The Hășmaș mountains falls within the center, the so called crystal-mesosoic sector, from among the three big geological units, which is bordered by the Giurgeului mountains in the west, and by the sandstone sector of the Cretaceous Age in the east. Its base rocks are more than four billion years old metamorphic rocks, on which it deposited a thick sedimentary rock bed during the Earth's History. This process happened as follows: by the end of the Paleosoic era a stratum of cristalline shales, which formed the base of the Hășmaș mountains, had drowned, and they warped up concentrically on the western, eastern, southern rim of the mountain. In this way was formed an enormous trough-like indenture, the geosynclinal of the Hășmaș-mountains, which had been overrun by the trias sea. Sedimentary, limy stratas of the Cretaceous Age were deposed in it. According to C. Grasu this process has happened in 6 cycles, which were followed by such periods, when the seabed had risen up, and it has become subaerial, and the erosive processes wore down a section of the strata risen to surface.
According to the Geologic Literature in the region of Hășmaș there are two classes of crystalline shales. The older, so called "Rarău or Hășmaș series" metamorphosed intensively; it is composed of mezometamorphites, of sharp gneisses and of mica-schists; which are put in strata by half-metamorphosed, vulcanogenic migmatites and amphibolites. This process had happened very long ago, in the Azoic Era.(M.D. Codarcea 1967). While on the southern rim of the Hășmaș geosynclinal the rocks of the Rarău series are present in a broader stripe, on the eastern rim they are present only in the form of a very narrow ribbon. Rocks silvern from the white mica can be found everywhere along the phreatic divide, so on the Pongrác peak (Bányai János), along the Vereskő brook, at the piedmont of the Calului mountain, Ciofronca, Hășmașul Mare, Piatra Singuratică, Ecem, Terchiu up to the Olt valley. But northeastwards from this gneissose base rocks grow up the chalk cliffs of Munticel, Licas, Vit, and over the Bistra Mică the Piatra Roșie.
The younger class of the crystalline shales are the so called "Tölgyesi series". They are composed from epimetamorphites, phyllites ( silver sericite shale, green chlorite shale, graphite shale), quartzite shale, and from acidic magmatite rock sequence. This, almost 3 kilometres wide, sequence forms the Giurgeului mountains(except the Ditrău syenites). It begins from the valley of the Békény, then along the serpentines towards the Pongrác peak reaches a 10 kilometres latitude; while in the eastern boarders it hardly reaches 1-2 kilometres. According to the Earth History it could have been formed in the late Proterozoic era.
Paradoxically, while the older Rarău-Hășmaș series are posturing on top, the younger Tölgyesi series are posturing below, in the point of view of spatial position. This reverse order had been formed at the time of the Hercynian diastrophism, 360 billion years ago, when a west-east bound orogenic movement-serial pushed on the older sequence to the younger one(structure with overthrust fold). The sedimentary sequence of the Mezozoic Era are composed from formations of the Trias and Cretaceous.
Stratum of trias on surface can be seen primarly from the western rim of the Hășmaș to the Red Lake. They can be met in two ways: in form of normal deposit and subordinate deposit. The lower trias is on the Rarău-type crystalline shales, and like every transgression begins with small pudding stones. A quartz sand layer comes to this, then a grey rag, which is broken with dolomite limestone deposits. Its thickness is maximum 20 metres. The mid-trias is formed by dolomites and dolomite limestones. This is the most significant sequence of the Triassic Period, its thickness is 400 metres. It is a white, mildly veined stone, which joint is filled with detritus. It formes not only the horsts of the mountains' eastern and western rims(Licas, Munticel, Calului, Ciofronca, but also the lower side of Hășmașul Mare-Ecem mountain chain's cliffs.
The rock sequence of the mid-trias turns up also at the eastern rim of the syncline, but not continuously, at the ridge insulating the valley of Bicajel and Domuk brooks. The rock sequence of the upper trias(composed from reddish sandstone, whitish yellowy limestone) turns up mainly around the Red Lake and at the bottom of the Piatra Roșie, along the Oii brook, on the western slope of the Domuc valley and in the cliff of the Ecem peak. Further improvement of the trias was broken off at the end of the upper Trias.
The sequence of the Jurassic Period begins with an other expansion of the sea, with lias. It is formed by reddish, sometimes greenish shales, psammitic marls, and at the top by reddish, marly, rimous limestone stratum. It doesn't form a continuous sheet, as sporadically it occupied the basins and indentures of denuded trias deposits; and so it breaks, but turns up very often and on many places. Its best-known occurrence is at the head of the Frumos brook or at the mountain-pass between the Piatra Singuratică and the Ecem. It is very rich in mollusc fauna( shell, snail, coral fossils). The sequence of the lower lias is usually reddish, irony stained, here and there yellowy faded limestone. and also dolomite limestone. It turns up at the bottom of the Piatra Roșie, on the western side of the Suhardul Mare and on the eastern slope of the Calului mountain. The dogger is composed of grey and reddish calcareous sandstones, psammitic limestones and rags. It is very extended in the region of the Red Lake, it can have a thickness of 200 metres.
The sequence of maim is the most significant formation of the Hășmașul Mare syncline, because the limestone-walls of the Red Lake and of the Bicaz gorge were formed in this period. The layer-sequence of the maim begins with a slim radiolarian silexite stratum. It is called also jaspis level. It turns up on the western side of the Piatra Roșie, in the mountain-pass of the two Suhards. Its best-known occurence is in the canyon of the Javardi brook.
The next sequence is 20 metres thick, it is equal to the Cimmerian stage. Its rock-matter is reddish nodular limestone, reddish mild limestone, marly limestone, greenish-grey sandstone. In this sequence we can find the mountain's richest fossil sites. Its type fossil is the Aspidoceras acanthicum, from which the whole sequence got its name. Its distribution: the bottom of the Piatra Roșie and Suhardul Mic, the rim of the Poiana Albă, the bottom of the Hășmașul Mare and Ecem cliff. The maim's upper formation is the Tithonian stage. Its sequence is the 400 metres thick Stramberg limestone. It is a homogeneous, whitish-grey coral limestone. The mass deposit of the corals points to the fact, that on this part of the maim sea could be a hot watered, shallow shore; which was barricaded from the eastern side of the syncline with a huge atoll. This sequence forms the whole mass of the Hășmașul Mare, Piatra Singuratică, Ecem, Piatra Roșie, Suhardul Mic and Cichibiuk. The upper part of the Bicaz gorge was formed in the Upper Jurassic Period; while its lower part in the Cretaceous Period.
The other Cretaceuos sequence southwards from the Hășmașul Negru, at the stria of the Hășmașul Mare is fallow yellow solid limestone. In the Cretaceous Period the atolls had grown so continuously, that it is very hard to distinguish between the two senquences.
The next sequence, which also had deposited continuously, is whitish-grey or yellowish pink; mildly reddish marbly solid limestone. This rock is the Barremian-Aptian stage. Its solid microcrystalline rocks can be found on the ridge of the Ecem, Hășmașul Mare, Hășmașul Negru and Piatra Roșie. Its hundreds of metres thick sequence forms the Piatra Altarului and the Piatra Bardosului. On both sides of the gorge the cliffs are formed of Urgonian limestone. The Bicajel canyon was formed of this, as well.
The geological happenings won't be complete if we wouldn't mention, that the sedimental rocks of the syncline were broken through by vulcanic rocks. Its presence points to big fault-lines, along which magmatic matters had risen to surface. Its matter are undersatured, metamorphic melaphyre and diabase. Because they cut through the Aptian-Albian stage, they could have been formed at the end of the Late Cretaceuos Period. Diabases and melaphyres can be found in the valley of Bicajel, under the Piatra Altarului, and in the Oii brook.