Boulder Hill

The greywacke sandstone and siltstone that forms the bedrock of the Wellington region and also underlies the main axial ranges of the North and South Islands, is mostly covered by grass or regenerating forest over the Western Hills. The greywacke was deposited on the deep sea floor off the coast of ancient Gondwana, before being squeezed and deformed by plate collision at the edge of the Gondwana continent. These deformed and mildly metamorphosed sediments were later submerged as Zealandia split off from Gondwana, then slowly uplifted as the modern plate boundary developed over the last 20 million years.
Erosion has preferentially removed the softer material (siltstone) leaving blocks of the harder sandstone to collect on the surface. At Boulder Hill, as well as many other locations on the Western Hills, the boulders scattered on the surface are often particularly durable concretions that formed within the sandstone layers.
During the last glacial period, the hills of this area, although not glaciated (except in parts of the Tararuas) were windswept and had very sparse vegetation. The groundwater was mostly frozen (permafrost) up to or just below the surface. However, a certain amount of thawing would have occurred in places on sunny days, with water penetrating into the ground, freezing again at night and thus wedging apart any cracks in the rocks. This 'freeze-thaw' process loosened up the bedrock and eventually lead to landslides and soil creep (solifluxion). The displaced debris blankets many of the lower slopes and valleys around the Western Hills. The mixture of soil and boulders can be seen overlying highly weathered in-situ bedrock in many of the road cuttings and stream banks around these hills.

Boulder Hill is a remnant of what was once a fairly flat landsurface (peneplain) that has been uplifted along the Wellington Fault over the last 750,000 to 1,000,000 years. In that time erosion has cut into the surface to create valleys that break up the originally fairly uniform peneplain.
Enjoy the naturally sculpted boulders which give character to the otherwise bare hill top of Boulder Hill.
Why do you think some of the boulders are quite angular in form, with flat surfaces and sharp edges, whilst others are more rounded in appearance?