Yale Peruvian expeditions, 1911-1915.
Publications no. 20.

Peru
_2Q_

for. the founding ef a. CoUege it? tfdi.CaUn.y"

[From The American Jodrnai, of Science, Voi,. XXXVI, August, 1913.]

The LA PAZ (BOLIVIA) GORGE

By Herbert R. Gregory

Results of the. Peruvian Expedition of J 912, under the auspices
of Yale University, and the National Geographic .Rtag^inei^

H. K Gregory— La Paz (Bolivia) Gorge. 141.

Art. XVI. — The La Paz (Bolivia) Gorge; by Herbert
E. Gregory.*
From the shores of Lago Pequeno, the nearly enclosed south
eastern portion of Lake Titicaca, the surface of the interior
plateau of Bolivia (the altiplano or altiplanioie of the Spanish
Americans) ascends toward the Cordillera Real. From Guaqui
to Viacha, forty-two miles, tlie rise is 120 feet and the railroad,
after following tlie irregular course of the Rio Tiahuanaco and
Fig. \f

&•"' ^¦-¦~izr*&&atL3k

Figs. 1. View from Alto looking eastward toward the Cordillera Eeal.
The position of the gorge, on tlie floor of which the city of La Paz rests, is
indicated by the arrow.
passing a group of low, mature hills, crosses the shallow
valleys of the Rio Colorado and the Rio Viacha. From Via
cha to Alto, the terminus of the steam railroad, the floor of
the altiplano is remarkably flat, and slopes westward at the
rate of forty feet per mile.f The drainage of this portion of
the plateau is sluggish and frequeutlyinterrupted by shallow
* Geologist of the Peruvian Expedition'ol
. \ Distances and elevations are as shown on the
Carril del Ser del Peru, a blue print of whichlwai
Corey, Chief Engineer.

142 H. E. Gregory— La Paz (Bolivia) Gorge.
depressions. No hills rise above the gravel-strewn floor which
appears to extend as an unbroken surface to the foot of majes
tic Illampu. At Alto a surprise awaits the traveler, for here,
without preliminary warning in change of slopes or eastward-
flowing streams, one finds himself on the brink of a canyon
cut entirely in alluvial deposits to a depth of over 1500 feet.
At the foot of the canyon wall lies the city of La Paz, whose
red tile roofs, cathedral spires and threads of streets, broken
by parks and traversed by streams and irrigation ditches com
pose a unique picture of singular beauty.
Fig. 2.

°'.' •'•p'-:?\\
;V-'i'-: >;;':^'.":-^'-*:5,1 >\
~-'-~..':rr.'.'° '»¦'-: f-y.*/.-'.-

*$$;

"1 ^

<*: ¦'.

^y. 'fa'gs.-T ¦

w

:'^>..!

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a, gravel, sand. 6, tuff, c, sands, gravel, clay, d, lignite, o, sands, gravel, clay.
Fig. 2. Generalized section of La Paz gorge, not drawn to scale.
As shown in the view (fig. 1), looking from Alto station
across the La Paz valley, the landscape gives no suggestion of
the presence of such a chasm and one is reminded forcibly of
the Colorado Plateau of Arizona, where impassable canyons
of great depth are revealed only when one is standing on their
rim. On descending the canyon walls it is found that the
floor is by no means flat, but is cut by streams which flow in
gorges one hundred feet and more in depth, between and over
which, resting on hills and terraces, the city is built. The
larger part of the buildings are distributed along two more or
less dissected terraces whose position with respect to the
valley walls is shown diagrammatically in fig. 2.
Between San Jorge and Obrajes the La Paz river has sunk
its bed into sands and clays whose eroded strata exhibit minia
ture " bad land " forms. Lying unconformably above these
finer deposits at San Jorge and northward through the city
are deposits of gravel whieh stand as nearly vertical walls fifty
to one hundred and fifty feet high. The material is exces
sively coarse and contains bowlders of white granite six inches
to six feet in diameter. Above the gravel terraces, forming
the knobs and benches and ridges of the western part of the

H. E. Gregorys-La Paz (Bolivia) Gorge. 143
city as well as the walls of the valley, and extending nearly to
the level of the altiplano, are beds' of gravels and sands and
clays eroded into a bewildering maze of forms. Needles in
groups or singly, columns unadorned or fluted or capped by
tables, rise five to fifteen feet on steep slopes and five to fifty
feet on knife-edged, dividing ridges. Innumerable sharply-
cut, miniature canyons with sheer walls five to two hundred
feet in height together with tunnels and pits in great variety
furnish passage for water. Landslides with slopes as great as
50°, frequently accompanied by open cracks, are numerous.
Here and there benches and tables composed of cemented
gravels and brown concretions project from vertical surfaces
or form the capping of columnar masses. The whole deposit
is ash-gray in general tone, but is beautifully striated by gray,
brown, light pink, bright yellow, purple and white bands from
a few inches to one hundred feet in thickness. Vegetation is
absent except for patches of wiry grass and tough shrubs
which find a foothold on the little flat-topped tables and gen
tler slopes. The bed's in general dip slightly to the south.
A closer examination of the strata exposed reveals the
presence of the following materials: (1) Sand, mostly fine,
some coarse, composed chiefly of quartz grains, and arranged
in beds several hundred feet in extent, or in short lenses. All
the strata are more or less cross-bedded, with laminae dipping
0° — 25°, (2) Gravel, composed of rounded pebbles from the
size of a small pea to three inches in diameter, arranged as
lenses which exhibit marked and sudden variation in position
'and size both horizontally and vertically. The gravels are
everywhere cross-bedded and frequently inclose lenses of sand.
The component materials in the upper part of the-section were
found to consist approximately of sedimentary fragments 75
per cent, igneous 15 per cent, metamorphic 10 per cent. The
following types of rock were recognized : gray sandstone, brown
sandstone, white granite, granite-gneiss, diorite-gneiss, garneti-
ferous granite-gneiss, black slate, mica or chlorite slate, gray
quartzite, brown quartzite. Quartz pebbles are rare and no
limestone or volcanic material was observed. All the pebbles
are worn, about half of them well-rounded, and a few are
faulted and veined. The gravel increases in amount and
becomes coarser toward the top, and along the electric railway
from La Paz to Alto contains angular, sub-angular and rounded
bowlders four inches to one and one-half feet in diameter. At
this locality the gravel forms beds of considerable thickness or
occurs as lenses embedded in finer gravels, sands and clays,
and resembles morainal deposits except for the irregular strati
fication. (3) Clays, rarely pure, usually highly arenaceous,
generally distributed as lenses within the finer sands. In the

144 If. E. Gregory — La Paz (Bolivia) Gorge.
localities studied by the writer, clay is relatively small in
amount, the larger beds being 100 to 200 feet in length and of
inconsiderable thickness. (4) Carbonaceous shale and two

Fig. 3.

y

-V- '¦ t ¦ . .,6.16. '¦¦ **
fij&i .J*w ' ^ &'& ¦ ¦-'

Fig. 3. View of deposits in La Paz gorge, about one mile west of the
American Institute.
layers or lenses of impure lignite three to six inches thick,
composed of comminuted plant remains too fragmentary for
determination. (5) Volcanic ash, eight to fifteen (at one point
twenty or more) feet thick, extending as a continuous white

H. E. Gregory — La Paz (Bolivia) Gorge. 145

band for over two miles on the west side of the valley and
reappearing on the east side at an elevation of about 12,600
feet. Microscopic examination shows the" ash to be dacitic.

Fig. 4

y

Fig. 4. View of deposits in La Paz gorge, about one and one-half miles
west of the American Institute.

The general appearance of the deposits and the arrangement
and alternation of strata are shown in figs. 3 and 4, with which,
for purposes of comparison, is inserted a typical view of the
Dakota bad lands (fig. 5).

146 H. E. Gregory — La Paz (Bolivia) Gorge.
The texture and structure of a portion of the beds taken
about midway between the top and bottom are shown in fig. 6,
Figs. 7 and 8 exhibit details and are fairly representative of a
large number of occurrences.
The profound gorge of La Paz with its great accumula-
Fig. 5.

Fig. 5. View of Bad Lands, South Dakota. Photo by Darton, TJ. S.
Geological Survey.

tion of unconsolidated sediments and striking erosion features
has naturally attracted the attention of scientists and travelers
alike. D'Orbigny* speaks of the La Paz deposits as "allu
vial," and notes that sandstone pebbles were more abundant in
the upper beds. He also recoguized kaolin deposits at Mira-
flores, — a suburb not visited by the writer. Forbesf assigns
* Voyage dans Amerique Meridionale, TomO III, 1842, partie 3, p. 120.
f Report on the Geology of South America, Quar. Jour. Geol. Soc. vol
xvii, 1860. '

R. E. Gregory— La Paz (Bolivia) Gorge. 147
to these beds a thickness exceeding 2000 feet. The band of
" trachytic tuff " " 300 feet below the surface of fhe plain,"

Fig. 6

y

"'Sfstub'.^—-

i

a, 15 ft., fine sand with thin lenses of gravel. 6, 4 ft., cross-bedded gravel, o, 24 ft.,
fine sand, consolidated In places and lenses of gravel, d, 14 ft., cross-bedded gravel,
with lenses of fine sand. e. 20 ft. , fine sand. /, 8ft., cross-bedded gravel with lenses
of fine sand, g, 200 ft., thin-bedded sands with gravel lenses ; portions of sand firmly
cemented.

Fig. 6.
to scale.

Section of a portion of the west wall of the La Paz gorge. Drawn

148

H. E. Gregory— La Paz (Bolivia) Gorge.

" 20 to 30 feet in thickness," Forbes considered as part of a
wide-sprea'd " diluvial formation," occupying a basin between
the Silurian rocks of the high Andes and the low Devonian
hills to the west, and believed the impure lignite to be an
extension of the carbonaceous strata exposed at the foot of
Illimani and also near Poto-poto. The material composing
Fig. *t<<

Fig. 7. Portion of gravel lens ; extent and thickness of gravel and sand
and orientation of cross-bedding laminse drawn to scale. __

Fig. 8.

a, sand with irregularly distributed gravel, b, gravel with two lenses of clay.
Fig. 8. Section on line of Arica-La Paz railroad, showing contact
between gravel and sand.' Drawn to scale.

these beds has its source, according to Forbes, in the Silurian
strata of the Cordillera Real ; with the exception of the vol
canic ash, which he assumes had been carried by streams from
Achacachi on the shores of Lake Titicaca. Zundt* assigns the
* Appendix to Spanish edition of D'Orbigny, La Paz, 1907.

H. E. Gregory — La Paz (Bolivia) Gorge. 149
granite bowlders to the Cordillera Real at the north, the quartz
fo the same area, and also to the mountains near Viacha and
Colquencha to the west, clays to the Ramos formation (Terti
ary?) underlying the altiplano, and the ash to the Letania
mountains. As to the condition of deposition of these deposits, D'Or-
bigny and Forbes appear to have held no definite views.
Evans* expresses the opinion that "the enormous deposits of
alluvium . . . represent not the alluvium of a lake, but the
infillings of a longitudinal valley." Zundtf speaks of the ash
deposit as carried by wind, dropped into a take or sea and
spread by the waves. Minchin^: speaks of lake beds, a part of
the floor of ancient Titicaca, now covered by glacial gravels.
D'Arlach§ speaks of floods induced by earthquakes which cut
the La Paz gorge aiid drained an interior sea. Posnanskyl
apparently considers the base of the La Paz beds as marine-
built, the upper portion deposited in an ancient sea detached
from the Pacific by uplift. Zundt, whose previous views
involved the existence of a salt sea, considers the deposits in
the La Paz gorge as lacustrine, — the fillings of a temporary
lake formed by blocking an ancient river which drained the
interior basin. The upper end of the La Paz gorge is believed
by this writer to have been excavated by glaciers. Bowman^
recognized the fluviatile origin of the La Paz beds, but in
speaking of the deposits as "the coarsest alluvium, the sort of
material that mountain torrents carry," evidently had in mind
the upper beds of the section and the material forming the
banks of the present stream, rather than the fine-textured,
stratified deposits to the west and south of the city.
From the general and detailed sections described and figured
above it appears that the deposits which line the La Paz gorge
are in no way typical of lacustrine formations. The absence
of continuous beds of thinly laid clays, silts and the finest
sands of uniform texture, the presence of cross-bedding and
channeling and the rapid alternation of gravels and sands both
horizontally and vertically argue against deposition in the quiet
waters of a lake. All the phenomena disclosed by the study of
the sections may be accounted for on the theory that the region
was traversed by low grade, piedmont streams. Such streams
with a shifting net-work of distributaries and interlaced chan
nels alternately depositing and cutting in a capricious manner
*Geog. Jour., vol. xxii, pp. 634-35, 1903.
t Loc. cit, 1907.
% Geog. Jour. vol. xxxvi, pp. 396-7, 1910.
§ Bol. Oficina National de Estadistica, No. 64-66, p. 756, La Paz, 1911.
| Bol. Oficina National de Estadistica, No. 64-66, pp. 689-702, 1911.
1[ This Journal, vol. xxviii, p. 400, 1909.

150 II. E. Gregory— La Paz (Bolivia) Gorge.
in response to seasonal rainfall would produce just such sedi
ments as the strata under discussion. Temporary lakes, —
annual or lasting for decades, — are normal features of such a
piedmont flood plain and are ample to account for the lenses of
clay and the thin, short bands of carbonaceous material occur
ring in the section.
The geologic history of the La Paz gorge can not be written
on the basis of the data at hand. The significance of the inner
gravel terrace pointing to a second, or perhaps a third, cycle of
filling and erosion, the conditions under which the remarkably
coarse upper beds were deposited, the character of the floor on
which the finer sediments were laid down, the extent of the
deposits and the pre-glacial history of the La Paz river itself,
are problems which will repay detailed physiographic research.

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