West Harnham Chalk Pit: A journey back in time, 66 million years & more.

October 2025

The city of Salisbury is built upon approximately 200 metres of chalk bedrock that formed during the Upper Cretaceous, a geological time period between 100 & 66 million years ago.  At that time, Earth was experiencing a warming climate, & rising sea levels meant that much of southern & eastern England was submerged beneath warm shallow tropical seas.

Living in the warm upper layers of the shallow tropical seas were billions of microscopic marine organisms, so small that they could only be seen using a SEM, or Scanning Electron Microscope. Among these organisms were a type of single-celled phytoplankton known as coccolithophores, & along with other marine organisms played an important role in the marine carbon cycle by removing dissolved carbon from seawater & using it to produce tiny calcite plates or shells, called coccoliths.  Under the SEM these plates resemble miniature hubcaps (see image below).

Figure 1. Coccolith viewed under SEM.

As these organisms died, their skeletal remains (plates) & so the carbon they were produced from rained down & settled onto the sea floor & over millions of years layer upon layer of this material built up into a soft sediment that eventually becoming compacted & hardened into the chalk we see today.​​​​​​​​​​​​

Figure 2.  Large area of chalk bedrock exposure during a recent housing development to the south west of Salisbury.

One of the best places to observe Salisbury’s underlying Upper Cretaceous geology is at West Harnham Chalk Pit, located on Harnham Hill, a few miles southwest of Salisbury city centre. This site offers a rare glimpse into the region’s deep geological past & is a SSSI or Site of Special Scientific Interest, a Natural England designation given to sites that have been identified to possess features of special interest. In the case of West Harnham chalk pit, the site was designated a SSSI because of its exposure of the Lower Campanian Newhaven Chalk Formation containing ​remarkable fossils including echinoids also known as a sea urchin, & belemnites. 

Figure 3. Exposed face within West Harnham Chalk Pit.

Many of the fossils from West Harnham were collected by Dr H. P. Blackmore, a local physician & amateur geologist. His interest in the natural world, archaeology & geology, shared with his brother William, led to an extensive collection from worldwide travels including fossils from Wiltshire, with the collection originally displayed in the purpose-built Blackmore Museum opening in 1867 in St Ann’s Place in Salisbury. 

Figure 4. Doctor Blackmore seen here in the Blackmore museum (Salisbury & South Wiltshire Museum).

Sadly, the Blackmore museum closed in the 1960s with the collection dispersed to various institutions including the Natural History Museum in London. The Blackmore Museum itself was later converted into flats.​

Figure 5.  Salisbury's former Mill with later 19th century extension incorporating large amounts of flint.

Figure 6. Samples of flint

Contained within the chalk bedrock are nodules of flint, a microcrystalline rock, thought to have derived from the silica contained within the skeletal remains of other marine organisms such as sponges, deposited via biochemical reactions soon after the deposition of the marine algae that produced the chalk.  Flint is formed of pure silica, very strong in contrast to the weaker but still relatively hard chalk & is the reason why the UK is dotted with hundred of chalk pits, dug since Neolithic times to extract both the chalk & also the flint to make flint tools such as knives & saws.  As exposed chalk bedrock gradually weathers & erodes away the harder flint it contains remains & these loose flints can easily be gathered from the surrounding land. This helps explain why so many historic buildings within Salisbury have incorporated flint: it was abundant, readily available, & quite literally free!​

Figure 7. View north east looking down onto West Harnham Chalk Pit with Salisbury city in the distance.

If you get the chance during your stay, a visit to West Harnham Chalk Pit is well worth the effort. It’s around a 30-minute walk from the city centre & offers lovely views down into Salisbury.  In the winter months, take a flask & enjoy a warm drink while soaking up both the scenery & the deep history beneath your feet.

You can drive there & park on the roadside for a limited time or walk from the city. Beware you will need to have a moderate fitness level