I lived in Minnesota for a while as a kid. The terrain is largely made up of glacial moraines with peat bogs in between them, or in many cases lakes and chains of lakes. Just like your marsh, when one walks on the peat bogs, it's like walking on a trampoline or a bed - all squishy and maybe your feet get wet if they push down too heavily on the peat. Once a crane dredged out a peat area nearby our house, and there were mountains of the brown peat around the open water area they cleared, maybe the size of two story houses with attics in height. I used to take bags and a handsaw, and cut out blocks out of the peat 'mountains' to bring home to shred-up and use in our garden.