Describe the geographical features of the kingdom of Brobdingnag?

Gulliver finds that much of the island is well cultivated, but to his surprise, when he comes across a hayfield, he realizes that the grass is more than twenty feet tall. Similarly, corn is at least forty feet high,

The kingdom is a peninsula, terminated to the north-east by a ridge of mountains thirty miles high, which are altogether impassable, by reason of the volcanoes upon the tops: neither do the most learned know what sort of mortals inhabit beyond those mountains, or whether they be inhabited at all. On the three other sides, it is bounded by the ocean. There is not one seaport in the whole kingdom: and those parts of the coasts into which the rivers issue, are so full of pointed rocks, and the sea generally so rough, that there is no venturing with the smallest of their boats; so that these people are wholly excluded from any commerce with the rest of the world. But the large rivers are full of vessels, and abound with excellent fish; for they seldom get any from the sea, because the sea fish are of the same size with those in Europe, and consequently not worth catching; whereby it is manifest, that nature, in the production of plants and animals of so extraordinary a bulk, is wholly confined to this continent, of which I leave the reasons to be determined by philosophers. ch 4