1 - Geoparque de Sobrarbe

55
Geo
route
PN
PN
LARRI
VALLEY
BIELSA-LARRI VALLEY
c Sobrarbe Geopark
Texts: The Geo-Routes PN are taken from the Geological Guide Ordesa
and Monte Perdido National Park" from the Collection of Geological
guides of the National Parks (IGME-OAPN, Roberto Rodríguez Fernández,
dir and coord.)
The texts of these Geo-routes have been developed by Alejandro
Robador Moreno, Luis Carcavilla Urquí, Josep Maria Samsó Escolá and
Ánchel Belmonte Ribas (Scientific Coordinator Sobrarbe Geopark).
Figures and illustrations, by Albert Martinez Ríus, and photographs by
Josep María Samsó Escolá, Luis Carcavilla Urquí, Alejandro Robador
Moreno and Ángel Salazar Rincón
Translation into French and English: Trades Services, S.L.
Design and layout: Pirinei, S.C.
GEOROUTE
NETWORK
CBC project Pyrenees-Monte Perdido, World Heritage (PMPPM)
of the 2007-2013 POCTEFA Program.
S
OBRARBE GEOPARK GEO-ROUTE
NETWORK
The Sobrarbe Geopark is located in the north of the province of Huesca and
coincides with the district of the same name. This area is noted for its many cultural and natural
values, most notably its spectacular geology.
Indeed, the Geo-Route network of the
Sobrarbe Geopark was created to learn about and
understand its geological heritage in greater depth.
This is a network of 30 self-guided routes that allow
visitors to access the most outstanding geological
sites in the district and understand their origin,
meaning and significance. All Geo-Route have
been designed so that they can be covered on foot
and are clearly signposted; in most cases they are
based on official short-route (PR) or long-route
(GR)except PN 1, PN 4, PN 5, PN 9, PN 10 and PN 11
that combine a stretch of road and vehicle with
trails paths. There is a brochure on each route in
order to facilitate the interpretation of each stop on
the way.
In addition, 11 of these geological routes
are located in the Ordesa and Monte Perdido
National Park, including the territory of the Geopark,
and 3 of the Geo-routes are of a cross-border
nature, allowing visitors to enjoy the geological
heritage of the Pyrenees-Monte Perdido, declared a
World Heritage Site by UNESCO.
In addition to the Geo-Route network, there
are mountain bike (MTB) routes in the Geopark,
some of which feature small information panels
along the way and there is also a brochure that
Taken together, these routes will enable visitors to enjoy the most beautiful parts of
the Sobrarbe district and also obtain further information on its long geological history
dating back over 500 million years.
T
HE SOBRARBE GEOPARK
In 2006 the Sobrarbe District was declared a Geopark and became part of the European
Geopark Network, sponsored by UNESCO. A Geopark is a district with unique geological features for
which a sustainable development strategy has been developed. Consequently, the key objective is to
preserve its natural and cultural heritage and promote development through the appropriate
management of the geological environment. There are currently 60 Geoparks in Europe and 100 in the
word. The Sobrarbe Geopark features an exceptional geological environment, with over 100 places of
geological interest that have been inventoried; many of which can be visited on the Geo-Route
network.
More info:
www. geoparquepirineos.com
1
M
AP OF THE SOBRARBE GEOPARK
GEO-ROUTE NETWORK
Gèdre
Aragnouet
Gavarnie
Bujaruelo
Víu
Torla
P.N. DE ORDESA Y
MONTE PERDIDO
Broto
Cin
ca
Viadós
Bielsa
PARQUE
NATURAL
NATURAL
DE
POSETSMALADETA
MALADETA
Escuaín
Fanlo
oA
ra
Río
A-138
Pineta
Monte
Perdido
Gistaín
Nerín
Rí
San Juan de Plan
Plan
Saravillo
Puértolas
Lafortunada
Laspuña
Fiscal
N-260
Ascaso
Escalona
Boltaña
Labuerda San
Victorián
Aínsa
San Juan de Toledo
Foradada
N-260
Campo
Bárcabo
Lecina
GEO 1 Geo-Route
PN 1
Palo
Samitier
Río E
Paúles
de Sarsa
Abizanda
A-138
PARQUE NATURAL
NATURAL
DE LA SIERRA Y
LOS CAÑONES DE
GUARA
Tierrantona
Embalse de
Mediano
Arcusa
sera
Las Bellostas
Embalse de
El Grado
Geo-Route in National Park of Ordesa and Monte Perdido
The various Sobrarbe geo-routes vary in length, difficulty, theme and duration. Consequently, almost
everyone will be able to find a route that suites them.
2
Nº
1
2
3
GEO-ROUTE
Geopark Interpretation Centre
Aínsa: a town between two rivers.
Urban geology
Geology: A bird's eye view
4
Inside the canyon
5
Breath-taking landscapes of water
and rock
6
Sobrarbe at your feet
7
Crossing the Jánovas Gully
8
Iron Age Elements
9
10
Whims of water for lonely mountaineers
A lake among the oldest rocks in
Sobrarbe
11
The hidden lake
12
13
14
A road with tradition
A privileged vantagepoint
Secrets of the Guara Mountains
15
Geology for the Saint
16
A passage between two worlds
17
Water inside the Earth
18
The Jewel of Cotiella
19
Treasures of the Posets-Maladeta
Nature Park
Nº
GEO-ROUTE IN NATIONAL PARK OF ORDESA AND
MONTE PERDIDO
TRAVEL
DIFFICULTY
DURATION
THEME*
Geopark area
-
1 hour
All
Aínsa
Low
Short
RTF
Samitier castle and
hermitages
Low
Medium
TF
Congosto de Entremón
Medium
Short
TR
Vero River canyon
viewpoints
Low
Medium
RF
Ascaso- Nabaín
Medium
Medium
TF
Near Jánovas
Medium
Short
TR
Viu-Fragén-Broto
Low
Short
GR
Ordiso Valley
MediumHigh
Long
GKR
Lake Pinara and Puerto Viejo
Low
Medium
GR
Lake Bernatuara
Medium
Long
RGT
Bujaruelo Pass
Medium
Medium
RGT
Fiscal-GradatielloPeña Canciás
High
Long
RT
Las Bellostas-Sta. Marina
Low
Long
FRT
Low
Short
RT
Medium
Long
RFT
Low
Medium
KR
Low
Short
GR
Medium
Long
GR
Espelunga de San Victorián
Collado del Santo
Badaín-Chorro de Fornos
Basa de la Mora
(Ibón de Plan)
Viadós-Ibones de Millars
TRAVEL
DIFFICULTY
DURATION
THEME*
PN1 Ordesa Valley
Torla-Cola de CaballoGóriz Shelter
LowMedium**
Medium
RGF
PN2 Mount Perdido
Góriz Shelter-Mount Perdido
High
Long
TRKGF
PN3 The Roland Gap
Góriz Shelter - Roland Gap
High
Long
TRKGF
PN4 Cutas Viewpoints
Torla-Viewpoints-Nerín
Low**
Medium
KRGFT
Low**
Medium
RGT
High
Long
FTG
Medium
Long
RGT
PN5 La Larri
PN6 Balcon de Pineta
Bielsa-La Larri Valley
Pineta-Balcón de Pineta
PN7 Añisclo Canyon (lower part)
San Urbez-Fuen Blanca
PN8 Añisclo Canyon (upper part)
Fuen Blanca-Añisclo Pass
High
Long
RGTF
Escalona-Puyarruego
Low**
Medium
RTK
Tella, Revilla-Escuaín
Low**
Medium
TK
Broto -BujarueloOtal Valley
Low**
Medium
GTK
PN9 Circuit Añisclo Canyon
PN10 Escuaín Valley
PN11 Otal Valley
* Theme: T- Tectonics; F- Fossils;K- Karst; R- Rocks; E- Stratigraphy; G- Glaciarism
** Combining vehicle and hiking
3
G EOLOGICAL HISTORY OF THE
The geological history of the Sobrarbe Geopark goes back over 500 million
years. Many geological events that have affected the current landscape and relief took
place over that vast period of time. The geological history of Sobrarbe can be divided into 6
different episodes, each of which includes significant moments that led to today's geological
landscape.
1
THE REMOTEST PAST
(between 500 and 250 million years ago)
Over a long period of the Palaeozoic, the land now
occupied by Sobrarbe was a seabed where silt, mud, clay and
sand accumulated. Today these sediments have become the
shale, sandstone, limestone and quartzite that form the northern
mountains and valleys of the District. These rocks were intensely
altered by the Variscan orogeny: an episode of intense tectonic
activity that affected much of Europe and resulted in a huge
mountain range. Numerous folds and faults attest to this past
together with granite that was also formed in that era.
Folds in Palaeozoic rocks
2
TROPICAL MARINE SEDIMENTATION
(between 250 and 50 million years ago)
The giant mountain range formed in the previous stage was
heavily eroded and almost disappeared. Once erosion has almost
swept away the mountain range, the resulting flat land was
covered by a shallow tropical sea. Coral reefs appeared and the
calcareous mud we see today in the shape of limestone, dolomite
and marl, containing abundant marine fossils, accumulated. The
sea fluctuated several times and there were many time when its
depth increased and decreased; however, it practically covered
the area throughout this episode.
3
Typical landscape of turbidites outcrops
Fossils of marine organisms in the
Cretaceous limestone
THE FORMATION OF THE PYRENEES
(between 50 and 40 million years ago)
The marine sedimentation process continued during this episode, but
under very different conditions to previous episodes. The sea, which
separated what is today the Iberian Peninsula from the rest of Europe,
gradually dried up. About 45 million years ago, as this sea became
narrower and sedimentation occurred on the seabed, thousands of
metres below the surface, on land, the Pyrenees began to develop. I
In Sobrarbe we can find exceptional examples of turbidites, rocks formed in that sea as it
accumulated huge amounts of sediments resulting from the development of the mountain range,
while the mountains continued to develop.
PALAEOZOIC
542 m.a.
Cambrian
EPISODES:
4
488 m.a.
Ordovician
443 m.a.
416 m.a.
Silurian
359 m.a.
Devonian
1
299 m.a.
Carboniferous
251 m.a.
Permian
SOBRARBE GEOPARK
4
THE SOBRARBE DELTAS
Conglomerates: rocks formed from rounded
fragments of other rocks
(between 40 and 25 million years ago)
The formation of the mountain range caused the
gradual disappearance of the sea, which was becoming
shallower and elongated. About 40 million years ago, a
system of deltas marked the transition between the area that
had emerged and later stages of this marine gulf. Although
this period was relatively short, huge amounts of sediment
accumulated, which can be found today in the southern part
of the District converted into marl, limestone and sandstone.
Once the sea had retreated definitely from Sobrarbe, the
relentless effects of erosion became all the more intense if
possible. About 25 million years ago, active and dynamic
torrents accumulated huge amounts of gravel that, over time,
became conglomerates, such as those that form the bulk of
Peña Canciás.
5
THE ICE AGES
(last 2,5 million years)
Once the mountain range and its foothills had formed,
erosion began to transform it. The river valleys widened and
the present river network began to be formed. On several
occasions during the Quaternary, mainly over the last two
and a half million years, various cold spells occurred,
covering the mountains with snow and ice. The last major
ice age reached its peak about 65,000 years ago. Huge
glaciers covered the valleys and mountains and shaped the
landscape, effectively eroding some places and
accumulating sediment in others. The landscape of the
entire northern section of the District was shaped by those
ancient glaciers.
6
Glaciers like the ones we see today in the Alps covered
the Pyrenees at that time
TODAY
Today, erosion processes are slowly and gradually
wearing away the mountain range. This erosion occurs in
many ways: through the action of rivers, erosion on the
slopes, karst dissolution, etc. The landscape that we see
today is only an instant in a long evolutionary process
that is on-going, but now with the participation of man
who is changing the environment like no other living
being is capable.
Río Cinca, agente modelador actual
MESOZOIC
CENOZOIC
199 m.a.
Tria
145 m.a.
Jurassic
2
65 m.a.
23 m.a.
Cretaceous
Palaeogene
3
4
2,5 m.a.
Neogene
5
Quaternary
6
5
E
PISODES REPRESENTED IN THE GEOROUTES
Nº
GEO-ROUTE
EPISODES
PN1
Ordesa Valley
2
PN2
Mount Perdido
2
PN3
The Roland Gap
PN4
Cutas Viewpoints
PN5
La Larri
PN6
Balcon de Pineta
2
PN7
Añisclo Canyon (lower part)
2
PN8
Añisclo Canyon (upper part)
2
PN9
Circuit Añisclo Canyon
3
6
PN10
Escuaín Valley
3
6
PN11
Otal Valley
5
6
3
5
6
2
3
5
6
2
3
5
6
3
5
3
5
6
5
6
1
1
3
3
5
5
6
Episode 1: Variscan orogeny - Episode 2: Tropical marine sedimentation - Episode 3: The formation of the
Pyrenees - Episode 4: The Sobrarbe deltas- Episode 5: The ice age - Episode 6: Today
Nº
GEO-ROUTE
1
Geopark Interpretation Centre
2
Aínsa: a town between two rivers.
Urban geology
3
Geology: A bird's eye view
4
5
EPISODES
1
2
3
4
5
6
3
6
2
3
6
Inside the canyon
2
3
6
Breath-taking landscapes of water
and rock
2
4
6
Sobrarbe at your feet
3
6
7
Crossing the Jánovas Gully
3
6
8
Iron Age Elements
5
6
9
Whims of water for lonely mountaineers
5
6
10
A lake among the oldest rocks in Sobrarbe
1
11
The hidden lake
1
2
5
12
A road with tradition
1
2
5
13
A privileged vantagepoint
14
Secrets of the Guara Mountains
2
15
Geology for the Saint
2
3
A passage between two worlds
2
3
6
16
17
18
19
4
Water inside the Earth
2
1
6
6
6
2
The Jewel of Cotiella
Treasures of the Posets-Maladeta
Nature Park
5
6
5
6
5
6
7
8
55
Geo
route
PN
PN
LARRI
VALLEY
BIELSA-LARRI VALLEY
An easy route up a wide forest
road closed to traffic as far as La Larri
Valley, hanging over 400 metres above the
Pineta Valley floor and with spectacular
views.
The hanging valley of La Larri
reflects the activity of glaciers in this sector
of the Pyrenees a few dozen thousand
years ago. However, it also allows us to
make interesting observations that are
essential to understanding the geological
configuration of this sector and deciphering the formation of the Pyrenees.
Photo Archive Sobrarbe County. Nacho Pardinilla
9
4 5
6 7
i
3
LEGÉNDE
N
500 m
i
1
Parking
Home Geo-Route
Tour Geo-Route
Walking Tour
Number of stop
2
1 i
5
LA GEO-ROUTE PN5
Pineta Valley is without doubt one of the best-known and most frequently
PN
visited areas in the Pyrenees. The spectacular glacial valley and the fact that
the Mount Perdido glacier can be observed from many points, make it a highly
attractive route. In order to understand the tectonic structure of the area, it is important
to begin the geological visit by observing Bielsa. Then, from the Pineta car park, next to
the State Hotel (Parador Nacional), the easy walk to La Larri Valley along a pleasant
forest track closed to traffic leads to a key point in the process of understanding the
geological origins of the Pyrenees: the La Larri tectonic window; the central point of most
of this excursion.
However, if this were not enough, this place is also a magnificent vantage point
overlooking the Pineta Valley and the Mount Perdido massif. In addition, it preserves key
geological data to understand the magnitude, age and evolution of the glaciers in this
sector of the Pyrenees during the last ice age.
The final part of this route takes place outside the boundaries of the National
Park, as the Park only includes a small section of the beginning of La Larri Valley and its
western slopes.
Stop 4: The Sucarraz spring forms a beautiful
waterfall over the red Permian-Triassic rocks.
Stop 6: At the beginning of La Larri Valley, there is a large
meadow with sediments forming a natural dam originally
built from Pineta ice and moraines.
Stop 2: Pineta Valley, a beautiful example of a glacial valley.
Stop 7: The only place in the Pyrenees where you can observe the lower
block of the Gavarnie mantle in a tectonic window.
Fig. 1. Route Scheme
11
stop
1
THE VICINITY OF BIELSA
Entrance to the town of Bielsa
WHAT WE CAN SEE
- Rocky ridges near the town
- The base of the Gavarnie thrust fault
We can leave the car at the southern
entrance to the town of Bielsa and enjoy an
excellent view of the ridges forming the
southern slopes of Pineta Valley. From this
point, we can see Punta del Cuezo opposite,
consisting of a grey rocky ridge that
corresponds to limestone from the early
Cretaceous (K1).
The northern slopes of the peak
feature another type of reddish rock from the
Permian-Triassic deposits and, below these,
there are igneous rocks: Palaeozoic granite.
All these rocks form the Bielsa tectonic unit
and constitute the structural unit on which the
Gavarnie mantle rests. A visit to the town of
Espierre or to the neighbouring valley of
Chisagüés will enable us to observe that the
rocks appear below Palaeozoic rocks, which
is not normal as some of them are more
modern than the rocks above. This
arrangement is similar to the one we shall see
in the final part of the route, in La Larri Valley,
where they form what is known as a "tectonic
window".
We can see the summit of Mount La
Mota in the background, which consists of a
layer of reddish rocks above extremely white
rocks. These are the red Permian-Triassic rocks
that
arranged
here
generating
an
unconformity above the Bielsa granite.
Fig. 2: Rocks from the
lower unit of the
Gavarnie thrust fault can
be seen at the entrance
to Bielsa.
Palaeozoic rocks
(granite) can be seen on
the right (north). Above,
and dipping to the south,
we can see the red
Permian-Triassic (Pg)
rocks. Above are
carbonate rocks from the
Late Cretaceous (K1).
The Gavarnie overthrust
involves these rocks;
where older rocks appear
above younger rocks.
They have eroded and
the picture interprets the
eroded area.
2
stop
JAVIERRE AND THE ENTRANCE TO PINETA
Pineta Valley, a beautiful example of a glacial valley
WHAT WE CAN SEE
- A large esplanade surrounded be rocky cliffs
- Example of a glacial valley
Pineta Valley is an example of the
morphology of a glacial trough (valley
excavated by ice). During the last glacial cycle,
at the time of the maximum expansion of the ice
that, in this part of the Pyrenees, took place
65,000 years ago, the glaciers from the Pineta or
Tucarroya cirque joined the glacier from La Larri
Valley and the smaller glaciers on the north side
of the Zucas ridge and, downstream from this
The thickness of the ice, caused by this
almost horizontal plateau, can be estimated at
over 450 metres; as indicated by some deposits
belonging to a lateral moraine discovered by
the geologists, Hernández Pacheco and Vidal
Box in 1946 on the northern side of the valley, in
the area known as "llanos de Diera" (to the east
of the town of Espierba).
Stop, they merged near Bielsa with another
great glacial system, the La Barrosa system,
which, in turn, resulted from the confluence of
other minor glaciers from the La Munia and
Punta Suelza massifs. Once merged, the great
Cinca glacier descended to the vicinity of of
Salinas de Bielsa, totalling a length of about 25
kilometres.
The view of the figure taken from the
track to the Añisclo Mountain Pass (GR-10),
displays the clearly glacial profile of the
valley, As the road nears Pineta, we can see
part of the upper cirque and the Tucarroya
mountains that preserve sharp summits that
are typical of peaks that have been shaped
by glaciers on all sides.
Fig. 3: View of Pineta Valley to the west with the classic U-shaped profile of glacial valleys.
stop
3
PINETA CAR PARK (LLANO DE LAS ARTICAS)
WHAT WE CAN SEE
- Valley floor surrounded by rocky cliffs forming a cirque.
- The cliffs feature numerous waterfalls
The "Llano de la Ártica", next to the
car park opposite the Parador Nacional
(State Hotel) and Pineta Chapel, offers a
spectacular
view.
Looking
at
the
surrounding rocky cliffs, it is possible to
identify
the
channels
along
which
avalanches flow after the winter snows. It is
also possible to identify forests devastated
by the avalanches, with fallen trees and
snapped trunks, as well as fragments of rock
that have accumulated at the foot of the
channels forming a cone or fan. Directly
above the hotel, there are some large
screes
covering
the
slope
almost
completely where it is possible to see flows
of rocks that are activated during the rainy
season.
Fig. 4 Appearance of the screes from Pineta car park
14
The valley also presents numerous
relicts of degraded moraines that correspond to
more ancient lateral and valley floor moraines.
The frontal moraine cordons, which were
abandoned as the glaciers retreated to their
present positions only survived in the highest
parts of the Pineta Cirque (excursion number
PN 6 to the balcony of Pineta), below the
Marboré and Mount Perdido glaciers. As more
recent processes, we can mention the action of
the river, with braided courses and small alluvial
fans that refills the area with layers of pebbles.
The route up to La Larri is on a good
forest track closed to traffic. The route is clearly
marked and if frequented by many visitors in
summer.
stop
4
SUCARRAZ SPRINGS (FUENTE DE SUCARRAZ)
The Sucarraz spring forms a beautiful waterfall over the red Permian-Triassic
rocks.
WHAT WE CAN SEE
- Intensely red rocks stand out from the rest
- These are ancient sediments from the Permian and Triassic
La confortable montée permet
d'obserThe comfortable route makes it
possible to observe the avalanche
channels and, a little higher up, the starting
point of the track to the Pineta Balcony
(Balcón de Pineta) on Geo-Route PN 6. It is
a few metres from the bridge over the river,
where the "Estrecho Limestone" has been
carved into a spectacular gorge.
Due to the effects of the
avalanches, this part of the route may have
to follow a path built in 2010 that connects
to the forest track after a half-hour walk.
We shall gain altitude through a pleasant
beech forest until we reach a new bridge
over the La Larri River. This is the Sucarraz
Fountain, where the red Permian-Triassic
sandstone stands out given the extreme
contrast of colours.
This waterfall is caused by the
difference in altitude between the floor of
the main glacial valley, Pineta, and the
floor of the secondary valley or hanging
valley, La Larri.
Fig. 5:At Sucarraz Springs, we can observe intense red rocks. Their age is unclear and is estimated between the Permian and Triassic. They are red
because they are continental sediments; ancient rivers with oxygenated waters that oxidized the iron minerals contained in the sediment, giving them
their typical colour.
15
stop
5
LAS VUELTAS
WHAT WE CAN SEE
- Accumulations of rocks next to the track
- Remains from a moraine located between the Pineta and La Larri glaciers
of ice, was the Pineta Glacier and it, therefore,
"blocked" the La Larri Glacier. Although the
moraine is located where the two valleys
converge, it is known as a lateral moraine
because of the position it occupied regarding
the main Pineta glacier.
This moraine has a characteristic
composition helps to understand its origin. At this
point, almost the easternmost sector, there are
many pebbles and Palaeozoic, granitic and
Permean-Triassic (red) rocks, mixed with
"Marboré Sandstone" and "Estrecho Limestone"
blocks. As we approach the entrance to the
valley, there is a predominance of large
"Marboré Sandstone" blocks above the other
rocks. The La Larri Valley does not contain
"Marboré Sandstone", only Palaeozoic rocks and
"Estrecho Limestone". Therefore, all the "Marboré
Sandstone" rocks that appear here come from
the glacier that descended from the Pineta
Cirque. This enables us to state that it was a
lateral moraine.
Fig. 6. There are rocks of different types and sizes on the track. These are
the sediments of the glacier's moraines. The existence of Marboré
Sandstone indicates that the moraine belongs to the glacier that came
from the head of Pineta and not from La Larri, as this type of rock does
not exist in the latter.
Next to the tightest bends in the forest
track, we will be able to observe the
composition of the La Larri Valley moraine. This
deposit of rock, sand and silt corresponds to an
accumulation of materials carried by the
glaciers that covered the La Larri and Pineta
valleys and that converged at this point. The
main glacier, which carried a far larger volume
16
Another privileged location to observe
this moraine is at the point where the La Larri
River cuts through it. We can reach this point
from the lowest part of the track, a few metres
before reaching the refuge (see the following
Stop). We must take a poorly marked path to the
west. We shall soon see the way down to the
river and a metallic gangway that crosses it. At
this point, erosion by the river has created a
natural cut in the moraine, where we can
observe the mixed sizes of the rocky fragments
that form it and its somewhat chaotic
appearance, with no clear separation between
layers or strata.
Fig. 7.Diagrams reconstructing the Pineta and La
Larri glaciers, with the location of the Stop and the
moraine sediments.
Fig. 8. The Pineta Glacier
stop
6
LA LARRI REFUGE
At the beginning of La Larri Valley, there is a large meadow with sediments
forming a natural dam originally built from Pineta ice and moraines.
WHAT WE CAN SEE
- A valley with a flat floor and steep slopes.
- It used to be a glacial basin and then a lake
Fig. 9. The La Larri Valley was a lateral glacier that contributed ice to the main Pineta glacier. The ochre-coloured sediments seen on the
left are river sediments that cover former grey lacustrine sediments. The latter were deposited in a lake that formed when the ice retreated
because the moraines in the main valley formed a natural dam.
The entrance to the La Larri Valley is
spectacular. If we look behind us, we shall see
the Mount Perdido glacier, above Balcón de
Pineta, which has been described in detail on
route 6. The La Larri Valley is a hanging valley as
it was carved by a tributary glacier that flowed
into the Pineta glacier, which, in turn, carved a
much deeper valley.
We have crossed the moraines and are
now entering a hanging valley, where we can
observe different types of sediments known as
blocking sediments that resulted from the
blocking effect of the Pineta glacier and its
lateral moraine regarding the La Larri valley
glacier. Observing these sediments, we can
easily imagine the landscape and the glaciers
during the last ice age.
18
The western margin of the valley
features a cut that is several metres high where
we can see two types of sediment. There are
mostly pebbles and gravel mixed with sand.
These sediments are very similar to those carried
today by the La Larri River and the torrents that
flow down the slopes. However, at the base of
this cut, we can identify another type of
sediment with a very different origin. These are
clear
grey
characteristics
silt
and
of
clay
sediments
with
that
typical
have
accumulated in high mountain lakes.
After the ice reached its maximum
expansion (65,000 years ago), the glaciers did
not retreat over night. On the contrary, the ice
advanced and retreated several times over tens
of thousands of years. During these phases
following the maximum expansion of the ice, the
La Larri Glacier, smaller and with a poorer
source, retreated more quickly than the main
Pineta glacier. Therefore, about 40,000 years
ago, the Pineta Glacier and its lateral moraine
plugged or blocked the La Larri Valley, not
allowing the runoff water and the water from the
thaw to flow freely. This resulted in a natural
dam, behind which a lake formed with a depth
of about 40 metres. Significant amounts of
sediment were trapped in this Paleo-lake and,
therefore, by about 11,000 years ago the lake
had been completely filled in with sediments
and the La Larri River crossed the floor of the
valley from end to end.
Consequently, the fluvial deposits
(pebbles, gravel and sand) completely covered
Fig. 10. Illustration showing the La Larri area. First, when it was covered
in ice. Secondly, when the ice retreated and the valley was blocked
(natural dam effect). Thirdly, the current aspect with the lacustrine
sediments almost entirely covered by river sediments.
the lower lacustrine sediments (silt and grey
clay). Subsequent erosion by the La Larri River
has excavated the moraine and the top part of
the blocking sediments, forming a "terrace" and
partially uncovering the top part of the
lacustrine sediments.
19
stop
7
ASCENT TO LA RIBARETA
The only place in the Pyrenees where you can observe the lower block of the
Gavarnie mantle in a tectonic window.
WHAT WE CAN SEE
- Rocky ridges with different aspects and colours.
- A key tectonic structure in the Pyrenees.
There is a good view from the refuge in
La Larri Valley, but you can achieve a better
perspective if you advance a little in the
direction of Ribareta. In this area, we can see
what is known as "the Larri tectonic window". At
the bottom of the valley basin there are granitic
rocks that are covered by 30 to 40 metres of
reddish
Permian-Triassic
rocks
(from
approximately 250 million years ago).
These, in turn, are covered in 50 metres
of "Estrecho Limestone" (which formed between
100 and 85 million years ago). So far everything is
normal: rocks of a certain age covered by others
that are younger. However, over the said
limestone, there is a fold of Palaeozoic rocks
(from the Devonian and Carboniferous, which
formed between 400 and 350 million years ago),
i.e. older rocks covering younger rocks. In turn,
"Estrecho Limestone" appears on top of these
again, in a layer that reaches the summit of
Mount Capilla.
This
anomalous
arrangement
corresponds to a thrust fault, which places
older rocks above younger rocks, repeating
the sedimentary series. It is the base of the
great Gavarnie thrust fault mantle (it receives
this name because it was discovered for first
time in this French town). Glacial erosion has
eroded the valley revealing this complex
tectonic structure that has allowed us to
rebuild the structure of this sector and of the
entire Pyrenean area.
Fig. 11.
Panoramic view
of the La Larri
tectonic window.
There are
Palaeozoic rocks
(granitic rocks) on
the valley floor
arranged above
reddish PermianTriassic rocks
(P9) and above
Late Cretaceous
materials (K1).
However, above
the latter there
are Palaeozoic
rocks again and
the normal
Cretaceous series
(K1, K2, K3
without PermeanTriassic). It is the
thrust fault of the
Gavarnie mantle
(see tectonic cut).
20
Fig. 12. Diagram to help understand the
structure of the La Larri tectonic window.
1.- The Gavarnie mantle places older rocks
above younger rocks.
2.- The erosion of the valley makes it
possible to observe a circular and closed
figure (tectonic window) of the lower block
of the thrust and, above, the upper block
where Palaeozoic rocks (older) are on top
of Cretaceous rocks (younger).
Hanging valley of La Larri. Photo Archive Sobrarbe County. Nacho Pardinilla
!
AN ANCIENT LAKE SECLUDED IN THE MOUNTAINS. HOW IS IT RESEARCHED?
Until recently, the presence
of sediments from a lake locked in
the La Larri Valley went unnoticed by
geologists; which is quite normal if
we consider the limited thickness of
the materials visible in the field. Its
existence was discovered when the
IGME
conducted
a
detailed
geological map of the Ordesa and
Monte
Perdido
National
Park.
Subsequently, as part of a research
project titled "Glacial, climatic and
vegetation dynamics in the Ordesa Monte Perdido National Park during
the Holocene", the Instituto Pirenaico
de Ecología (CSIC) and the IGME,
with the cooperation of other
institutions, conducted an in-depth
study of the said sediments.
The erosion caused by the
La Larri River has uncovered just over
two metres of sediments from an
ancient lake, which comprise silt and
clay with fine particles of sand
interbedded. The most typical
characteristics of these lacustrine
sediments are their colour, grey,
which indicates sedimentation in
poorly oxygenated waters, and the
fact that they present very fine
layers, millimetres thick, that indicate
that the material settled in a quiet
sedimentation environment, without
currents.
There are also very fine
interspersed layers of find sand with
undulated
stratification
that
geologists call ripples, and that
indicate occasional contributions of
sand with a the effects of a slight
current. There are also layers that
have slipped, indicating unstable
conditions on the bottom of the lake.
Fig. 13. Image of laminated clay, typical of ancient lake sediments, exposed on a
recently excavated slope in the La Larri Valley.
In order to discover the thickness of the sedimentary backfill, a geophysical survey
was conducted based on the so-called electrical tomography method. The measurements
are obtained by introducing a current in the soil through a pair of electrodes and measuring
the difference in impedance created using another pair of electrodes. The electrical resistivity
of geological formations, which is the specific electric resistance of each material to the
passage of current, is related to the lithology, porosity, pore fluid... By knowing the local
geology, it is possible to establish a relationship between the resistivities obtained and the
nature of the materials in the subsoil. This method was used to determine that the thickness of
the lake sediments at La Larri was about 40 metres.
However, in order to observe and sample the materials in the subsoil, it is necessary
to drill test holes and obtain so-called "cores", which are cylinders of geological materials
obtained at various depths. At La Larri, three test bores between 10 and 17 metres deep were
drilled, obtaining almost continuous cores of lacustrine materials similar to those found in the
cut or outcrop.
A key aspect required to interpret the geological significance of the materials, but
always difficult to obtain, is the age of the sediments. The La Larri lacustrine sediments have
been dated using radiocarbon, a dating method that uses the carbon-14 isotope (14C). The
oldest cores obtained from the lower part of the test bores are about 30,000 to 35,000 years
old and the youngest, obtained at the surface outcrop, are 11,000 years old. Therefore, we
can connect the blocking of the La Larri Valley with a cold phase identified at several points
in the Pyrenees and that began about 40,000 years ago.
The presence of filled lakes or paleo-lakes is more frequent in the Pyrenees than
would be imagined. For example, according to works published by Jaume Bordonau and his
collaborators in 1989, the famous town of Benasque, near Mount Aneto, rests on one of these
paleo-lakes whose sediments reach a thickness of 300 metres.
Fig. 14. Core from one of the test bores conducted in the La Larri Valley, displaying teh
aspect of clay that has settled in an ancient lake.
!
PYRENEES-MONTE PERDIDO,
A TERRITORY BORDER WORLD HERITAGE
In 1997, UNESCO added the
Pyrenees-Monte Perdido site to its World
Heritage List in recognition of its natural
and cultural importance. The site covers
a cross-border area and includes the
Gèdre, Gavarnie and Aragnouet valleys
in France and the district of Sobrarbe in
Spain.
This remarkable mountainous landscape
straddles the border between France
and Spain. At its centre lies the limestone
massif of Monte Perdido. The PyreneesMonte Perdido World Heritage Site
extends across 31,189 hectares and
includes the municipalities of Torla, Fanlo,
Tella-Sin, Puértolas, Bielsa and Broto in
Sobrarbe on the Spanish side and the
Gèdre, Gavarnie and Aragnouet valleys
in the Hautes-Pyrénées Department on
the French side. The entire Ordesa and
Monte Perdido National Park in Spain lies
within the listed site, while in France the
area is also protected by the Pyrenees
National Park.
Cultural and natural heritage
Monte Perdido from Mountain Sesa
Photo Archive Sobrarbe County. Nacho Pardinilla
Gavarnie Cirque.
Photo Archive Sobrarbe County. Nacho Pardinilla
The Pyrenees-Monte Perdido is home to deep canyons and cirques with spectacular
walls: three canyons and a gorge on the southern side in Spain (Ordesa, Añisclo, Pineta and
Escuaín) and four large glacial cirques on the northern side in France, which is steeper
(Gavarnie, Estaubé, Troumouse and Baroude).
The karstic, glacial and valley landscapes contrast with the almost flat-topped peaks and the
underground waters that have formed extensive galleries, chasms and grottoes.
This single site thus harbours outstanding cultural and natural aspects: The geological
and biological characteristics of the site make it an extremely interesting place for science and
conservation, as it includes numerous endemic species of flora and fauna.
It is an outstanding cultural landscape that combines the beauty of a matchless
natural setting with a socio-economic structure that dates far back into the past and illustrates
ways of life typical of mountainous areas that are disappearing in Europe.
People have developed their way of life, their relationship with the environment and
their bonds with others in this area since prehistoric times. In the Middle Ages, an unusual form
of economic and social organisation came into being. In both Spain and France, the families,
towns, villages, valleys and regions on each side of the Pyrenean chain managed to conquer
the 'impassable wall' of the mountains and so were able to engage in trade, make business
agreements and forge alliances and cultural ties based on peace and a sense of fellow-feeling.
The landscapes we see today are the result of the legacy left to us by our forebears, who
worked hard to keep alive a basic system of farming and animal husbandry that would ensure the
survival of generations to come and their traditions, rituals, festivals, music and legends.
www.pirineosmonteperdido.es
Brèche de Roland
Photo Archive Sobrarbe County. Pierre Meyer
LARRI VALLEY
BIELSA - LARRI VALLEY
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
PRACTICAL INFORMATION
ROUTE: Bielsa - Pineta Valley - Larri Valley.
TYPE OF ROUTE: Linear (return along the same route).
DIFFICULTY LEVEL: Elemental
DURATION: 2 hours (up) and 1 hour down.
DISTANCE: From Bielsa: 23 km. From the hotel "El Parador" to Llanos de La Larri: 3,2 km.
GRADIENT: 500 metres
i
STARTING POINT: Bielsa.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
COMMENTS
This tour combines vehicle from Bielsa to Pineta Valley parking and trekking until the end of the route.
This Geo-Route runs through the Ordesa y Monte Perdido National Park, part of the transboundary
site Pyrenees-Monte Perdido, declared World Heritage by UNESCO.
Information Point of the National Park in Torla. Tel: + 34 974486472 and in Bielsa Tel:+ 34 974501043
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1750
1500
1
2
PROFILE GEO-ROUTE
4 6
7
5
3
1250
1000
5GEO-ROUTE
PN
of Sobrarbe
www.geoparquepirineos.com