Crevices between blocks that are large enough to be entered.
Due to fundamental forces of planet Earth, there are continents that rise above sea level and are thus exposed to wind and weather and subject to erosion. The process of erosion also involves rainwater cutting valleys into the rock in the form of rivers. The combination of plate tectonics and erosion is the reason that our earth's surface is mostly not flat.
But the greater the differences in altitude, the greater the effect of gravity at these points. And so it happens that rock faces, valley slopes or escarpments collapse from time to time, so-called landslides. The fallen rocks form a fan of rock debris at the base of the cliff. The rock fragments, usually broken off at fissures in the rock, are irregularly shaped and angular. They are thrown on top of each other by the rockfall.
Thus, crevices between the blocks are very common, as their irregular shape means that they do not fit together without gaps. But only if the blocks are big enough, these gaps are big enough to form a cave, i.e. to be accessible by a human being. Because they are formed by boulder they are also called boulder cave.
From this we can deduce some peculiarities of this type of cave. They form very quickly, in minutes or even seconds, so there is no evolution of the cave and once formed it will change little or not at all until the next landslide. As these caves usually have no cave rive, they can fill in over time through alluvium, wind transport and animal activity. Material transported into them is not transported further. The rock is usually hard; in a landslide, the largest blocks are also the most stable rocks. Soft rocks usually disintegrate quickly into gravel and sand. Typical rocks are granites and crystalline rocks, which in turn are not water-soluble. Therefore, dripstones or other speleothems rarely form in these caves. The length of the cave is usually limited by the diameter of the rock block. In rare cases, several blocks coincide to form a bigger cave. In this case, the transitions between the blocks are often particularly narrow and difficult to navigate.