CityofRocks
City of Rocks State Park, establishedin
1952, encompasses
a square mile of scenic
desert region in southern New Mexico at an
elevationof 5,200ft. Facilitiesincludepicnic
units, campsites,fresh water supply, restrooms, a playground, and hiking trails. A
desertbotanicalgardencontainsa wide variety of cacti and other Southwestflora. Wildlife includes owls, other native birds, and
numerousdesertanimalsand reptiles.Highlighting the scenicattractionsof the park are
rows of great stone blocks sculptured by
nature into a fanciful representationof rows
of housescrowdedalong narrow streets,as in
a medievalvillage. To understandthe origin
of these rocks, we must trace the geologic
record back to the Tertiary Period-more
than 33 million yearsago.
GeologY
New Mexicowas
What is now southwestern
then undergoingone of its severestbaptisms
of fire by cataclysmicvolcaniceruptionsthat
spewedout glowing lava flows, massesof
coarserock fragments,and extensiveblankets
of volcanicasi. Rocks formed by compaction
and consolidation of this ash are calledtutfs;
threemajor varietiesarepresentin the vicinity
of the park. Ash-fall rzls resulted from explosivevolcaniceruptionsin whichcolumnsof
ash and pebble-sizedfragments were ejected
into the air by steamand hot gases.Falling to
earth(with somewinnowingand wind drift), a
wedge-shaped
depositof looselyconsolidated
tuff was laid down. Ash-flow tuffs are particularly significantin the origin of rocks in
the park. In contrast to the brief explosive
eruptionsthat createdash-falltuffs, ash flows
originated from extensive avalanchesand
clouds of incandescentparticles that swept
swiftly acrossvast areasabout the volcanoas
a highly fluid sheet.In someinstances,rocks
formed by ash flows are similar 'to oir-fall
however,the flows retuffs. ln otherinstances
tained sufficient heat and plasticityto facilitate a high degreeof compactionand welding
February 1980
NewMexico Geologlt
of the particlesto producea dense,hard rock rocks, such as the weldedtuff in the park, a
similar to that of cooled lava flows; these weatheringprocessknown as exfoliation rerocks are weldedtulfs, The City of Rocksis a sults in spalling and scaling of thin layers
sheet of along curved surfacesthat lead to progressive
small segmentof a once-extensive
rounding of the column, ultimately creating
moderatelyweldedtuff.
The third category, water-laid tuffs, is a spheroidalforms. Noticethe seriesof straight
product of the erosion and redepositionof pathwaysforming the "streets" of the citylooseair-fall and ash-flow tuffs in streamsand theseare the coursesofjoints widenedby erolakes,usuallyresultingin thin-bedded,slabby sion. Softerspotsin the rock are more rapidly
rocks that resemblesedimentaryrocks.Tuffs eroded, forming cup-shapedcavities and alof this type are well exposedin stonequarries coves. The cavities provide nesting sites for
along NM-61, about 2 mi northeastof the kingbirds and swallows, while the alcoves
park accessroad. The air-fall, ash-flow,and provide shelter from the elements for park
water-laid tuffs in this area constitute a visitors.
A closelook at a freshlyspalledsurfacewill
geologic formation-the Sugarlump Rhyolite
Tuff, which ranges in thickness from about revealthe componentmineralsthat distinguish
a rhyolite. Although theseare visible to the
200to 1,4@ft.
Welded tuffs are resistantto erosionand nakedeye,a handlensor magnifierwill prove
commonlyform sheercliffs atop slopescut in helpful. The shiny, black to bronzy crystals
the weaker,nonweldedtuffs and other softer are hornblende.Lessconspicuousbecauseof
rocks. The two cliff-forming layersvisiblein their paler color are grayish-whitegrains of
Table Mountain about I mi northeastof the quartz, and the still whiter, glassygrains of
park are examples.Another common feature sanidineand plagioclasefeldspar. Although
is their tendencytoward columnar jointing, the matrix enclosingthesecrystalsor phenoalso seenin Table Mountain and in the park. clasts is composedof fine-grained,angular
Joints are simplefractureplanesalong which particles of volcanic ash compressedand
dislocationof the rockshasoc- welded together, these features can be seen
no appreciable
curred.Jointscommonlydevelopdue to cool- only undera microscope.
ing strainsfollowing compactionand welding
Panoramicview
of the ash flow. Wheretwo or morejoint sets
panoramic
diagramidentifiessomeof
The
of
a
set
into
intersect,the rock is divided
polygonal columns. The widths of the col- the prominenceson the skyline visible from
umns dependupon the spacingbetweenthe the park; the more distantonesrequirea clear
joint planes.In the City of Rocks,thejointing day (typicalof New Mexico).Orientationmay
is widelyspaced,hencethe columnsarebroad. be madewith referenceto the north arrow or
Onceexposedto weatheringby erosionof the by aligning with nearby Table Mountain' a
overlyingvolcanicrocksthat formerlycovered shortdistanceto the northeast.
the tuff beds,chemicaland physicalprocesses To the north-northeast,the prominentpeak
attack the tuff, penetratingreadily along the is Mimbres Feak on WhitehorseMountain,
joints. Probing roots of low-crownedEmory composedentirely of volcanicrocks that inoaks find moisture and nourishmentalong cludebasalt,andesite,and rhyolitelava flows,
thesefractures.Chemicaldecompositionand breccias,and tuffs. Possiblyvisibleon the disphysical disintegrationof the walls of the tant skyline beyondis the crestof the Black
joints coupledwith erosionby waterand wind Range,one of New Mexico'smajor mountain
progressivelywiden the joints, isolating the ranges.
Nearbyto the northeastis Table Mountain
columnsinto monoliths.In massive.resistant
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-with an upper and a lower cliff of welded
rhyolite tuff and slopes of softer bedded
rhyolite tuffs, all part of the SugarlumpFormation:
To the eastis Round Mountain, composed
of andesiteand latite flows, breccias,and
tuffs. Unseen-midway betweenthe park and
Round Mountain-is the courseof the southward-fl owingMimbresRiver.
Cooke's Peak, a prominent landmark for
early travelers,piercesthe skylineto the eastsoutheast.The peakis a massivebody of granodiorite, an intrusiveigneousrock similar in
appearanceto granite. The granodioritewas
injectedas a hot, moltenmassinto a thick sequence of sedimentaryrocks consistingof
limestone, sandstone,and shale. Some of
thesesediments,particularly the limestones,
containfossilsindicatingthat seascoveredthis
regionnearly5@ million yearsago. Clustered
about the foot of the peak are a number of
small lead, zinc, and copperminesand prospectholes.
Just to the right of Cooke's Peak, and
slightly more than 4 miles away, is the intrusive rhyolite dome of Taylor Mountain-at
the westernfoot of which lies the channelof
the MimbresRiver.
On the horizon to the southeastare the
jaeged peaks of the Florida Mountains jutting abruptly upward from the flat surfaceof
the Deming Plain. The higher peakson the
north are sculpturedfrom volcanictuffs and
brecciasoverlyinglimestone,shale,and sandstone.The bulk ofl the range, however,consists of ancient igneous rocks @recambrian)
probablywell overa billion yearsold.
Nearer,to the south-southeast,
is the basaltcapped volcanic peak of Black Mountain.
Volcanic peaks cluster about Grandmother
Mountain to the south-southwest;
farther to
the southwest the highest prominence is
Soldier's Farewell Hill. The old Butterfield
Trail passedthe foot of the poignantlynamed
hilt.
Nearlydue weston the horizonis the broad
a
city of Raks sure Park
earliestmilitary expeditionswas that of the
Mormon Battalion under Captain Philip St.
GeorgeCooke, whoseroute westwardpassed
but a few miles south of the City of Rocks.
on landmarksof
Cooke'snameis emblazoned
the area: a prominent mountain range, a
mountainpeak,and a largespringthat slaked
the thirst of many weary travelers. John
Russell Bartlett, during his survey of the
southern boundary of the United States,
visitedthe area in May l85l and sketcheda
group of columnarrocks nearbyon the MimbresRiver.
The ButterfieldOverlandMail Routebegan
service in 1858 and brought a stream of
travelersand regular mail serviceacrossthe
History
dustytraceof Cooke'swagonroad. Although
The muterocksof the park havewitnessed
a serviceended in 186l becauseof the Civil
colorful paradeof visitors down through the War, the silencewas soonto be brokenby the
centuries.Among the earliestwere the Mim- sound of the bugles.Fort Cummings(from
bres Indians, inhabitants of the Mimbres 1863-1873
was established
and 1880-1886)
at
River Valley area from the beginningof the Cooke's Spring to protect travelersand setChristianera to about 1250A.D. The Mim- tlers against Apache bands that roamed the
brefios,a major branch of the Mogollon cul- countryside.Not until the mid-1880'swere
ture, lived in small villagesalong the stream theseelusiveraiderssubdued,makingpossible
whose clear waters irrigated fields of corn, the settlement by miners, cattlemen, and
squash,beans, and cotton. Arts and crafts homesteaders.
flourished; most noteworthy were pottery
With the westward expansionof railroads
decoratedwith geometric and animal figure reachingDeming in 1881,a swelling flood of
designs unexcelled by any other group of settlersspreadover the land. ColonelRichard
AmericanIndians.
Hudson,a veteranof the Civil War who came
During the earlyhistoricperiodof the Span- to New Mexico with the California Column,
ish conquestof the AmericanSouthwest,sol- purchased
land with a fine
a plot of homestead
diers and priests pressednorthward out of mineral hot spring a few miles southwestof
Mexico in the 1500's,but did not settleuntil the park. This spring became one of the
much later. Thesehardy colonistsbeganmin- famous western resort spas; its waters were
ing copperat SantaRita in l8(X and continued bottledand widelydisuibuted.
for some40 years.Mule packtrainsburdened
With main trade centersat Silver City and
with the red metal trailed yellow ribbons of
Deming, mining, ranching, and farming are
dust southwardacrossthe sandy plains en- now the principalindustriesof the region.It is
route to the mints of Mexico, where copper a land of clearskiesand warm sun, crisp,cool
wasin demandfor coinage.
nights,and distanthorizonsin which one can
During and after the Mexican War of recapturethe drama of the turbulent eventsof
18461847,trails wereblazedto link the newly thepast.
tr
-Robert H. Weber
acquired territories with easterntrade centers
and the seaportsof California. Among the
SeniorGeologist,NMBM&MR
crest of the Big Burro Mountains, a large
uplifted fault block stripped of its former
mantleof youngerrocksto exposethe ancient
Precambrianbasementcomplexof graniteand
metamorphicrocks.The palebandof mill tailings from the Hurley mill of KennecottCopper Corporation is to the west-northwest.
Here the copperoresfrom the Chino open-pit
mineat SantaRita areprocessed
to yieldmuch
of New Mexico's current copper production.
In the middledistanceto the northwestarethe
volcanic Cobre Mountains overlooking the
Chino mine on the north side.Beyond,on the
distantskyline,is the PinosAltos Range,also
composedof volcanicrocks.
New Mexico Geolog!
Februaryl9E0