Twenty minutes east of Hamilton lies the town of Penshurst, home to the extinct volcano Collorrer (Mount Rouse). The mountain is a massive accumulation of scoria, rising 100 metres above the surrounding volcanic plain. Its high relief offers an important vantage point from which to view the lavas and adjacent volcanoes of Mount Eccles and Mount Napier.

Collorrer (Mount Rouse) is built mainly of red and brown scoria with thin, interbedded basalt lava flows. The scoria forms an arcuate mound opening towards the south-west, giving the appearance of a breached cone.

To the south of the main scoria cone is a deep circular crater with a small lake and a smaller shallow crater rimmed with a small lake and a smaller shallow crater rimmed with basalt. Past lava flows from Collorrer (Mount Rouse) followed shallow, gently sloping river courses, extending at least 60 kilometres south. A thin basalt lava flow contained in the scoria cone has been dated at approximately 1.8 million years old. If this is accurate, then Collorrer (Mount Rouse) marks the beginning of the second (younger) phase of activity in the Newer Volcanics Province.

The Collorrer (Mount Rouse) summit has track access and provides panoramic views of the lava plain and surrounding district. At its base is a deep circular crater and lake rimmed with blotchy spattered basalt.