The Wellington fault, which passes through the Hutt Valley, has been active for around 2 million years. This activity has caused the underlying bedrock to shatter and weaken, making it particularly susceptible to weathering by water. Over time this has led to the formation of the Hutt River and its associated floodplain.
The original vegetation of the Hutt Valley varied depending on its proximity to the river and the coast. A sandy flat near the seashore supported the native sedge pingao. Behind this flat were swampy marshes which extended about 3 kms up the valley. This wetland was full of raupo, tall flaxes and toetoe, and was criss-crossed by winding tidal creeks. Virtually the whole of the rest of the Hutt Valley was occupied by tall podocarp forests dominated by kahikatea but also included matai, pukatea, rata and rimu. Drier sites were flavoured more by totara, tawa and beech forests.
The biggest environmental change in the Hutt Valley over the last 200 years has been the clearance of most of the native vegetation and its replacement with pasture and later houses. Remnants of the original vegetation of the Hutt Valley do remain, usually near the banks of the river. These areas were susceptible to flooding and consequently were unattractive for building or farming.
A remnant of the large kahikatea wetland in the upper valley remains at Trentham Memorial Park, Upper Hutt, as Barton’s Bush. Another significant area of native vegetation is present on the lower hills at Keith George Memorial Park. Some large rimu and rata remain along with tawa, matai and beech.
While not showing the diversity of the original forest, remnant totara trees are found in Harcourt Park and Poets Park. Beech trees also line the riverside near Birchville.