I agree with TheWinWizard regarding the prey items such as worms, salamanders, frogs and toads.
That being said, burrowing is quite common for garters as they would burrow to find worms and salamanders, and further they are a denning snake in the wild. It's just what they do.
I have an Eastern Garter living in the recesses of my brick porch currently who occasionally sits up on top to bask in the sun. It can be quite a surpirse when you step out the front door unsuspecting. At least my wife, daughters, and mother in law who lives with us, are all OK with that. Its just our outdoor pet.
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