"The Ambarawa Plain, which is located approximately 475 metres above sea level, is one of Java's ancient lakebeds. To the West, its boundaries are the Jambu mountains; to the East, its boundaries consist of the marl-hills of South Semarang. It became a lake when the volcanoes Merbabu and Telamaya (or Telomoyo), which are located to the South, and Ambarawa, which is located to the North, erupted and poured out volcanic material.

The lake eventually acquired a gradual discharge to the West, and, after several centuries, this River Tuntang has largely succeeded in draining the plain. (This is puzzling, as the Tuntang flows to the East, and this must be considered to be a mistake.)

Part of the plain which is still submerged forms Rawa Pening, an oval swamp, 4km long and 2.5 km wide, into which a number of small rivers deposit their mud and silt. Rawa Pening however, has a bottom which largely consists of soft peat.

In the past, the swamp was much larger in size. Evidence for this statement is provided by the names of several villages around Rawa Pening, all of which have names with "Rawa" in them.

In 1838, 1845, 1865, 1868, and 1885, the development of gases in the area caused small hummocks, a few metres high to be formed in the lake, most of them on the Western side. Some half-decayed trees were then seen to appear."

"The Tuntang river - A river located in Central Java, the Tuntang river carries the discharge from Rawa Pening (see Pening [Rawa]). Afterwards, the Tuntang breaks through the hills which close the basin of Ambarawa in the Northwest, to form a beautiful waterfall. (Again, this is puzzling, as the Tuntang flows to the North East from Rawa Pening).

Upon reaching the coast, water from the Tuntang is used in the Glapan Dam to irrigate the Demak Division. Nearer the sea, the Tuntang splits into the Demak River, its main arm, and the Banjaran River".