A frustum. Specifically, a frustum of a triangular pyramid -- as one could also have a frustum of a cone, square pyramid, etc.
Read more: What_do_you_call_a_triangular_pyramid_with_the_top_cut_off
square
It is a frustum of a rectangular pyramid.
Depends where you make the cut Across the planes it is a CUBOID Across the corners it is PYRAMID .
A triangle
the humidity may have contributed to it somehow
Not exactly. A frustum would be a pyramid with the apex cut off. A truncated pyramid would be a pyramid with all vertices cut off.
It is called a frustum.
It is the net of its solid shape. If the top of the pyramid is cut off parallel to the base, then it is a truncated pyramid, or a frustum.
It depends on how you cut it. It might be a frustrum, it might be a (somewhat differently shaped) pyramid, or it might be a wedge.
square
It would be a wooden cube that has been cut in half and painted red.
I would imagine the huge bricks that they are built of would have been cut out of sandstone, at a nearby quarry.
How to make a pyramid out of poster board?
It is a frustum of a rectangular pyramid.
Depends where you make the cut Across the planes it is a CUBOID Across the corners it is PYRAMID .
This would be called a gemstone.
It is the net of its solid shape. If the top of the pyramid is cut off parallel to the base, then it is a truncated pyramid, or a frustum.