The geological collections contain specimens in the following forms:
- Chalk fossils and samples from Sussex
- 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.