"aux amarres rompues" means that the canons used to be moored to their places and that, probably following a storm, the moorings went torn, so the canons began rolling on the floor.
"labourer" is an image to express the fact that the wheels of the canons rolling on the floor of the ship were so heavy that they were digging a sort of grove on the boards (since they couldn't turn, their va-et-vient was rolling always exactly on the same line, so that in the end you could see the marks of the wheels like a furrow on a corn field).
"semblent rouler dans la crasse" means that, to the eyes of the pilot, the mountains seem to be moving instead of the plane, as if the plane was still. Then, "dans la crasse" means that the landscape must be full of dust, probably because of the wind, and "St-Ex" sees the mountains moving through and above that dust, as if their movement was the cause of the dust.
I hope I've helped you make it clear.