World War 2 occurred in the late 40's, so this is most likely why there is a dip in that age group.
That is because metamorphic rock is formed by parent rocks that are either sedimentary rock or igneous rocks and we can use the former rock type to distinguish the type of metamorphic rock, marble was igneous rock exposed to high pressure or temperature.
While the other two type of rocks are formed initially by sedimentation and volcanic movements and has no parent rocks.
That would be the Gobi desert.
To get more rocky and it will get bigger and taller
