“Revisitas” de regiões esquecidas no tempo - “Plant Hunting Regions” - a partir de uma obra de grande valor para o especialista e amador de botânica como da Natureza em geral.
Por
Horst Engels, Cecilia Sousa, Luísa Diniz, Nicole Engels, José Saraiva
da
Associação “Trilhos d’Esplendor”
I .2 Relevo, Geologia, Clima e Vegetação da Península Ibérica
1.2 Relevo, Geologia, Clima e Vegetação da Península Ibérica
1.2.1b Relevo e Geologia - orogenia hercínica
Polunin & Smythies
escrevem (p.3-4)::
The
crystalline massifs
are formed of the most ancient rocks of granite, gneiss, and some Palaeozoic sediments. Today they form a grey landscape with rounded undulating hills, and fairly steep valley slopes. The hard rocks have resisted denudation despite the heavy rainfall, particularly in north-western Spain, and the general level is still high, with mountains up to 1500 m or more, and with steep V-shaped valleys with fast rivers which have cut into the ancient plateaux. Where these rocks form the present-day coastline, steep cliffs alternating with flooded valleys form a deeply indented coastline with fiord-like
rias
typical of north-western Spain, where the sea penetrates many miles inland. Despite the great age of this landscape in Spain, it is not of great importance either as a refuge for ancient plant species or as a site for new speciation, and in consequence the north-west of the peninsula does not figure in the accounts of the botanical regions. The
Massif Central,
which is twice the size of Switzerland, is of much greater importance in this respect. Not only does it comprise a relatively high mountain area with subalpine conditions, which acts as a 'stepping stone' between the Alps and the Pyrenees, but it also has an extensive recent volcanic area, an important site for endemic plants, of considerable botanical interest. In the formation of the Massif Central the ancient crystalline rocks have acted as a rigid mass against which the softer rocks, formed on the floor of the Tethys Sea, have buckled and folded during the alpine earth movements. As a result of these enormous pressures the old rocks were tilted slightly in a north and north-westerly direction, and intense volcanic activity followed, continuing almost into historical times. Pinnacle-like volcanic plugs, piles of ash, streams of black lava, clusters of volcanic cones, give the country an almost lunar landscape. The Plomb de Cantal is the remnant of a large volcano thirty miles across. Outside the volcanic areas, high rounded hills with thin acid soils and deep, often heavily wood ed valleys, produce a typically 'old ' landscape.
The central plains. or mesetas
are the vast, often extremely flat tablelands which cover nearly half of the interior of Spain.They are composed of Tertiary sedimentary rocks which overlay the ancient core and are largely formed of clays, marls, limestones, and sandstones. The Central Sierras traverse the Central Plains diagonally and divide them into two distinctive regions; those of
Old Castile
to the north and those of
New Castile
to the south. Old Castile is about half the size of England. It is a huge upland basin lying at the average altitude of about 800 m and drained by the tributaries of the Douro river. These cut broad, shallow, but steep-sided valleys into the meseta and form fertile alluvial plains known as
campiña
. But away from the few rivers the landscape is flat, monotonous and parched, relieved only by low ranges of hills known as
cuestas
or
cerros
. They result from the erosion of an upper horizontal stratum, revealing the strata below; they are very steep and abrupt, 20-100 m high, with an eroded, serrated front. Seen from below they stretch across the horizon in tawny serried ranks of low hills. Elsewhere, flat isolated tablelands, known as
paramos
, with steep-sided cliffs stand out from the plains; they are arid and uncultivable and fit only for goats and sheep. Bleak frosty winters and hot dry summers have made this a 'monotonous treeless plateau, falling at long intervals into a bluff-edged alluvial valley, repeated again and again'. Man and his animals have, in ll probability, been largely responsible for the present-day aridity of this region. Remains of pine and oak woods testify to a once extensive tree cover where today only a low, sparse scrubland or matorral covers vast areas, whilst in the driest regions only steppe vegetation can survive. The
meseta
of New Castile falls into two contrasting regions. To the west is the region of
Estremadura
where the ancient rocks lie close to the surface resulting in a more varied, undulating landscape, and where the rocks break through they form the low rounded hills of the Montes de Toledo and Sierra de Guadalupe. To the east, there is the same monotonous featureless plain stretching for mile after mile that occurs in Old Castile. South-east of Madrid is a distinctive region known as
La Mancha
, where surface water is very scarce and where there is an accumulation of salt in the surface soil layers as a result of evaporation, forming the typical
salt-steppes
of semi-desert regions. The Spanish salt-steppes are uniqu e in Europe and have a unique flora. A number of plants found there occur nowhere else in Europe, the nearest members of the same species growing as far away as North Africa, eastern Turkey, or even the Caspian Sea (which all have similar salt-steppe areas).
The old-fold mountains
were probably formed during
Hercynian folding
and are composed of rocks of the ancient crystalline core. Characterized by the presence of granite, they have remained as relatively high summits owing to the very slow rate of denudation. The two most important old-fold ranges are the
Central Sierras
- the
Serra da Estrela-Sierra de Guadarrama group
- which are such a dominant feature of the Iberian peninsula, and the
Iberian mountains
with two main highland areas centred around
Soria
in the north, and Teruel in the south. Both ranges ar e of great interest botanically as they act as refuges for montane and alpine plants, and as centres of diversification of recent species, being considerably isolated from other comparable montane regions. Of lesser importance are the
other old-fold mountains
, which include the Sierra Morena, the Montes de Toledo, and other lower ranges, where there are fewer endemic species. In general, these old-fold mountains have rounded summits often strewn with granite blocks and deep rocky glens, alternating with gently sloping hillsides.
As
montanhas de foldagem (orogenia) antiga
formaram-se provavelmente durante o período do
orogéno hercínico
e são compostas de rochas do núcleo cristalino antigo. Caracterizadas pela presença de
granito
, mantiveram-se como picos relativamente altos com uma taxa lenta de denudação. As duas mais importantes cadeias centrais da dobragem antiga são o
grupo da Serra da Estrela, Gredos e Guadarrama
(
Sistema Central
), parte tão importante da
Geografia de Espanha
, e o
grupo do
Sistema Ibérico
(Cordilheira Ibérica)
com duas áreas montanhosas principais centradas à volta de Soria no norte, e à volta deTeruel no sul.
A Serra da Estrela, Portugal (Sistema Central)
Pico Almanzor (Serra de Gredos) acima do rio Tiétar
Face Sul do Pico Almanzor, Sierra de Gredos
La Sierra de la Demanda. Ao fundo o Monte San Lorenzo.
Laguna Larga
na Sierra de Neila (Burgos).
Lagunas de Neila: Laguna de la cascada, antes de Los Paulazos
[2]
Picos de Urbión
A laguna Negra de Urbión
Ambas as regiões do Sistema Ibérico (Norte e Sul) são de grande interesse botânico porque servem de refúgios para plantas das montanhas e plantas alpinas e formam centros de diversificação de espécies recentes, consideravelmente isoladas de outras regiões montanhosas do género.
Sierra de Albarracín
Sierra Calderona
Montes Universales
De menor importância são outras cadeias de montanhas de foldagem antiga como a
Serra Morena
, os
Montes de Toledo
e outras de altitudes baixas onde se encontram menos espécies endémicas de plantas. Em geral, estas montanhas de foldagem antiga possuem picos redondos atenuados, frequentemente fortalecidas por blocos graníticos e com ravinas rochosas alternando com colinas suavemente declivadas.
The Sierra Morena at
Despeñaperros
Veja à seguir: 1.2.1c Relevo e Geologia - orogenia alpina
[1]
Imagens e figuras são tiradas de Wikipédia se não for indicada outra fonte
[2]
www.panoramio.com/photo/634142
Sem comentários:
Enviar um comentário