A son of "Mikjell Olsen" and "Anna Tomsasdatter" will NOT be named Edmundson. By traditional Norwegian naming practice and pattern (patronymic naming) he would have been a Mikkelsen which could be spelled Mikjellsen by some.
Perhaps Edmundson is some name used for him after he emigrated from Norway... it would not be used in Norway.
What is your source for this information that you have?
Where is Garretson? That place is mentioned in Norway references to that family after 1882 and it is unknown to me.