Logging on rollback?

I have question on logging during a rollback and aborted long transaction. My understanding was that activity during a transaction is written to a logical log and if that transaction is rollbacked, the rollback is also logged. Thus, if you have a transaction involving 1 Meg of activity, then rollback the transaction, there will be approx 2 Megs of logical logs used. Also, I assume no logs ( involved with the begin transaction ) can be freed until the rollback is completed. Our config has high-water mark set at 50%. During an alter table operation ( not by me!), data involved was just over 50% of avail. logical log space which triggered an abort of long transaction. By looking at the engine log, I could see that 18 logs ( of 37 logical logs) were filled, then long transaction abort. BUT after that, logs weren't filled during what I assume is a rollback. I have less than a year of experience and am no expert so any help would be appreciated ! I just want some clarification.. Thanks, -Lin