How can older rocks be found on top of younger rocks in a cross-section?​

So If the older rock is sedimentary, such as a stream deposit, the layers will indicate which way was up when deposited. When you find this layer overturned on top of a younger rock, it’s likely part of a fold structure and should have a trend matching other rocks showing a fold.

You already know from surveys that the complete section of the area has a layer of A below a layer of B. If you find A above B, some event overturned it

There is a marker bed such as a layer of volcanic ash that records a single event in the area, then you have a reference point in time.