Garter Snake Burrowing?

[spaztik]

Alright, I was at my friends house last night for his birthday, when my dad calls me up, asking if i moved my snake anywhere. I said no, it's in the tank, look under stuff. So he did, and he couldn't find it. Then i had to go home and find it....well, i took out every rock, stick, plant, and he wasn't there. I stir around the substrate (peat moss by the way) and still can't find him. Then my dad tries, and ends up finding him burrowed all the way in the back corner...I woke up this morning, and he burrowed again, (but this time there's a little opening in the substrate, so i know he's there)
Okay, here's the question, is it normal for Garter's (I have a Butler's Garter, I believe), to burrow? How do i get him to stop burrowing? He's really boring when he's underground and i can't see him.