Rivers and mountains

In the study by A. Almeida, the granite complex of Cabeceiras de Basto does not have a single grain size. The classification uses three average intervals: 0,5–1 millimeter for fine grain, 2–4 millimeters for medium and 5–7 millimeters for coarse grain. In the last group, the author also mentions rare larger crystals, with 8–10 millimeters.

This allows distinguishing two observations: the usual size of the grains and the presence of some crystals that stand out. The description does not transform each visible point of a photograph into a measurement. To compare sizes in an image, a scale is required; a magnification on the screen may make small grains look large.

The study refers to three genetically independent series, although they have similar chemical composition ranges. Appearance is an entrance to research, not all explanation of origin. On a wall or a façade, the grains arouse curiosity, but do not prove for themselves where the stone came from.

Among the granites, the Archaeological Charter identifies two cores of Paleozoic metasediments. The Vila Nune extends to the northwest, following the alignment of the Peio; the Gondiães flanks the Beça. These are references to understand the diversity of the subsoil, not visiting points or collection sites. The landscape gains another layer when you look at what it is made of.

To understand the name, it is worth separating two stages. Sediments can accumulate, be buried and consolidate into sedimentary rock. If this rock undergoes metamorphism, it undergoes changes caused by conditions such as high temperature and pressure. It is this sedimentary origin followed by transformation that the term metasediment marks.

Metamorphism does not mean melting: USGS explains that the transformation happens without the rock passing the magma. Minerals can reorganize or give way to others. In some rocks, elongated or flattened minerals are oriented and produce a structure in planes, called foliation. Not all metamorphic rocks present it.

They're tools for interpreting the language of the map. They do not allow each stone of Vila Nune or Gondiães to be assigned a mineral, an exact age or a specific process by appearance only. For this, it is necessary to relate the observation with cartography and geological study.

The tutor of Adam Mendes, of the Portuguese Association of Professors of Biology and Geology, relates the granular texture of granite to the slow cooling of magma in depth. Among its characteristic minerals are quartz and feldspates; there may also exist mica. This general explanation helps to look at the stone more carefully, but recognizing colors and grains is not enough to identify the origin of a sample.

If granite has formed in depth, how do we see it in an outcrop? The British Geological Survey relates the exposure of these rocks to the survey of land, erosion and change over millions of years. The history of formation and the history of surface exposure are different stages.

It is also convenient to distinguish rock and mineral. In granite, different minerals appear associated in crystals that fit. Recognizing this combination is different from observing a single isolated mineral. The relative sizes of the crystals and their disposition help to describe the texture; an isolated color does not tell the whole story of the rock.

The Municipal Archaeological Charter describes large granite outcrops with erosion marks. The relief descends from the southern periphery of the Cabreira towards the basin of the Tâmega, in a succession of heads. Looking at these forms helps connect the mountain to the valley: the stone is part of the history of the territory, even when it was not worked by anyone.

A rock can change without leaving the place. It is called meteorization to this change: there can be physical fragmentation, chemical transformation of minerals or intervention of living beings. The British Geological Survey gives simple examples: roots that widen cracks, water that freezes and expands in fractures, or rainwater that reacts with minerals. They are general mechanisms; they do not allow to attribute a cause to every block that is in the Cabreira.

When the materials are removed and transported by water, wind or ice, erosion enters the scene. The distinction helps to imagine two steps: the rock changes; part of its materials can then travel. It is not necessary that the two happen at the same rate, nor that a fragment reaches directly into the river.

The reading tour ends in the veigas of the Tâmega, where the municipal source describes deposits of sand and clay. Between the exposed rock and a lowland field there are thus different questions: of what is made the stone, how is changed and where do the transported materials end?

The work of A. Almeida refers to a dating of the complex of Cabeceiras de Basto by the uranium-chumbo method, using the minerals zircon and monazite. The result cited is a minimum age of 311 ± 1 million years. It is a geological reference to the rock formation studied: it does not date the construction of a wall, nor establishes when a hillside has acquired the form that we see today.

The article integrates the minutes of the V National Congress of Geology, published in 1998. Upon reaching the value, Almeida refers to a work of geochronology with other authors, indicated in the bibliography. This chain of references allows us to understand where the number comes from: it is a result of investigation, not an age estimated by the color or size of the grains.

Reading a landscape forces you to separate time scales. One thing is the age attributed to the rocky complex; another, the transformation of relief; another, the moment when a stone was used in a construction. The three stories can cross on the same wall, but need different evidence.

The study published by the Museum in 2011 describes two levels in the mills of Cabeceiras: above is the mill; below, in the cabouco, the motor mechanism. The form of the building accompanies the operation of the mill. The survey records rectangular plants of varied dimensions, with the exception of a circular plant sour.

The granite dominates the walls, but not all the stone receives the same treatment. The study highlights the points subject to constructive requirements: corners, doors, windows and elements through which the water passes. In the wedges, the stones help to lock the construction; pads and shoulder pads frame the spans. An irregular wall can thus coexist with carefully chosen pieces.

This description belongs to the historical study, not to an inspection of the current buildings. To read the landscape, start with the route of the water and then ask where were the motor force and the work of grinding.

The survey of 4 March 2008 describes the water of Levada de Víbora divided into eleven shifts of 24 hours, called couples. Each shift began at sunset and ended at sunset. The initial order changed from year to year.

The names came from houses of Abadim: six couples were associated with the Village of the Tower and five to Santo António. The granite gutter led water from the stream of Busteliberne for irrigation and grinding. Thus, the levada had a visible construction and an equally important time organization. These data belong to the inventory of 2008, not to a current watering scale.

The brook of Cavez is born on the border of the municipality with Montalegre and ends at Tâmega. Between the mountain and the mouth, the municipal description highlights levadas, mills and irrigation mills: buildings and fields that show different ways to enjoy the water.

On the trail beginning in Vilela, Rio Douro, the narrative goes from the descent to the riverside to the levada and the landscape of the lameiros, then following rural paths and the slope. The return to the village brings another material to the history: the granite masonry of the houses. See the route chart to prepare the walk; this reading explains the landscape, without confirming the conditions of the way on the day of the visit.

In the lowlands near Tâmega, the Archaeological Charter describes accumulations of sand and clay linked to some of the vegetation with the greatest agricultural potential. The contrast is clear: on the slopes dominate large granite outcrops; by the river, deposited materials help explain another way of cultivating. The same presentation shows a landscape of heads that descend south, with water lines that seek the Tâmega and its tributaries. It is not a soil map of each field, but a key to read the set.

The Municipal Archaeological Charter refers to a geological environment rich in volframite and cassiterite. The two names add a mineral dimension to the landscape of granites and metasediments: they talk about the composition of the subsoil, beyond the form of the hills.

The volframite contains iron, manganese and tungsten. Virtual Microscope, a project with participation from Open University and the Royal Cornwall Museum, explains its importance as tungsten ore. Cassitis is a tin oxide with a formula SnO2, and is an important commercial source of this metal, according to USGS. They're two different minerals, associated with different metals.

A mineral and the metal you can get from it are not the same thing. Volframite is not a piece of metal tungsten, as cassiterite is not ready to use. The interest of these names is precisely in this connection between the matter found in nature and the materials transformed by people.

The municipal description confirms the local geological reference. International sources help explain the minerals, but their examples of exploitation belong to other territories. None of these explanations identify here a visitable mine, an active farm or a collection site.

The inventory of 27 of February 2008 located the mill of Quinta de Covas in Parada, in the parish of Cabeceiras de Basto. The water came from a levada with a weir in Peio. Before arriving to the castor, it passed through a gutter excavated in a trunk: wood was also part of this hydraulic construction.

The building brought together an olive oil mill, a mill and a wine mill. The form associates the decommissioning of the mill and the mill in 1994 to the entry into operation of a hydroelectric power station, which conditioned the available motor force. It is the explanation recorded in that survey, not an independent evaluation of the operation of the plant.

The document describes private heritage and equipment already without the necessary elements for grinding. This story does not indicate public access or current conditions of conservation.