oldest WAL files not removed

Hi,

Looking at WAL folder after a crash, I noticed that new files after
restarting overwrite the more recent files before the crash and not the
oldest, which was what I expected.

Is that normal ? I got only one file marked .deleted. Does that happens
when a WAL file hase been completed updated in the database and if then
while all oldest files aren't marked .deleted after restarting ?

Example :

Crash occurs Aug 31 22:03 which is the more recent Wal file, the oldest file
is Aug 30 17:20 (and 105 files between those two)

After restarting Aug 30 17:20 is still there, Aug 31 22:03 disappeared, one
new file is Sep 1 12:15 marked .deleted (restarting date) and one new Sep 1
12:36 which I guess is normal. Right now, I see an new wal file and the
previous one marked .deleted which is ok.

Why are the oldest wal files still there ?? Can I remove them ?

Hope I'm clear enough and thanks for explanations,

Olivier