Alcañiz grew between the reeds of the Guadalope and the walls of Calatrava

The Guadalope encircles Alcañiz with a meander that is not only a feature of the landscape: it also appears in the account of the city’s origin. Along its banks, reeds or reed-stalks were plentiful, to the point that one of the explanations given for the name Alcañiz links it to the Arabic al-qannis, “the reeds or reed-stalks”. The other proposed interpretation comes from al-kanā'is, “the churches”. Neither of the two is presented as definitive, but the first directly links the name to the water that surrounds the settlement.

The subsurface helps to understand why that fluvial corridor could be so recognisable. The geotechnical information brings together fine sands and silts, gravels and compact sands, as well as more resistant sedimentary formations. Not all of those materials have to crop out on the same bank or belong to a single episode: the data describes the municipal whole. But it does draw a territory in which water, sediments and changes in relief have conditioned the space available for living, moving through and building defences.

The river that could have given it its name

Fine sands and silts occupy 6.76 % of the analysed subsurface, while gravels and compact sands account for 5.20 %. When this type of material is associated with watercourses and flood events, it tells a story of currents that transport particles, leave them behind when they lose speed, and set them moving again during floods. This is the mechanism that turns a channel into a changing strip: a riverbank may offer moisture and vegetation, but it may also be exposed to the dynamics of the river.

Historical documentation places the current settlement of Alcañiz in the Muslim period, although its origins as a stable settlement are unclear, and the deserted settlement of Alcañiz el Viejo has also been considered as a precedent. The link between the name and the reed-stalks does not, by itself, show why exactly that site was chosen, but it makes visible a specific advantage of the location: the Guadalope formed a continuous physical reference and a recognisable riverbank.

Water does not appear only at the surface either. The permeability classification included in the technical data corresponds to class A-1, described as characteristic of generally extensive, highly permeable and productive aquifers. This category does not allow each spring or each parcel to be attributed to a specific layer, but it reinforces the image of a territory in which the circulation of water is an essential part of the landscape. Alcañiz is not understood solely as a city alongside a river: it is understood as a settlement in permanent contact with a system of water and sediments.

A mixture of rocks for a territory of contrasts

The largest group within the subsurface is made up of sandstones, conglomerates and flysch: it accounts for exactly 36.81 %. Each name translates a different setting. Sandstones may come from ancient beaches, deltas or rivers with a tranquil current; conglomerates preserve the imprint of far more energetic courses, capable of transporting rounded pebbles; and flysch consists of an alternation of sands and clays deposited at the foot of a continental shelf.

The figure is important precisely because it does not speak of a single rock. It speaks of a composed ground, made up of different sedimentary environments. Where conglomerates predominate, the landscape may preserve resistant surfaces and cemented pebbles; where alternations of sand and clay appear, the ground’s behaviour changes. Geology suggests that this variety may have helped to separate passage corridors, low-lying areas and more suitable sites for controlling the environment, although there is no specific lithological identification in the information assembled beneath the castle.

Alcañiz’s history shows how a recognisable physical space ended up becoming a space of power. In 1157, Ramón Berenguer IV reconquered the settlement and expanded its territory to deal with Muslim rule, although he lost it again before the second reconquest carried out by his son Alfonso II of Aragon. In 1179, Alfonso II ceded the castle to the Calatrava religious-military order.

The order imposed its model of settlement and promoted an initial walled enclosure, the so-called high wall, whose symbol was the castle. This was not a geological decision in the strict sense, but rather a human response to a place that had to be organised, protected and dominated. It is likely that the combination between the Guadalope corridor and the available sedimentary relief helped make that hierarchy readable: below, the city linked to the water; above, the fortress that expressed who was in command.

Gypsum preserves the memory of a closed lagoon

The second major component of the Alcañiz subsurface consists of gypsum, anhydrite and salts, at 27.43 %. These materials do not tell the story of an open river flowing to the sea, but of a closed lagoon that evaporated under an arid climate. As water diminished, dissolved substances became concentrated and eventually precipitated. In that sense, gypsum and salts are the mineral outcome of a landscape that lost water again and again.

The presence of evaporites adds a second scale to the landscape of Alcañiz. The Guadalope riverbank explains the vegetation that may have given the place its name; the gypsum and salts recall that the territory also experienced dry and enclosed environments. This is not a contradiction. The same municipality can bring together traces of running waters, evaporating lagoons and former marine or continental environments, provided the ground changed during its formation.

There is also a current consequence that should not be confused with an anecdote from the past. The risk map identifies areas of moderate karstification in carbonate formations, with possible significant and scattered karsts, and indicates areas with expansive clays: from a non-clayey substrate to sectors of low to moderate risk and others of moderate to high risk where clays are locally predominant. Limestones, dolomites and marbles represent 4.98 % of the subsurface. The data does not, by itself, explain any historical building, but it shows that beneath a single city, materials with very different responses to water can coexist.

From a Calatrava villa to a town of the council

The Guadalope and the ground did not determine Alcañiz’s politics, but they provided the material framework on which that politics left its marks. During the 15th century, the medieval council gained pre-eminence in Alcañiz and in Bajo Aragón, supported by different kings of Aragon, by the appearance of a local nobility and by the progressive deterioration of the military orders after the end of the Reconquest.

Urban transformation was associated with a new ring of walls, the so-called low wall, and with the enlargement and reorganisation of the town. The Casas Comunes, the plaza Mayor —the current Plaza de España— and the lonja expressed the growing weight of the council. The building that inherited the function of the Casas Comunes in the 16th century is the current town hall, where the Mural de la Justicia is preserved.

Also in that process, the local landscape became the setting for decisions that affected the entire Crown of Aragon. In 1411, Alcañiz was the seat of the parliament of the Concordia, created to choose the successor to Martín I el Humano. On 15 February 1412, Catalonia and Aragon signed the Concordia of Alcañiz there: nine compromise-makers, three for each territory of Aragon, Valencia and the County of Barcelona, were to meet in Caspe and decide who would occupy the throne under specified voting conditions.

The tremors recorded in the present

The episodes that organise the documented historical chronology of Alcañiz are the conquest, the presence of Calatrava, the expansion of the walls and the Concordia of 1412. The recent seismic record offers another kind of account, measured not in changes of power but in small, localised jolts.

Over the last 30 years, 13 events with a magnitude equal to or greater than 1.5 have been recorded. The maximum recorded magnitude is 2.3 ML. The most recent earthquake and also the most intense of the series occurred on 7 February 2025 at 12:34:26 UTC, with magnitude 2.3 ML and an indicated depth of 1 kilometre. The previous record corresponds to 29 January 2025, with magnitude 2.1 ML.

These data do not make it possible to turn Alcañiz’s history into the history of a major earthquake, because the documentation used does not link its main urban transformations to a destructive seismic event. They do, however, require that we remember that the ground remains active and that its differences matter: carbonate formations can present dissolution, while certain clays can change volume with moisture. The river, the sediments and the evaporites are not only chapters from the past; they continue to form part of the site’s physical conditions.

Alcañiz thus preserves two overlapping landscapes: that of the Guadalope, with its reeds and its meander, and that of a sedimentary subsurface made up of sandstones, conglomerates, flysch, gypsum, anhydrite, salts, sands, silts and gravels. Between them, the castle, the walls and the council city were built. As you move through it, the landscape no longer ends at the façade: it continues beneath the layout that the water and the ground made possible.