Oh that one is easy.
In the last ice age the continent was covered with a couple miles of ice. That pushed a lot of stuff downward.
As the ice melted, the sea rushed in allowing for the animals you saw to die and fall to the floor.
Finally as the glaciers receded completely, the earth under them springs back up. A couple hundred feet is nothing when compared to huge glacier.
According to what I’ve read sea level rises during that period are measured in the tens of meters, not hundreds. And I’m not so sure there was ice in this area. I’ll have to check.