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So they followed him wherever he went and when he began to bump into things, they began to bump into things, too. Finally he came to a concrete highway and he started up the middle of it and all the other creatures followed him.

What does the underlined part refer to? Does it mean "he started going toward the middle of it" or something else? Thanks.
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he started up the middle of it = he started walking along the middle of the highway.
Comments  
Mister Micawberhe started up the middle of it = he started walking along the middle of the highway.

Thanks, Mister.

Your reply is crystal clear.