Geology & terrain

The Gorbeia massif forms the back-bone of this whole area, and the basis for the delimitation of the 20 official districts of the Basque Autonomous Community. However, the "cuadrilla" of Zuia can also be classified according to other physical and socio-economic criteria.

Geologically, most of the area is dominated by cretaceous rocks, mainly sandstone, clay, loam and chalk. The east-west axis from the Altube valley to the Elgea hills is marked by a broad strip of chalk and sandstone dating from the Lower Cretaceous period around 140 million years ago. The main rivers of the area run crosswise over this strip.

The highest points in the area (some over 1000 m in altitude) consist generally of chalk and other erosion-resistant sedimentary rocks on Gorbeia, at the top of the Aramaio valley and in Elgea.

In the south of the district, from the Gibijo hills to the Ullibarri reservoir there is a strip of loam and chalk dating from the Higher Cretaceous (100 million years ago), from which roughly flat highlands protrude, ranging from 800 to 1000 m (the hills of Gibijo, Badaia and Arrato).

The Cretaceous rocks are broken up at some points by Triassic gypsum and clay (200 million years old), mainly in the Zuia valley and the eastern side of Urkabustaiz. On top of these materials lie later deposits of sandstone and other materials from the tertiary Period (5 million years ago).

Over the last 2 million years (since the Quaternary Period) the rivers of the area have deposited fine gravel, sand and pebbles throughout the centre of the Zuia valley and in the south of the municipalities of Zigoitia and Arrazua-Ubarrundia.

The intrusion of light material into pre-existing rocks to form what is known geologically as a diapir is much in evidence in the region, in the outcrops of Triassic rocks and the subsequent development of the Zuia valley. The chain formed by the mountains of Gorbeia, Urkiola/Arangio and Elgea form the southern flank of the external anticline of Bizkaia, while the dips in the land to the south are part of the Zuazo and Apodaka anticlines.

The type of substrate combined with the effects of tectonic movements and of erosion in particular have resulted in a mosaic of different types of terrain, so that the parts of several different geological features extend into the area:

• The western hills in Urkabustaiz, southern Zuia and southern Zigoitia: the Gibijo, Badaia and Arrato hills.
• The external anticline in much of Zuia, Zigoitia, Legutiano, Aramaio and Arrazua-Ubarrundia: the Gorbeia massif, the Arangio, Legutiano and Elgea, the valleys of Altube and Aramaio.
• The southern river basins on the southern banks of Zigoitia, Legutiano and Arrazua-Ubarrundia and the plains.
• The Murgia diapir in the centre of Zuia, the eastern end of Urkabustaiz and the Zuia valley.

The highest point in the district is the summit of Gorbeia, at 1482 m, and the lowest is Ibarra de Aramaio, at 301 m.

The highest peaks are in the Gorbeia massif (Gorbeia 1482 m; Arroriano 1343 m; Berretin 1226 m; Oketa 1031 m; Nafakorta 1017 m) and in the hills of Arangio/Urkiola (Orixol 1129 m; Santikurtz 1113 m; Izpizte 1061 m).

The peaks of the southernmost hills are mostly between 800 and 1000 m in altitude, and include Txobita (873 m) in Gibijo; Ganalto (900 m) in Badaia; Peñas de Oro/Atxabal in the heart of the Zuia valley; Armikelo (888 m) in Arrato; Jarindo (896 m) in the hills of Legutiano; and Usakoaitza (912 m) in the hills of Elgea.

Much of the land is at elevations of between 500 and 800 m, including the bottoms of many small valleys and broad expanses of open land such as the plains of Alava/ Araba. Most of the major towns and villages are in these areas, including Izarra (631 m), Murgia (618 m), Ondategi (600 m), Legutiano (575 m) and Durana (541 m).

At elevations of between 300 and 500 m there are two exceptionally beautiful valleys: Altube (335 m) and Aramaio (301 m).

Many of these areas have sites of exceptional geological, educational and recreational interest, including the Nervión and Goiuri waterfalls, the karsts of Gibijo, Badaia and Arrato, the caves of Mairuelegorreta and the area around Peñas de Oro/Atxabal.