The Beskids are built of flyschAlternating hard sandstones and soft claystones, laid down on the floor of a Cretaceous sea and later thrust up into mountains. – layers of hard Godula sandstone interleaved with soft claystones. The claystones act as slip surfaces: sandstone blocks slide slowly down the slope on them and the ridge comes apart.
Where the massif tears, cracks are left behind, and out of them grows a fissure cave. Cyrilka is one: a network of cracks on three levels linked by squeezes. In places the passages are barely a few tens of centimetres high, elsewhere up to eight metres, and here and there they narrow to tight joints. This is what sets the Beskid caves apart from karst ones: their passages follow vertical cracks in the sandstone, so the walls tend to be almost straight. That is why people long suspected them to be human work rather than the work of nature.
The Radhošť ridge is full of such cracks – they have long been called the Radhošťské ďůry, the Radhošť holes. Cavers have counted about thirty of them here.