Vlădeasa Mountains in the North of ANP have quite a complex geologic structure, mostly being made of magmatic rocks and cristalofien strata and less of sedimentary rocks. In Vlădeasa Mountains the platform is better preserved, the dominant character of the relief is due to the very long crests, almost flat with a dull relief. Deep valleys with very steep sides border them. The volcanic, rough rock manifests in some places by abrupt sides, with huge debris and slope fractures in the river basin, which produce waterfalls, the most representative being Răchiţele fall.
ANP includes 80% of the total amount of karstic rocks of the Bihor – Vlădeasa Mountains. The lithologic and tectonic complexity of the region created the necessary conditions for the formation of an exquisite karst relief, where all karst types in temperate climate region can be found.
The Exokarst includes forms such as the endoreic basin of Padiş – Cetăţile Ponorului, karren plains (Bătrâna – Călineasa), chaotic sink hole plateaus (Lumea Pierdută, Groapa de la Barsa), deep pans (Bălileasa), karstic valleys (Gârdişoara – Gârda), canyons (Ordâncuşa), defilés (Arieşul Mare), and, not at last, two of the deepest sink holes of Europe: Cetăţile Ponorului and Coiba Mare.
The Endokarst includes over 1,500 caves and potholes with special morphogenetic characteristics for each of the three major geomorphologic units, which include karstic rocks: The Padiş – Scărişoara plateau, the Someşul Cald graben and gthe piedmont region (towards the Beiuş depression).
In the Padiş – Scărişoara area there are a lot of descendant cavities (potholes and sink holes), which reach in depth down to the pressure level of underground waters, the access being closed by deep and long syphons. This is the case at the V5, Lumea Pierdută and Şesuri potholes or at the caves of Cetăţile Ponorului and the Zăpodie system. A special case is represented by the Gârda Seacă basin which shelters a series of superlatives such as Hodobana (the most labyrinthine cave in Romania) – 22.1 km long, Izbucul Tăuz (the deepest underground cave at -85 m) or in Zgurăşti cave, which shelters the largest underground lake from all the caves of Romania – 85 000 m3 of water.
To the Someşul Cald graben one associates large caves, with huge interior spaces and quite horizontal (generally the oscillations of level do not surpass the value of 150 m). This is the case at the Humpleu cave (without the Poieniţa pothole), Cerbului, Rece caves etc. Remarkable are here also two hydrological punctures, both with a second opening (the Poienita and the “cu Vaca” potholes) to the plateau, from which the aquifers are draining the water.
The piedmont area includes the relief between Budureasa and Băiţa Bihor. In this area, the severe erosion of the relief did not allow the formation of some large hydrokarstic systems, but there is in this area a great number of important caves both from their landscape and paleonthologic point of view, such as Urşilor, Micula, and Măgura caves.
The karst from the area of Cârligate – Valea Rea is special because of the first poligenetic endokarst of Romania. Valea Rea cave (21 km long) began its evolution due to postmagmatic hydrothermal substances, associated to the eruptive area of Vlădeasa. These created a mineral hydrothermal paleokarst and a relict hydrothermal karst. As a consequence of the end of postmagmatic activities and of the lifting of the northern Bihorul block during the Pliocen age, the pre-conditions for the formation of a classic endokarst of cold water, which partially re-organised the previous spaces, appear.
The cave from Valea Rea, besides the fact that it is one of the most complex caves of the world, is also considered as one of the top 10 cavees on Earth from the mineralogic point of view. Inside the cave there are over 35 minerals in the shape of speleothemes (cave pearls, gyps, quartz, celestit, malachite, rhodochrosite, metatyuyamunit etc.), a lot of these being described for the first time in speleologic environment.
From the landscape point of view, the most important caves from ANP are the following: Valea Rea, Piatra Altarului, Micula and Pojarul Poliţei caves; from a paleonthologic point of view: Urşilor and Onceasa caves; from an archeologic point of view: Vârtop and Rece caves.
ANP further shelters over ten important underground ice caves. Scărişoara Glacier (equipped for tourists) and Focul Viu Glacier being the most famous, among other glaciers such as Borţig, Barsa, Zăpodie, Vârtop (that from the Casa de Piatră area – do not mix up with the Vârtop Cave), and from Poiana Vârtop (the one in Ponor Valley) etc.
A unique phenomenon in Romanian landscape is represented by Groapa Ruginoasă, a ravine with a diameter of about 450 m and a depth of 100 m, made by water erosion in the layers of sandstones and red-violet clays of permo-werfian age, of the Ţapu Mountain. Being a proof of active erosion, the morphology of Groapa Ruginoasă is quickly developing, in rainy years the ravine increases by some meters. During rainfalls the water picks up rock fragments from the hole causing a noise that gets louder due to the echo and terrifies visitors. Due to the lithologic substratum it has a rusty shade that gives the name of Groapa Ruginoasă (Rusty Hole).
