Fisherman -
Can you think of a way that these prints could have been made before these sedimentary layers were deposited?
Can you think of a way that these prints could have been left in solid rock?
No you can not.
Therefore these prints were made sometime after the sediments being deposited and before they were buried under subsequent layers.
Therefore if the deposition rate is such that this will occur within the margin of error for the particular dating method we can date the prints by testing the rocks.
You are wrong because we can date these prints within a margin of error by testing the rocks.
You are basically asking for nothing to be tested (i.e. the lack of rock that makes the footprint visible) and that is very similar to the creationist who demands the names of all transitional species from the beginnings of time to now before they will accept evolution.