showcaves.com specializes in underground attractions, and a gorge is not necessarily underground. But anyone who has ever been in one of the many gorges in the Alps knows how close a gorge can come to a cave. If such a slot canyon is only a few metres wide but maybe a hundred metres deep, you can not see the sky for a long time. And also daylight shafts and cave ruins often have no roof and are still to be attributed to the caves. Thus, it is a borderline case.
Another common feature is that gorges and river caves have a similar genesis, both were created by the erosion of a river. And both represent a unique and spectacular geotope. We have therefore decided to include these gorges, the very narrow and extraordinary canyons, as well.
There are obviously millions of valleys in the world, large and small. As long as they are not really exceptional and also interesting for tourists, they are not eligible for this category.