The geological collections contain specimens in the following forms:

• Chalk fossils and samples from Sussex
• Fossils
• Mineralogical and petrological slides
• Rock samples of sedimentary, metamorphic and igneous rocks
• Maps, catalogues and diaries of geological collectors.

They have been obtained by various collectors from around the world. The specimens include collections from:

• George Bax Holmes’ Horsham focused collections including a large amount of Iguanodon fossil material.
• Sir Alexander Crichton’s mineralogical collections from Russian and Eastern Europe.
• Type specimen chalk fossils from a variety of locally based collectors.
• Fossil insect collections from Dr Andrew Ross, now at Edinburgh Museum.

The museum holds chalk fossils and sub-fossils from the elephant beds which represent the ice age and late cretaceous environments of Sussex.