The base of the pyramid is a square. If the crosssection is parallel to the base, the shape formed would be that of the base and it is a square
If the crosssection is perpendicular to the base, the shape formed would be that of the triangular face and it is a traingle
The other shape that can be formed when the cross section is perpendicular to the base but not actually at the vertex, the shape formed is a trapezoid
The shapes are
square
triangle
trapezoid