I believe CoA's response is in the right direction. Pragmatically, the writers may be presuming a fan base such that they don't have to depict some details in the script any more.
What was the explanation of the 'zombie pit' again? Anyone catch that? Something about a group that had left the safety of the town and started another place?
The sense I got was that a few employees barricaded off the quarry with trucks and tried to establish a safe community with a secure perimiter there. However, they got turned into zombies. As time went on, more zombies fell into the quarry pit and could not (easily) escape it. So, over a period of months, with the zombie arrival rate being somewhat larger than the zombie departure rate, the quarry became a densely populated with zombies. The writers seem to expect and require an audience grasp of reasoning basics that are a bit higher than the norm. In almost every episode, the writers present dilemnas to the ensemble characters and then deliberately eschew cliched and trite hollywood style resolutions (eg good guys always wear white hats, good guys always prevail, etc). In that sense, the show is a bit refreshing.
It is interesting to contemplate the show's zombies as being metaphors for modern day liberals. Both zombies and liberals are slow-witted. Both zombies and liberals congregate and move in herds. Both zombies and liberals are easily guided in a given direction. Both zombies and liberals are easily distracted by loud noises. Etc. The metaphorical interpretation seems to me, on the whole, to be about as valid as the metaphorical interpretation that the pod people in "Invasion of the Body Snatchers" allegedly represented rabid anti-communist conservative blowhards during the 1950s era of the so-called "Red Scare."
Thanks for the quarry explanation.
Like you, I appreciate that TWD allows us to use our brains and make our own connections.