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.
A triangle
If you were to cut a cube in half along any plane passing through its center, you would get two equal halves of a cube. Each half would still have the shape of a cube with the same dimensions, just split into two separate pieces. The resulting solid figures would be two smaller cubes.
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
A square pyramid with the top cut off is called a frustum of a pyramid. Specifically, when referring to a frustum that has a square base and a smaller square top, it is often called a square frustum. This shape retains the characteristics of a pyramid while having two parallel square bases instead of one pointed apex.
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.
If you cut a pyramid perpendicular to its base, the resulting cross-section will be a polygon that is similar to the base of the pyramid. The specific shape of the cross-section will depend on the height at which the pyramid is cut; as you move higher up, the cross-section will gradually decrease in size while maintaining the same proportions as the base. If the cut is made at the very top, the cross-section will be a single point, while cuts at varying heights will yield progressively smaller similar polygons.
How to make a pyramid out of poster board?
It is a frustum of a rectangular pyramid.
This would be called a gemstone.