The Waitakere Limestone is a shallow-water temperate algal carbonate ranging from upper Eocene (Runangan) to lower Oligocene (Whaingaroan) in age. The principal components of the rock are coralline algae 37–72%, benthonic foraminifera 1–9.2%, bryozoa 1–10% and echinoids 1–4%, set in a spar matrix 13–32%. Glauconite l–9% is ubiquitous.
Surface temperatures may have been as high as 20°C, but possibly as low as 12°C; water depth was 1–12 m, with adjacent sea grass probable. However, no evidence was obtained for the widely recognized temperature drop at the end of the Eocene. known elsewhere in New Zealand. The limestone fits well into the cooler-water foramol type, and within this the bryozoan assemblage.