Moria is old.

According to The Simlarillion, it was founded by Durin I sometime after the elves awoke, during the Years of the Trees. This was a time before there was recorded time, before there was a sun and a moon, when all light in Middle-Earth came from two magical trees that grew in the far west, in the land of the Valar.

Tolkien was unclear as to how long time periods in the early ages of Middle-Earth were, not least because he kept changing the conversion factors. However, I found several estimates online that put the Years of the Trees at about 10,000 modern years. Durin awoke toward the end of this period, but this still gives him and his descendants a couple of thousand years to get starting mining the mountains.

We then have the First Age. Again, a modern time estimate is difficult because of the changes Tolkien made to his calendars but Wikipedia says it lasted between about 5,000 and 65,000 years.

Next there's the Second Age. We can be more specific at this point because the calendar used corresponds to the modern one, and it lasted about 3,500 years. The events of Lord of the Rings take place at the end of the Third Age which was around 3,000 years. It was during the Third Age that Moria was abandoned, about 1,200 years before the book begins.

So even the most recent rubble from the delvings of Moria has had 1,200 years to weather in the storms of the Misty Mountains. If you look at most monuments of that age that still stand in the world today, there's not a great deal left of them, even though they were built to stand and there have been strenuous efforts to preserve them. How quickly, by contrast is a slag heap going to be reduced to local pebbles and boulders?

To hammer the point home, that's the most recent excavations. Moria was a great city many thousands of years before that point and it seems a safe bet that most of the construction was done back in the glory days of the dwarves and elves. The remains of that have had anywhere between 10,000 and 70,000 years to weather away.

Moria is old.

According to The Silmarillion, it was founded by Durin I sometime after the elves awoke, during the Years of the Trees. This was a time before there was recorded time, before there was a sun and a moon, when all light in Middle-Earth came from two magical trees that grew in the far west, in the land of the Valar.

Tolkien was unclear as to how long time periods in the early ages of Middle-Earth were, not least because he kept changing the conversion factors. However, I found several estimates online that put the Years of the Trees at about 10,000 modern years. Durin awoke toward the end of this period, but this still gives him and his descendants a couple of thousand years to get starting mining the mountains.

We then have the First Age. Again, a modern time estimate is difficult because of the changes Tolkien made to his calendars but Wikipedia says it lasted between about 5,000 and 65,000 years.

Next there's the Second Age. We can be more specific at this point because the calendar used corresponds to the modern one, and it lasted about 3,500 years. The events of Lord of the Rings take place at the end of the Third Age which was around 3,000 years. It was during the Third Age that Moria was abandoned, about 1,200 years before the book begins.

So even the most recent rubble from the delvings of Moria has had 1,200 years to weather in the storms of the Misty Mountains. If you look at most monuments of that age that still stand in the world today, there's not a great deal left of them, even though they were built to stand and there have been strenuous efforts to preserve them. How quickly, by contrast is a slag heap going to be reduced to local pebbles and boulders?

To hammer the point home, that's the most recent excavations. Moria was a great city many thousands of years before that point and it seems a safe bet that most of the construction was done back in the glory days of the dwarves and elves. The remains of that have had anywhere between 10,000 and 70,000 years to weather away.