I did this walk with Michael G, a CBC mate, who was recce-ing this walk  Drive from Weston Creek via the southern suburbs through Tharwa.  Turn right into Orroral Road just after the top of Fitz's Hill and drive to the former Orroral Valley Space Tracking Station.  All sealed road, about 50 km and 1 hour's drive.

The main feature of Cotter Rocks is identified as Split Rock Pinnacle in ACT Granite, p 193.

This walk is number 12 in Graham Barrow's book Namadgi and Tidbinbilla Classics: Tough Bushwalks in Canberra's High Country .  It is rated as 20km walk distance, 610m total climb and 7 hours walk time.  We left Canberra at 7am and started walking at 8am.

Walk from the car park to the Walk Register, then up the path to the Cotter Hut Road.  Turn right (north-west) onto the Cotter Hut Road for easy walking and views onto Orroral Valley.  There were plenty of kangaroos early in the morning.  After around 4km, the road sharply turns to the south-west and climbs the valley wall.  As the road flattens again, look for the weather station in the bush on the right, before crossing Prarie Dog Creek.  500m further on, take the sign posted Australian Alps Walking Track off to the left.

This leg of the walk is through delightful (recovering from Jan 2003 fires) bush, the AAWT little more than a foot pad and easier on the eyes and feet than the fire trail.  Cross Sawpit Creek, then climb up to Cotter Gap, a well defined saddle.

The other indicator that we'd arrived was our first sight of 'the bum' (Split Rock).  Unmistakable on the still bare hill, a vertical fissure likens it to a body part.

Taking a hint from the Graham Barrow book and the lie of the land, we left the AAWT and headed so as to approach the climb up the hill from the east.  Crossing a swampy creek tributary we began sidling up the hill.  It was steepish, but open vegetation made the going ok.  Marvellous wild flowers.

We came around the base of Split Rock from east towards the west.  The 'bum' fissure was obviously not the split.  Climbing higher and further round, we came to the lower entrance to the split - truly astonishing!  We walked in and gazed around - walls around an arm's span apart, a carpet of shattered rock on the floor and, say, 15 metres high.  At the end was an L turn, with another split leading up to the top of the feature.  This was obviously an easier entry point - to climb to the top of the hill on the side and come into the split from the top.  We climbed onto the adjoining rock platform, took some photos down into the top of the first part of the split and had some lunch.

We came off the Cotter Rocks area a little to the west, rejoined the AAWT and endured the slog back to the car at the former Orroral Valley Space Tracking Station, where we arrived at around 3.10pm.

Distance: 20.4km  Time: 8am-3.10pm, with 50 minutes of stops.