The geology here reads like a compressed textbook of deep time. The Hasmas Massif sits on a crystalline basement that dates to the Proterozoic, more than half a billion years old. On top of this ancient foundation, the Mesozoic seas deposited layer upon layer of conglomerate and hard limestone over six major phases, each interrupted by periods when the land rose above the waterline and erosion stripped away what the sea had built. Volcanic intrusions penetrated these sediments at the end of the Lower Cretaceous, adding another layer of complexity. Fossils from the Jurassic and Cretaceous periods are abundant throughout the park, traces of the marine creatures that once swam where mountains now stand. The tectonic forces of the Paleozoic pushed older Proterozoic rocks over younger Cambrian formations, creating a geological puzzle that researchers have been untangling for generations.

The Bicaz River begins its journey at Red Lake, a natural dam lake formed by an 1838 landslide just upstream from the gorge, and from there it descends through the canyon over a significant elevation drop. The main gorge stretches approximately ten kilometers, with the national road DN12C threading through it on twelve hairpin turns, some beneath overhanging rock, connecting Transylvania to Moldavia through a passage that has served as a natural corridor between the two Romanian provinces for centuries. But the Bicaz is not alone in its carving. Tributaries have cut their own side gorges into the limestone: Laposului, Cupasului, Sugaului, and Bicajelului, each decorated with rapids, waterfalls, and erosion potholes. The Sugaului gorge features petrifying springs that deposit calcareous tuff and travertine, building new stone even as the water dissolves the old.

The karst landscape within the park reads like a catalog of what limestone and water can create when given enough time. Vertical and overhanging walls, limestone towers like Piatra Altarului, and the solitary spire of Piatra Singurateca, the Lonely Stone, rise from the ridgeline. Clint fields stretch across Poiana Tarcau, their surfaces etched into geometric patterns by acidic rainwater. Below ground, pit caves like Licaas drop into darkness, while conventional caves thread through the massif. Above, the rounded limestone ridges of the Hasmas Mountains take on a peneplain appearance, their gentle profiles contrasting sharply with the precipitous gorge walls that drop away from their edges. Subalpine meadows cover the highest ridges, giving way to compact spruce forest on the slopes, mixed with beech, fir, and occasional larch at lower elevations.

The national park divides its territory into two management zones: a special conservation zone covering 78 percent of the area, and a protection zone encompassing the remaining 22 percent. Within these boundaries sit several distinct natural reserves. The Bicaz Gorge reserve alone covers 3,728 hectares split between Neamt and Harghita counties. Red Lake is protected as a separate reserve of approximately ten hectares. The Hasmasu Mare Massif, along with the Lonely Stone and Hasmasul Negru, forms another reserve of 800 hectares. Brown bears, gray wolves, and Eurasian lynx roam the dense forests, while golden eagles and peregrine falcons nest on the towering cliffs. The park's biodiversity reflects its position at the intersection of multiple habitat types: subalpine meadow, montane forest, limestone cliff, and river gorge, each supporting its own community of species adapted to the particular demands of life on, in, or above the stone.