This pic is from the same IOD link to pics of the Yehliu area.
We see something like these "wash tub" formations in "freestone" rivers of the western US.
The current swirls a small rock around inside a depression, and over time the "tub" forms.
But look closely at the center in these formations.
It looks as tho there are rocks in there, but they each have cracks
... and each of these cracks is oriented in the same direction !
Anyone have some sort of explanation for these formations ?