The basement of the core mountains is formed by the Tatric Unit, which is composed of dominantly hercynianmetamorphic and igneous rocks and locally also a sedimentary cover of various thickness. In the Paleozoicclastic and volcanoclastic sediments deposited in the area, but later during Hercynian orogeny, sediments were metamorphosed in the greenschist and amphibolite facies. Regional and contact metamorphism was induced also by intrusions of granites, which accompanied the Hercynian collision. Basement rocks were later exposed due to erosion and at the end of the Paleozoic affected by marine transgression that lasted till the end of Mesozoic. During this period sedimentary rocks, known as Tatric cover units, deposited. They generally consists of clastic and carbonate rocks. In the Upper Cretaceous area of Tatric Unit, was affected by Eo-Alpine of Alpine Orogeny, that caused movement of large masses of rocks to the North and Northeast. To the South of the Tatric area a 2 superficial nappes, so called Subtatric Nappes, were detached and thrust over the Tatric. These nappes are generally composed of limestone, marl, dolomite and less by sandstones and slates. The lower nappes is called Krížna Nappe and the upper is Choč Nappe. Krížňa Nappe is dominantly composed of rock, that were formed in the Zliechov basin. It was situated in the Fatric area between the Tatric and Veporic units. Fatric had the same basement as the Veporic unit and is considered as former northern part of Veporic, but later during the thrusting its substratum was consumed and subducted. Surface occurrences of Fatric basement remained only as a remnants in the northern part of Veporic Veľký Bok Unit and few other areas. Choč Nappe, was thrust from the southern area, but its roots were probably consumed entirely, so its paleogeographic position is not exactly known. Choč Nappe is considered as rootless nappe. After the end of main Eo-Alpine orogenic movement, the area of Western Carpathians was still not so mountainous. In the Paleogene subsidence along the normal faults caused formation of grabens - intra-mountain Inner Carpathian Paleogene Basin and horsts present daymountain ranges. In the Neogene, further orogene movement in the external part of Carpathian arc caused formation of back-arcPannonian Basin. Northern branches of Pannonian Basin were penetrating the Tatra-Fatra Area and formed inter-mountain basins. Simultaneously with the immersion of the horsts, ongoing and intense erosion is denuding the mountains and supplies the basin with clastic sediments. Among all of the rocks, the most erosion resistant were the crystalline schists and granites of the Tatric. Similar fold and thrust model of origin is known from the other areas of alpine system. In the English terminology, similar model is called Fault-block mountain, it is typical in the area of Basin and Range.
