The only individual of hybrid Polystichum aculeatum x braunii i found within Raabklamm till now. A clearly intermediately shaped fern of large size at silicatic rocks, close to the path and river Raab.

The lower half of so called "Große Raabklamm", or saying the southeastern part as here below the castle Gutenberg, does consist of metamorphous rocks, rich in silicates and +- acidic. Polystichum braunii, Braun's shield fern is present there, but not in great numbers as in other sites. Hard shield fern, P. aculeatum is present in low numbers at few scattered places, thus chances for hybridisation do seem to be low.

The upper part to the northeast is made of metamorphous limestone ("Schöckelkalk", of marble consitence) downwards to the small river Raab.