The punctuation mark that is not used internally in a sentence is a period (.). Periods are only used when ending a sentence therefore, it should only put at the end. Periods signal that the sentence has already ended and no continuation needed.
Erosion is when it rains and the rain wears down rocks making it bumpy, smaller, and deformed. With the floor incorporating rock the rain would make the earth deformed causing damage with the ground changing which can cause earthquakes and such. I hoped this helped you understand a little
