Also known as Inland Rock, quarries and cliffs contain exposed rock surfaces which are almost devoid of vegetation. The Belfast Hills are rich in limestone and basalt with numerous quarries but few natural cliff faces. Impressive natural cliff faces may be seen at Cave Hill. These were formed by a series of ancient lava flows. Quarries, both existing and abandoned, can be very important as they provide extra vertical faces for birds such as peregrine falcons and ravens to nest safely. Spoil and clay often collects at the bottom of faces and results in poor soil, ideal for unimproved grasslands. This is why many quarries have good orchid populations. Woodland perimeters have often been planted in the past to minimise impact on landscape and now provide cover and nesting areas for woodland birds.
Finally the lower stretches of older quarries in particular often have quite extensive ponds and wetlands, great for newts, wetland plants, waterfowl and aquatic insects such as damselflies. All these features, plus the fact that working and abandoned quarries are often comparatively secure and secluded for wildlife, combine to play an important role in the biodiversity of the Belfast Hills. Clever restoration of quarries can therefore be particularly rewarding, as long as they make the most of the above habitats and ensure that invasive species such as Japanese knotweed or Himalayan balsam, often found in abandoned quarries, are kept out.