I have before remarked in speaking of the allotments of the gods, that they distributed the whole earth into portions differing in extent, and made for themselves temples and instituted sacrifices. And Poseidon, receiving for his lot the island of Atlantis, begat children by a mortal woman, and settled them in a part of the island, which I will describe. Looking towards the sea, but in the centre of the whole island, there was a plain which is said to have been the fairest of all plains and very fertile. Near the plain again, and also in the centre of the island at a distance of about fifty stadia, there was a mountain not very high on any side.
I have described the city and the environs of the ancient palace nearly in the words of Solon, and now I must endeavour to represent the nature and arrangement of the rest of the land. The whole country was said by him to be very lofty and precipitous on the side of the sea, but the country immediately about and surrounding the city was a level plain, itself surrounded by mountains which descended towards the sea; it was smooth and even, and of an oblong shape, extending in one direction three thousand stadia, but across the centre inland it was two thousand stadia. This part of the island looked towards the south, and was sheltered from the north.
There are not two plains mentioned in this part of the narrative.
The plain on the island Atlantis was 3000 stadia East-West, and 2000 stadia North-South. The south border of the plain was on the south coast of the island, and centrally located on that coast (the centre of the island). Again, central to the south coast of the island and 50 stadia inland was a small, solitary 'mountain' (a hill). On the East, West and North, the plain was surrounded by mountains.
The hill was where the city of Atlantis was founded and the area around it was enclosed by the canals the Atlanteans dug - one of which reached to the sea.