They are sedimentary, yes. In a lake bed setting, imagine clay particles settling out of suspension and forming a thin layer of mud ontop of the original rock. Now imagine that the lake persists for a million years, that deposit of mud on the lake floor has now grown hundreds of metres thick.

Billions of years later, the rover visits the site. The lake water is long dried up and that mud has turned into stone, which has been heavily eroded. The rover landed at the lowest point in the crater, where you can see where that very first layer of mud sitting ontop of the original rock. Therefore, we first encountered mud formed at the very beginning of the lake's history. In the years since, the rover has driven steadily uphill, climbing through sequentially higher mud layers and so 'further in time'. So, as the rover ascends upwards, we see progressively younger layers. We are now at the time when the lake dried up.