The middle mountainous and highlands region is the largest of the geographic regions and covers about three-fourths of the state. The region mostly comprises the hills and mountains of the Eastern Ghats which rise sharply in the east and slope gradually to the eroded plateaus in the west running from north-west (Mayurbhanj) to the south-west (Malkangiri). Geologically the formation is quite old. It is a part of the Indian Peninsula which was a part of the ancient landmass of Gondwanaland. The rivers of Odisha and their tributaries have cut deep and narrow valleys in this region. The Odisha highlands are also known as the Garhjat Hills. This region is well marked by a number of watersheds. A number of river valleys of varying width and flood plains dissect the Eastern Ghats in this region. The average height of this region is about 900 meters above the mean sea level. The highest peak is Deomali.