I fist saw the spider August 17th after we had been away for a week.
Over the next 10 days, the spider got fatter and fatter. This picture was taken on August 27th.
Then, a few days later I went out and the spider and the web were gone. In it's place was this thing that I feared might be an egg sac.
I called Tim out to look at it and he found the spider nearby with a new web. Just to be safe he removed the sac. Good thing, too because a little we research showed that this sac probably contains 1000 eggs. Though I don't mind spiders too much, I would not like to be swarmed with 1000 of them so close to the garage.