comma before present participle: they disappeared leaving a trail

[sam1234]

Does the following sentence need a comma where the * is marked? If so why?

We watched the dolphins for about 15 minutes; in the end they disappeared *leaving a sparkling trail like a shooting star.


Thanks for any help.

(Whoops, spelling mistake there!)

 

[sam1234]

Also, I'm pretty sure there is a comma needed before 'so' in the next sentence, but can't work out why. That is, what 'type' of comma is it?

The wind dropped and we were making slow progress, so I went to sleep in the bow cabin.

Cheers

 

[chamyto]

In my opinion comma should be used after "disappeared", because you are adding information.

Am I right ?

 

[sam1234]

Actually, I think you're right.

Anyone got any ideas about the next one?