>We can tell the difference between the two.
I don't exactly know who the "We" is, but I know that for many generations
the literature has not distinguished them, even after they were recognized
as sedimentary features. The best example of synaresis (thanks, Glenn!)
cracks I know of are the giant ones at the base of the Coconino in Grand
Canyon, although they are not described as such, because they have not to
my knowledge been studied since subaqueous mud cracks were described in the
1970's. They are one generation, irregular in shape, not polygonal, very
deep with parallel sides, and filled with the overlying Coconino Sand.
Art
http://chadwicka.swau.edu