In an earlier episode in season 4 of Game of Thrones, Jon Snow suggested to Ser Alliser to seal the tunnel through the Wall before Mance's army arrives, which Ser Allise dismissed.
My question is How would they have sealed the tunnel? The tunnel consist of an outer thick 4 inch rolled steel gate, while the inner gate is a thin slotted one. So what were they gonna seal the tunnel with?
From S04E07:
We need to prepare - Jon
We have been preparing - Ser Alliser
We should seal the tunnel. Block it with rocks and ice, flood it and let it freeze - Jon
Jon might have said "flood it with rocks and ice" but I thought it made more sense the way I quoted it.
In ADWD, Jon III Bowen Marsh says:
Let them return to their villages and fight the Others there, whilst we seal the gates. It will not be difficult, Othell tells me. We need only fill the tunnels with chunks of stone and pour water through the murder holes. The Wall does the rest. The cold, the weight... in a moon's turn, it will be as if no gate had ever been.
I'm not sure if/where the tunnel is described in more detail, but I remember seeing it suggested that there is a second tunnel above the main one which is how they would use the murder holes.
In the same conversation with Ser Alliser Throne Jon suggests a way to do so:
We need to prepare.
We've been preparing.
We should seal the tunnel. Plug it with rocks and ice. Flood it and let it freeze.
Game of Thrones, Season 4 Episode 7, "Mockingbird"
The reasons for not doing so are also stated:
And how would we range north?
We wouldn't.
Coward. You would cut off our legs, pluck out our eyes, leave us cowering behind the Wall hoping for the storm to pass?
...
First Builder Yarwyck, Lord Snow here recommends sealing the tunnel, leaving us unable to carry out our duties as sworn brothers of the Night's Watch. Do you agree with him?
No.
Game of Thrones, Season 4 Episode 7, "Mockingbird"
