied 

the senior author during the well tests. Mr. Irving McNeil, Jr., superintendent, El Morro National Monument, 

other members of his staff, and land owners in the area 

provided valuable information on wells in the vicinity of 

the monument.
^iW^^ift " _^- ^ *^*fc ^3^
In the land of enchantment, between Gallup and Grants, 

N. Mex., near the Zuni Mountains, a huge sandstone bluff rises 

abruptly 200 feet above the plain. The Spaniards called it "El 

Morro," which means "the headland" or "bluff." Around it are 

other mesas and canyons and stands of pinon and ponderosa pine. 

Other great rocks are nearby, but none are as popular as El Morro, 

and none have been as important to the traveler.

For at El Morro there is water.

In that country, water is scarce and precious. In the old days, 

travelers from Santa Fe would tell each other about the pool of 

clear, refreshing water at the base of the huge rock. This is the 

story of the great bluff, its water supply, and the rocks around it.

In the late summer of 1849, an American lieutenant of the 

Topographical Engineers, James H. Simpson, accompanied infantry and artillery troops on a reconnaissance march from Santa 

Fe into the Navajo Country. On September 18, at the urging of 

one Mr. Lewis, an Indian trader, Lieutenant Simpson left the 

main party in order to see "half an acre of inscriptions" x upon a 

huge rock (fig. 1). Although somewhat dubious, the Lieutenant 

had allowed himself to be persuaded by Lewis that the trip was 

worthwhile. Taking with him an artist named R. H. Kern, another man by the name of Bird, and Mr. Lewis as guide, he set off 

through miles of desert country, filled with huge red and white 

sandstone rocks, "some of them looking like steamboats, and others 

presenting very much the appearance of facades of heavy Egyptian 

architecture." 2

As they drew near the huge mass of rock, they dismounted and 

went up close. There were the inscriptions. "Perhaps not half 

an acre," wrote Lieutenant Simpson, cautiously. But there were 

enough to be an exciting find: "One of them dating as far back 

as 1606, all of them very ancient, and several of them very deeply 

as well as beautifully engraven." 3 Simpson tried to remain objective, but excitement and interest kept breaking through. He 

knew he had found something of enormous interest to historians 

and to Americans generally.

He and his friends decided to stay overnight, in order to copy 

the inscriptions. Simpson knew that without facsimiles their 

report might not be believed just as he himself had been doubtful 

of Lewis' story.

1 Simpson, 1852, p. 98. a Op. cit., p. 99.

3 Simpson, 1852, p. 99. Some inscriptions date further back than 1606.

1
Figure 2. The pool at El Morro as it is today. National Park 

Service photograph, by Jack E. Boucher.
 4 Figure 1. Inscription Rock, El Morro National Monument,

New Mexico.

Looking for water and a place to camp, they rounded a corner 

of the rock and came upon the pool, "canopied by some magnificent rocks, and shaded by a few pine trees, the whole forming an 

exquisite picture." 4 Kern has left us a charming sketch of it. 

This lonely little scene had the wild and primitive beauty which 

Europeans looked for in the novels of Fenimore Cooper, and 

which Americans were learning to love and recognize as their own.

Lieutenant Simpson thought the water came from springs, but 

actually the pool is fed by rains and melted snow. After improvements, it is 12 feet deep when it is full, and holds about 200,000 

gallons of water (fig. 2).

Lieutenant Simpson was the first American official to visit El 

Morro. Although it is a military report, his account is delightful 

to read, full of personal touches and expressions of a sense of 

beauty and wonder not characteristic of official reports. With 

Bird and Kern, he copied all the inscriptions they could find, 

carefully noting that there was not a single inscription in English 

upon the rock. Then they too left their names inscribed on the 

rock (fig. 3), and set off to rejoin the troops and tell the story of 

their discovery.

4 Simpson, 1852, p. 100.

Figure 3. Facsimile of Simpson-Kern inscription.

712-880 O-65-2
From that day to this, Americans have been visiting El Morro. 

A few years after Simpson's visit to El Morro, a Captain Sitgreaves 

passed by the rock on an expedition down the Zuni River. It 

is briefly mentioned by his surgeon, Dr. Woodhouse, in a description of the natural history of the region.5

In 1858 Edward F. Beale, in the course of a survey of the 

wagon road from Fort Defiance to the Colorado River, stopped 

briefly at the rock:

When at 9 a.m., we reached Inscription Rock, I was tired of exclaiming, as every hundred yards opened up some new valley, "how 

beautiful." The rock itself seems to be a centre from which radiates 

valleys in all directions, and of marvelous beauty. It rises grandly 

from the valley, and the tall pines growing at its base give out long 

before they reach the top of its precipitous face. Inscriptions, names 

and hieroglyphics cover the base, and among the names are those 

of the adventurous and brave Spaniards who first penetrated and 

explored this country * * * the rock is some three or four hundred 

feet in height, and the spring almost hidden in the cavity of it * * * 6

Beale's trip was especially interesting, because he commanded 

a caravan of camels that were imported as an experiment in 

transportation for the arid Southwest. However, the railway 

train soon made its appearance and put an end to camel trains.

Long before the Spaniards arrived, the Zuni Indians had 

constructed a village on top of the headland, and carved a path 

on the face of the cliff between the village and the waterhole. 

Ruins of this village, fragments of pottery, and interesting carvings on the face of the cliff gave one a glimpse into the lives of 

these prehistoric inhabitants (fig. 4).

For nearly 200 years, El Morro was the principal wateringpoint for Spanish travelers between the villages of Acoma and 

Zuni. Here they found shelter from the sun and from storms, 

and at the pool, fresh water for their horses, and for their own 

needs.

The first written record of El Morro dates from 1583. A 

Spanish visitor, Diego Perez de Luxan, mentions in his journal, 

"El estanque del penol," "The Pool by the Great Rock." 7

5 Sitgreaves, 1853, p. 35.

8 Beale, 1858, p. 85.

7 El Morro National Monument: U.S. Dept. Interior Pamphlet.
The earliest inscription by the Spaniards was that of Don Juan 

de Onate in April, 1605, who wrote, "Paso por aqui el Adelantado Don Juan de Onate del descubrimiento de la mar del sur 

a 16 de Abril de 1605." 8 (Passed by here the Governor Don

Figure 4. Prehistoric Indian village on top of Inscription Rock.

Juan de Onate, from the discovery of the Sea of the South, on 

April 16, 1605). It is impressive to think that Onate's name 

was inscribed on the rock 15 years before the pilgrims landed at 

Plymouth! We do not know whether other explorers preceded 

him and left no record inscribed on the rock. Many other 

Spanish travelers who visited the waterhole between Onate's 

visit in 1605 and the last recorded Spanish visit in 1774, left inscriptions on the sandstone cliff.

8 El Morro Trails: Southwestern Monuments Association, Globe, Ariz., 

p. 4-5.


Because fresh water was so important to all travelers in this 

area, the trail past El Morro became the principal road across 

the region. Names of wagon-train emigrants, traders, Indian 

agents, soldiers, surveyors and settlers, were inscribed on the rock 

in the years following 1850 (fig. 5).

After the transcontinental railroad and U.S. Highway 66 were 

constructed 25 miles to the north, the road by the watering place 

lost its prominence and became little used for a time. Improvement of the road past El Morro (State Highway 53) was begun 

in 1955, and by 1962 construction was almost complete.

El Morro National Monument was established by Presidential 

proclamation in 1906 to preserve the rock carvings and the pool 

of water that had contributed so much to the exploration and 

development of the Southwest. There were always some visitors 

to the monument, but after the improvement of the road, they 

began to come in greater numbers. In 1961, nearly 20,000 

tourists visited El Morro. Ironically, the pool of fresh water 

that gave prominence to El Morro for 300 years is now inadequate 

to supply the large numbers of visitors, tourists, and the monument staff. Eventually, most of the water supply had to be 

hauled by truck from San Rafael, 38 miles northeast of the monument, and later from a well 3 miles to the south. Although its 

charm remains, Lieutenant Simpson's "exquisite" pool has outlived its usefulness as a water supply.

The waterhole, gouged by rain and snowmelt that cascaded 

off Inscription Rock through the centuries, is protected from the 

bright sunlight of this semiarid region. Precipitation in the area 

averages about 12 inches a year, according to records of the 

U.S. Weather Bureau. Streams do not exist in the area, except 

for a short spring-fed stream in Water Canyon, 6 to 7 miles northeast of the monument. Wells are scattered widely, and failures 

have been common.

It is often thought that this region was at one time green and 

verdant and that the present desertlike appearance is the result 

of overgrazing. However, the accounts of the expeditions of 

Simpson and Beale and those by other trustworthy travelers reveal that, although grass did grow well in certain areas, "there 

were also many areas * * * where grass was so poor that forage 

for a string of horses could hardly be obtained." 9 Rainfall tends

'Leopold, 1951, p. 295.
to run off the land or evaporate very quickly where there is not 

sufficient plant life to catch and hold it. Some of it, of course, 

does sink into the soil and eventually seep into the rocks under the 

soil cover. There, under the earth's surface, is the source of 

more water for the monument what we call ground water.

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View eastward from top of Inscription Rock. 

The steep southwest slope of the Zuni 

Mountains forms the distant skyline; the 

valley plain is underlain by volcanic flow 

rocks.
El Morro National Monument lies in a wild and beautiful 

region of sheer cliffs, high mesas, steep mountains, and narrow 

valleys and canyons. The altitude at the monument headquarters 

is 7,200 feet. North of El Morro, the forest-covered slopes of the 

Zuni Mountains rise higher, to altitudes of 8,000 or 9,000 feet. 

Many streams in the Zuni Mountains have carved deep canyons 

in the slopes. Unfortunately, none of these streams cross the lava 

plain to run near the monument because all the water sinks into 

the lava very quickly.

The inscriptions and Indian relics at El Morro are young in 

comparison with the rocks. The history they record is only an 

infinitesmal part of the millions of years of history that can be read 

in the rocks of the region. Through the billions of years of its 

history, the appearance of the earth has changed many times. 

The mountains and valleys that we recognize did not always exist 

where they are now. The crust of the earth has been pushed and 

pulled by subcrustal forces, which made the crust sink in some 

places, and rise in others. At places it has been ripped by 

earthquakes.

In ages past, many of the rocks which now are mountains were 

formed at the bottom of the sea. We know this because fossil 

remains of sea animals are found in the rocks on mountain tops. 

Then powerful forces from within the earth folded and raised these 

rocks to their present position. As soon as the mountains rose 

above the sea, however, streams, wind, and glaciers began tearing 

them down. Billions of tons of sediment (minute particles of 

rock) were eroded from the land by rivers and added to the oceans 

or the stream valleys along the way. Some of the particles were 

whisked about by the winds and accumulated in dunes on the 

land. As the ocean floor piled up with sedimentary material, 

subcrustal forces gradually lifted the sediments out of the water 

until they became mountains, and the process of erosion started 

all over again (fig. 6).

These movements of the earth's crust the wearing down and 

the bulging up have taken place during many million years. 

At El Morro, some of the sedimentary rocks are almost 300 million 

years old. The sedimentary rocks lie on a complex system of 

igneous and metamorphic rocks, which are more than 600 million 

years old.

712-880 O-65-3 H
In Precambrian time layers of sediment accumulated and eventually

formed rock.

layers of rock were squeezed and crumpled into mountains. 

Molten rock from below moved upward through cracks and formed 

volcanoes.

During many millions of years the mountains were worn down by 

erosion, and a nearly level plain little higher than the sea was 

left.

12 Figure 6. The process of mountain building and erosion.
In Paleozoic and Mesozoic time, new layers of sediment accumulated in seas and streams and as sand dunes in broad expanses 

of deserts.

E

At the end of Mesozoic time the new layers of rocks were bowed 

upward above the seas in a broad arch that formed the Zuni 

Mountains.

In Cenozoic time, erosion sculptured the landscape as we see it 

today, modified slightly by new outpourings of lava.
The history of the earth can be read in these rocks as clearly as 

the writing on Inscription Rock. Sedimentary rock is made from 

materials that have been carried by water or wind to their place 

of rest and turned into rock. Rain and ice grind rocks into sand 

and mud, which settles into the bottoms of rivers, lakes, and seas. 

As this sand and dirt piles up into layers, it is squeezed and pressed 

into sedimentary rock. Some characteristic sedimentary rocks are 

sandstone, limestone (rocks made from sea shells), shale (rock 

made from clay and mud), and pudding stone or conglomerate 

(mass of different-sized pebbles cemented together).

The word igneous comes from the Latin word for fire. The 

inside of the earth contains molten rock, or magma. In some 

places, this magma is closer to the surface than in others, and the 

molten rocks push through crevices in the earth's crust. This 

rock, when solidified, is called igneous. Granite, basalt, pumice, 

and obsidian are igneous rocks. Nearly everyone knows what 

granite looks like. Basalt is hardened lava, and pumice is the 

hardened froth from volcanoes. Both basalt and pumice have 

bubbles or air holes in them. Obsidian looks like dark, heavy 

glass.

The word metamorphic comes from the Latin word meaning 

to change or be changed. Metamorphic rocks have been changed 

by heat and pressure from the way they were originally. They 

may have begun as igneous or sedimentary rocks, but heat, time, 

pressure, and sometimes water, have changed the rocks into other 

kinds. For instance, sandstone is metamorphosed into quartzite; 

limestone into marble; shale into slate.

All three of these main kinds of rock are present at El Morro. 

The layers of sedimentary rocks that once were horizontal, or 

nearly so, have been tilted from their original plane, as in the 

diagram. The layers of rock are tilted, as though bent by a giant 

hand (fig. 7). You will learn more about these rocks as you 

read further.

Geologists rarely think in terms of years or even hundreds of 

years; they usually reckon time in millions of years. To make their 

work easier, they have divided time into eras, and the eras into 

periods. Geologists tell us into which eras and periods the rocks 

belong, usually starting with the oldest and working up to modern 

times, as in other history books. If you will refer to figure 8 on 

geologic time divisions, as you read, the descriptions of the rocks 

will be easy to follow and understand.

14
AGE

CRETACEOUS

70-135 

million years ago

JURASSIC 

135-180 m. y. ago

TRIASSIC

180-225

million years ago

PERMIAN

225-270

million years ago

PENNSYLVANIAN 

270-310(?)m. y. ago

PRECAMBRIAN

FORMATION

Mesaverde Group pi I, gas, coal

Mancos Shale

Dakota Sandstone

pi Gray slopes and tan cliffs £Qf

 ' '.' ' .' ' ' ' '!." '.' .' .' -.' ! ' '.'." ''.' '. ' '>' ' '-'. . ? '' '.'-'.I

[LrGray slopes =^^f~2§iE^=~ ~^^

Zuni Sandstone

Wingate Sandstone

Chinle Formation 

(shale and sandstone)

San Andres Limestone

Glorieta Sandstone

Yeso Formation 

(shale, sandstone, 

limestone and gypsum)Abo Formation 

(shale and sandstone)

(shale and limestone) 

million years ago- 

(granite)

ndian ruins

:\'. White and red cliffs.):;:^;/;;:^^-'^:^''/-;;':/ ' ;\y

£=Red cliff s%^==S

Inscriptions Ground water in 

r rock openings

Basalt and alluvium 

/ (Quaternary age)

Figure 7. Tilted layers of rocks at El Morro.
I QUATERNARY and TERTIARY 

I CRETACEOUS 

I JURASSIC 

I TRIASSIC 

PERMIAN

| DEVON I AN 

SILURIAN

ORDOVICIAN Naytilokh

CAMBRIAN Trilobi.e:

400 500

MILLION?; OF

1OOO

_YEARS AGO.

Figure 8. Division of geologic time.

How do geologists know how old different rocks are, or how old 

the earth is? Until recently, they had to guess, basing their guesses 

on physical and biological evidence. Geologists know roughly 

how long it takes for layers of sediments to be laid down. Remains 

of animals and plants are found at many levels, and study of these 

fossils can be correlated with the evidence of the rocks themselves. 

Such dating, however, only establishes the sequence of events; it 

does not tell how old the earth's rocks actually are.

In the present century a fairly reliable method of age determination has been developed. This method depends on measurements 

of radioactive minerals in rocks. The atoms which compose 

uranium, thorium, and other radioactive minerals decay or break 

up into smaller atoms at a regular rate, and gradually end up as 

atoms of lead. As the years go by, a rock that contains uranium 

will have less uranium and more lead in it. By comparing the 

amounts of lead and uranium, scientists can tell how old the rock 

is. Use of the radioactive method of dating has shown that the 

earth is more than 3 billion years old.

In using the table of geologic time, one must remember that 

these divisions and subdivisions are more or less arbitrary. We 

use them for convenience in dealing with such long periods of time. 

The divisions are based on the time estimated for accumulation of 

sediments and the building of mountains. The passage of time

16
from one period to another was very long, and the changes which 

developed in the earth's surface were very gradual and overlapped 

from one period to another.

Ground water is the water stored in the rocks under the surface 

of the earth. If geologists know the kind of rock in a given area, 

they can usually tell approximately how much water the rocks are 

likely to yield. For this reason, all the rocks underlying the area 

have been carefully listed and described.

The person willing to leave the main roads may see all these 

rocks a short distance from El Morro.

Precambrian Rocks

The oldest rocks in the El Morro region are igneous and metamorphic rocks of the Precambrian Era. (See fig. 7.) These 

Precambrian rocks could be 600 million to 3 billion years old. 

They have had a turbulent history. They were squeezed, contorted, and torn, and great masses of magma and narrow bands 

of quartz from the interior of the earth were injected into them. 

Some of the magma spewed out on the surface as lava flows. The 

intense heat and pressure subsequently converted the sediments 

and lava into metamorphic rocks. Lofty mountains were built 

and then eroded away. No traces of primitive life are evident in 

these rocks, but that does not necessarily mean no life was ever 

present. Such traces may have been destroyed by the intense 

heat and pressure to which the rocks were subjected.

The bands of quartz are more resistant to erosion than the granitic rocks that enclose them, and several can be seen in low ridges 

along the road across the Zuni Mountains between Ice Cave and 

Grants. (See fig. 5.) They are easily discerned because of the 

large amount of white quartz fragments strewn over the ridges.

Rocks of Paleozoic Age

The Paleozoic Era of geologic time began about 510 million 

years ago and lasted about 330 million years. However, only a 

small part of this time is recorded by the rocks in the vicinity of 

El Morro. If rocks were deposited in the region during the first

17
300 million years of the Paleozoic Era, they must have been later 

completely removed by erosion, as there is no evidence of them 

now. However, rocks of one of the later subdivisions of the Paleozoic Era, the Pennsylvanian age, lie over the Precambrian rocks. 

(Seefig. 7.)

The Pennsylvanian rocks range in age from about 235 to about 

205 million years. The central interior of the United States was 

a vast lowland at the beginning of Pennsylvanian time, and part of 

the continent was submerged intermittently below the seas. During much of this time, El Morro region was a highland surrounded by seas, but the sea advanced temporarily into the area of 

the Zuni Mountains. We know this because marine fossils have 

been found in sedimentary beds of limestone and shale.

The brown and gray Pennsylvanian rocks are in areas of low 

relief bordering the central part of the Zuni Mountains on the 

southeast. The secondary road between Ice Cave and Grants 

(fig. 5) crosses some of these rocks about 10 miles north of Ice 

Cave. Pennsylvanian rocks do not exist beneath El Morro itself.

The latest subdivision of the Paleozoic Era is the Permian, 

which began about 205 million years ago and lasted roughly 25 

million years (fig. 7). The central interior of the United States 

was a vast basin during Permian time. El Morro region remained 

above the seas during all of Early Permian time, but eventually 

it too was drowned by an advancing sea.

Rocks of Permian age in this region have been divided into four 

major sedimentary units, based on differences in the rocks that 

were deposited in the changing environment. These four units 

have been named, from bottom to top, the Abo Formation, the 

Yeso Formation, the Glorieta Sandstone, and the San Andres 

Limestone.

The Abo Formation consists of thin beds of brown to brownishred sandstone, shale, and conglomerate. Fossils are sparse, except 

for a few species of small plants. These features indicate that the 

formation was deposited on the flood plains of streams on a nearly 

flat surface.

The formation is soft to very hard, and it forms alternating 

slopes and cliffs where it has been eroded. The best exposures of 

these rocks are in the lower parts of high escarpments and in areas 

of low relief bordering the central part of the Zuni Mountains. A

18
few outcrops can be seen along the secondary road between Ice 

Cave and Grants, N. Mex. (fig. 5); better exposures can be seen 

along the road between Ice Cave and McGaff ey.

The lower part of the Yeso Formation consists of pale-red sandstone. It is soft to hard and forms low rounded ridges or steep 

cliffs where it has been eroded. The physical character of the 

sandstone indicates that it was deposited by the wind as massive 

sand dunes in a wide expanse of desert.

The upper part of the Yeso Formation consists of red sandstone 

and shale and gray limestone. Gypsum, a soft white rock that 

formed on the bottom of shallow seas as the water evaporated, is 

also present as thin beds and a dispersed material in the sandstone 

and shale. Where eroded, the Yeso consists of nearly vertical 

cliffs. The beds of limestone contain the remains of marine shells, 

which show that the region was submerged below the sea during 

part of the time when the Yeso Formation was being deposited. 

The beds of shale record intervals during which the region was still 

submerged below the sea or was a little above the sea, and deposition was on broad mudflats or deltas.

The Yeso Formation is best exposed in the walls of deep canyons 

on the slopes of the Zuni Mountains and in high cliffs around the 

central part of the mountains. The lower part of the formation is 

best exposed in areas of low relief circling the central part of the 

mountains. The formation is well displayed along the roads between Ice Cave and Grants and McGaff ey (fig. 5), and the striking red color of these rocks adds much to the scenic beauty of the 

Zuni Mountains.

The Glorieta Sandstone is light-gray to buff, and in some 

places it is red. The grains of sand are uniform in size, and they 

have been firmly cemented with minerals that make the rock very 

hard. This sandstone was deposited along the coast or in shallow 

water as the seas again covered the region.

The Glorieta is best exposed as the caprock on high cliffs and 

on timbered slopes in the Zuni Mountains. It can be seen in the 

canyon walls along the mountain road between Ice Cave and 

Grants. (See fig. 5.)

The San Andres Limestone is medium to dark-gray limestone 

and limy sandstone. Remains of seashells are common in the 

limestone. This marine limestone attests to the complete submergence of the region beneath the Permian seas.

19
The limestone is best exposed on broad slopes on the north, 

east, and south sides of the Zuni Mountains. The steep, southwest-facing mountain front that can be seen from El Morro is 

capped with the San Andres. The formation can be seen also 

along the road betwen Ice Cave and El Morro and in canyon 

walls between Ice Cave and Grants. (See fig. 5.)

The San Andres Limestone furnishes large supplies of water to 

irrigation, municipal, and industrial wells near Grants.

Rocks of Mesozoic Age

The Mesozoic Era is sometimes referred to as the age of reptiles, for it was during this time that the great dinosaurs roamed 

the earth. The era began about 180 million years ago and 

lasted about 120 million years. An incomplete record of its 

three geologic periods Triassic, Jurassic, and Cretaceous has 

been found near El Morro.

The Triassic Period began about 180 million years ago and 

lasted about 30 million years. (See fig. 7.) The Triassic rocks 

have been divided into two formations of sedimentary rocks 

in the vicinity of El Morro the Chinle Formation (shale, sandstone, and limestone), and the Wingate Sandstone. These rocks 

were formed during a long interval in which the land rose above 

the seas again, and rivers and streams deposited material in the 

valleys and on broad plains.

The Chinle Formation consists of a colorful sequence of red, 

gray, and purple shale, pebbly sandstone, and, in the uppermost 

part, impure limestone. Gypsum is dispersed throughout the 

formation. The beds of sandstone are firmly cemented, and 

they form low cliffs and even slopes. Because most of the Chinle 

is easily eroded, it commonly is exposed in areas of low relief in 

broad valleys. Perhaps the most widely known of the Chinle 

exposures are those at the Painted Desert and Petrified Forest 

National Monuments in Arizona. Equally impressive exposures 

can be seen on the flanks of the Zuni Mountains.

Petrified wood (wood that has turned to stone) is common in 

the Chinle Formation. At the Petrified Forest National Monument, Ariz., the wood is brilliantly colored. However, at El 

Morro it is mostly drab in color. The grain of the wood can be 

distinctly seen in the rock.

20
The Wingate Sandstone on the north flank of the Zuni Mountains was deposited as dune sand in a desert. The Wingate 

south of the mountains, in contrast, was deposited as sand, clay, 

and pebbles on flood plains, and in lakes or inland basins. It 

consists of pale-red, very fine grained sandstone. The best exposures of the formation are in the lower parts of cliffs on the 

slopes of the Zuni Mountains and south of State Highway 53 

from 5 to 10 miles east of Zuni. (See fig. 5.) The bright-red 

cliffs of Wingate Sandstone east of Zuni contrast sharply with 

the green of juniper and pinon trees. Many of the houses in 

Zuni are made of stone quarried from the Wingate Sandstone.

The Jurassic Period began roughly 150 million years ago and 

spanned 25 million years. (See fig. 7.) Rocks of the Jurassic 

age (also sedimentary) on the north flank of the Zuni Mountains 

have been divided into several units as follows (in ascending 

order): Entrada Sandstone, Todilto Limestone, Summerville 

Formation, Bluff Sandstone, Zuni Sandstone, and Morrison 

Formation.

A short distance south of U.S. Highway 66, the units lose their 

distinctive characteristics, and near El Morro they are all called 

simply Zuni Sandstone. This Jurassic-age unit is the sandstone 

on which the inscriptions are carved at El Morro.

The Zuni Sandstone consists of white, light-gray, and buff to 

pale-red very fine to fine-grained sandstone, containing some intermixed clay and silt. It also contains some coarse sand and 

fine gravel in the uppermost part, but in general the particles are 

uniform in size. This rock was formed from massive sand dunes 

and stream deposits in a broad expanse of desert. The Zuni is 

soft to hard and it stands in cliffs. The high red cliffs extending 

eastward from Gallup, N. Mex., for many miles just north of 

U.S. Highway 66 are Entrada Sandstone.

The Morrison Formation, another unit of the Jurassic rocks 

north of U.S. Highway 66 (fig. 5), contains about 65 percent of 

the uranium ore that is known to exist in the United States. 

Most of the ore is found north of Laguna and in the vicinity of 

Ambrosia Lake. Other important deposits are a few miles east 

and northeast of Gallup.

The Cretaceous Period began about 125 million years ago and 

spanned a time interval of 55 million years. (See fig. 7.) Only 

a part of the Cretaceous rocks are represented in the region. In

21
the vicinity of El Morro, these rocks have been divided into 

three sedimentary units the Dakota Sandstone, the Mancos 

Shale, and the Mesaverde Group (shale and sandstone), from 

bottom to top. These units reflect a marked change in the environment of the region from that of the Triassic and Jurassic 

Periods. Once again the seas gradually rose over the land and 

then receded. The Dakota Sandstone was deposited in streams, 

lagoons, and in the margin of an advancing sea. The Mancos 

Shale was deposited in a sea that covered the entire region, and 

the Mesaverde Group was deposited along the coast and in low 

swamplands as the land once again emerged from the sea.

As shown in figure 7, the Jurassic unit is the Morrison Formation, and the Cretaceous unit is the Dakota Sandstone. The 

change from one kind of rock to another was accompanied by a 

change in the environment, during which time the sea covered 

the land. In many places, the contact between the two formations is marked by the presence of kaolin, a white clay. In some 

areas the kaolin appears by itself; in others it is mixed with the 

sandstone as a cement. This whitened zone is very distinctive 

at Inscription Rock. The kaolinized cementing material probably 

came from weathering of other minerals during the long interval 

of time betwen the deposition of the Morrison and Dakota sediments. Where it appears almost pure, it may have been "formed 

by local concentration of the fine, white clay in small, shallow 

lakes on the old surface." 10 It indicates that a moist climate 

prevailed during a part, at least, of the time betwen Jurassic and 

Cretaceous deposits in this area. It is from clues like these that 

geologists put together the puzzle of the earth's history.

The Dakota Sandstone consists of uniform grains that have 

been firmly cemented into very hard rock. It is relatively resistant to erosion, and it forms the caprock on Inscription Rock and 

on other mesas near El Morro.

At places the Dakota Sandstone contains medium- to darkgray shale and coal that were deposited in swamps ahead of the 

advancing sea. The Dakota Sandstone is the reservoir rock for 

much oil and gas in San Juan and Rio Arriba Counties, N. Mex.

The Mancos Shale consists of thin beds of medium- to darkgray shale and fine-grained sandstone, containing marine fossils. 

Because the beds of soft shale in the Mancos are easily eroded, it 

commonly is exposed in areas of low relief in broad valleys.

10 Leopold, 1943, p. 62. 

22
The Mesaverde consists of light- to dark-gray shale and lightgray to reddish-brown sandstone. It also contains several beds 

of coal, where great amounts of vegetation accumulated in the 

swamps.

Coal has been mined from the Mesaverde Group at many 

places in western New Mexico. Notable were the mines near 

Gallup, which were operated for many years during the early 

part of the century. Farther north and northeast, the Mesaverde 

Group yields great amounts of oil and gas.

Rocks of Cenozoic Age

Our landscape as we see it today was sculptured during the 

Cenozoic Era, which began about 70 million years ago. During 

this era, all the seas were drained from the continents, and many 

mountain ranges of the world rose to their present, or greater, 

heights. We say "greater" heights because mountains are constantly being worn down by the process of erosion. The geologic 

history of the last few thousand years can be read in the traces 

left by man, as well as in those left in rocks by other forms of life.

The Cenozoic Era has been divided into two periods: the Tertiary and the Quaternary. Rocks of the Tertiary Period have not 

been identified in the vicinity of El Morro. Either they have 

been completely eroded or were not deposited at all. The 

Quaternary Period began about a million years ago. In many 

ways it is the most difficult period to interpret near El Morro, 

although it is the youngest of the geologic periods and extends 

into present time. This period is difficult to interpret because 

most of the Quaternary rocks consist of igneous rock and fine to 

coarse sediments that were deposited in small lakes and stream valleys. Neither of these types of rock is favorable for the preservation of fossils, and the geologist needs fossils to help him make 

age determinations.

The Quaternary rocks in the vicinity of El Morro have been 

divided into two units: basalt (an igneous rock) and alluvium 

(unconsolidated very fine to coarse sand, clay, silt, and gravel). 

Basalt and alluvium cover all but the we:tern part of El Morro 

National Monument. The thickness of these units has not been 

determined except at a few places. At least 200 feet of Cenozoic 

rocks have been penetrated by wells near El Morro.

23
Figure 9. Volcanic cinder cone 14 miles east of El Morro.

The igneous rock came from several volcanic cinder cones 12 

to 15 miles east of El Morro. Most people assume that volcanoes 

have only one opening the crater but actually they generally 

also have smaller vents around the sides. Piles of volcanic ash 

form around these vents; these are called cinder cones (fig. 9).

In the vicinity of El Morro, lava flowed through the vents 

down the valleys and solidified. The fronts of the flows are 

commonly marked by large masses of broken black rock. North 

of El Morro, the lava formed a plain 2 to 3 miles wide and about 

10 miles long. Similar lava flows followed other valleys south of 

El Morro and spread across a few hundred square miles of plains 

to the south and west.

24
 K_ rCf

'KgX -^Sa^V'^ & 

%.-^^

25
'J f,f i \ ' ' 1 A*** < 1. ]^4/^M W'ZMtSjfJh
From rocks we obtain many valuable products for our everyday 

use. Near El Morro some of the rocks contain fluoride minerals, 

coal, oil, natural gas, and uranium. Many of the barren rocks 

cannot be excelled for natural splendor even artists may fail to 

capture their beauty in the changing light of day. As far as the 

monument is concerned, however, the most important product 

of the rocks is ground water.

Ground water is not found in underground lakes or streams, as 

many people think. The water clings to the rocks and between 

them. When it rains, part of the water is absorbed by the soil; 

some runs off the soil into rivers and lakes, and the rest sinks 

deeper into the ground until it eventually reaches and wets the 

rocks beneath the soil cover. The rocks at some depth below the 

earth's surface nearly everywhere are wet, but geologists looking 

for water have to find a supply that is free to come into a well. 

Certain rocks lack adequate openings between the rock grains, 

through which water can move. There may be water in these 

rocks, but it cannot move fast enough to be of much use. In 

other rocks the openings are larger and the water will move into 

wells.

Each of the rock units in the vicinity of El Morro was carefully 

examined to determine whether or not it was capable of yielding 

adequate supplies of water to a well. Some rocks, such as shale, 

can be ruled out immediately because the spaces between the 

grains of rock are too small to permit movement of much water. 

Also, shale generally contains a variety of minerals that are easily 

dissolved. Only small supplies of water containing much dissolved material are generally found in shale.

Sand and sandstone differ widely in their capacity to store and 

transmit water. Uniformly coarse sand generally yields large 

supplies of good water to wells. Beds containing mixtures of 

different sizes of sand grains yield less than those containing sand 

grains of uniform size. Beds containing mixtures of sand and clay 

yield even less than those of pure sand. Sandstone is made of 

grains of sand cemented together by various minerals. Because 

the spaces between the grains are partly sealed up, sandstone 

yields less water than sand. The pores in a very fine grained 

sandstone or one in which grain sizes are mixed may be completely sealed and yield little or no water to wells.

27
Limestone may be so dense that water does not move easily 

through it, or it may contain many openings which permit easy 

movement of water. The openings in limestone are caused by 

solution of the rock along natural breaks. These openings become larger as water circulates through them, and eventually 

may become caverns. Wells of large yield may be drilled in 

beds of cavernous limestone, but the water generally is hard because of a high content of calcium and magnesium carbonate 

that may have been dissolved from the rock.

Metamorphic rocks and some igneous rocks are generally too 

dense to store and transmit water. Volcanic-flow rocks (igneous 

rocks) may be very dense, or they may contain many openings 

caused by contraction of the rock during cooling. Intensely 

broken volcanic-flow rocks are generally capable of yielding large 

supplies of water to wells.

Superimposed on the igneous and metamorphic rocks underlying El Morro is an accumulation of sedimentary rocks. Most 

of them are not very permeable. However, the San Andres 

Limestone, of Permian age, and the Chinle Formation (sandstone 

and limestone), of Triassic age, 4 to 7 miles north of El Morro, 

yield potable water in ample quantities.

A nearer source of water is the alluvium of Quaternary age. 

Alluvium is the name given to sand and gravel washed away and 

deposited elsewhere by streams. The Quaternary alluvium is the 

only formation within 2 miles of the monument capable of yielding adequate supplies of potable water to wells. The water in 

the alluvium is of good quality, according to chemical analyses. 

Alluvium is discontinuous in the area, and test drilling is necessary 

to locate favorable occurrences of it and to determine its waterbearing properties. The prospects of adequate water supplies are 

best in the deeper buried valleys; that is, in areas where the alluvium has been covered by lava flows.

Much of El Morro lies in a valley which widens to the northeast between Inscription Rock and a high cliff to the east. This 

valley, which has been partly filled by alluvium and basalt, is 

tributary to a larger valley that trends westward of the monument.

Five test wells were drilled in the northeast valley on the monument land. Alluvium was found below the lava flows in only 

one of the five test wells. The yield of the alluvium in the well 

was only 3 gallons per minute, which is far from adequate to

28
supply the monument. The test drilling indicates that the buried 

valley that crosses the monument is very narrow and that an adequate supply of water probably cannot be obtained from wells on 

monument land. However, the alluvium in a larger buried 

valley just outside the north boundary of the monument seems to 

contain a larger supply of potable water.

The Importance of Water

El Morro is a dramatic record of the unique mixture of cultures 

in the old Southwest, as well as an interesting display of chapters 

in the geologic history of our earth. Its story is a perfect illustration of the importance of water to any civilization. The Indians 

settled on top of El Morro because of the pool at the base of the 

rock. Spanish and American travelers planned their journeys 

to stop at Inscription Rock because of the pool.

Now we have completed a full circle. People used to come to 

the rock because of the water; now water must be brought to the 

rock because of the number of people. So many visitors come 

to see the inscriptions left by those who stopped to take advantage 

of the pool that an additional supply of water had to be located. 

We know from news stories that the distribution of our national 

water supply may pose serious problems by the year 2000. Yet 

it is hard to grasp this huge problem and understand it on a 

national scale. Here at El Morro we can see it demonstrated as 

though under a microscope. The more numerous the population, the more rich and complicated our civilization becomes, the 

more we need ample water supplies. Water is an indispensable 

resource, even for a National Monument.

29
FOR MORE INFORMATION ABOUT

. . . . Geology for Children:

The How and Why Wonder Book of Our Earth, by 

Felix Sutton: Grosset & Dunlap, New York, 1960, 

48 p.

Our Earth, What It Is, by F. H. Dough: Whitman 

Publishing Co., Racine, Wis., 1961, .'59 p.

Rocks and the World Around You, by Elizabeth 

demons: Coward-McCann, New York, 1960, 109 p.

Rocks, Rivers and the Changing Earth, A First Book 

About Geology, by Herman Schneider and Nina 

Schneider: W. R. Scott, New York, 1956, 175 p.

Rocks and Minerals and the Stories They Tell, by 

Robert Irving: Alfred A. Knopf, New York, 1956, 

175 p.

. . . . Geology for the Layman:

Introduction to Physical Geology, by C. R. Longwell 

and Richard F. Flint: John Wiley & Sons, New York, 

1962, 504 p.

The World of Geology, by L. D. Leet: McGraw-Hill 

Book Co., New York, 1961, 262 p.

The Face of the Earth, by G. H. Dury: Penguin 

Books, Baltimore, Md., 1959, 220 p.

. . . The History of El Mono Region:

Wagon Road from Fort Defiance to the Colorado 

River, by E. F. Beale: 35th Gong., 1st Sess., Ex. Doc. 

No. 124,1858, 87 p.

Journal of a Military Reconnaissance from Santa Fe, 

New Mexico, to the Navajo Country, by J. H. Simpson: Lippincott, Grambo & Co., Philadelphia, 1852, 

140 p.

Report of an expedition down the Zuni and Colorado 

Rivers, by Captain L. Sitgreaves, Corps Topographical Engineers: 32d Gong., 2d Sess., Ex. Doc. No. 59, 

1853,198 p.

Missions and Pueblos of the Old Southwest, by E. R. 

Forrest: Arthur H. Clark Co., Cleveland, Ohio, 1929, 

386 p. 

30
The Scientific and Technical Background:

Triassic stratigraphy in the State line region of westcentral New Mexico and east-central Arizona, by M. E. 

Cooley, 1959: in New Mexico Geol. Soc. Guidebook, 

10th Field Conf., west-central New Mexico, 1959, 

p. 66-73.

Preliminary geologic map of northwestern New Mexico, by C. H. Dane and G. O. Bachman, 1957: U.S. 

Geol. Survey Misc. Geol. Inv. Map 1-224.

Scenic trips to the geologic past, No. 4, Southern 

Zuni Mountains, by R. W. Foster, 1958: New Mexico 

Inst. Mining and Technology, State Bur. of Mines 

and Min. Resources Div.

Stratigraphy of the uppermost Triassic and the Jurassic rocks of the Navajo Country, by J. W. Harshbarger, C. A. Repenning, and J. H. Irwin, 1957: 

U.S. Geol Survey Prof. Paper 291, 74 p., 3 pis., 38 

figs.

A revised geological time-scale, by Arthur Holmes, 

1959: Edinburgh Geol. Soc. Trans., v. 17, p. 3, December, p. 183-216.

Pennsylvanian rocks on the northeast edge of the Datil 

Plateau, by F. E. Kottlowski, 1959: in New Mexico 

Geol. Soc. Guidebook, 10th Field Conf., west-central 

New Mexico, 1959, p. 57-62.

Climatic character of the interval between Jurassic 

and Cretaceous in New Mexico and Arizona, by 

Luna B. Leopold, 1943: Jour. Geology, v. 51, no. 1, 

p. 56-62.

Vegetation of Southwestern watersheds in the 19th 

century, by Luna B. Leopold, 1951: Geol. Rev., 

v.41,no. 2, 295-316.

El Morro New Mexico's historical headland, by E. P. 

Lohr, 1959: in New Mexico Geol. Soc. Guidebook, 

10th Field Conf., west-central New Mexico, 1959, 

p. 149-153.

Comments on some of the ground-water problems at 

El Morro National Monument, by R. A. Maxwell, 

1940: National Park Service open-file rept., 6 p., 8 figs.

Report on the geology of El Morro National Monument with special references to underground water, 

by H. E. Rothrock and H. H. Hawkins, 1939: National Park Service open-file rept., 35 p., 4 pis., 6 figs.
Geologic map of Inscription Rock quadrangle, Valencia and McKinley Counties, New Mexico, by C. T. 

Smith and others, 1958: New Mexico Inst. Mining 

and Technology, State Bur. Mines and Min. Resources Div., Geol. Map. no. 4.

Jurassic rocks of the Zuni Mountains, by C. T. Smith, 

1959: in New Mexico Geol. Soc. Guidebook, 10th 

Field Conf., west-central New Mexico, 1959, p. 74-80.

32 U.S. GOVERNMENT PRINTING OFFICE : 1965 O 712-880