Geologists will find Thompson Island interesting for its Paleohelikian gabbro dikes and sills, formed approximately one billion years ago. These dikes and sills were created when pressurized volcanic lava deep within the earth's crust was thrust through vertical and horizontal crevices in the bedrock. The lava or magma hardened into gabbro, a coarse-grained dark rock. The nature reserve also features raised cobble beaches, and Arctic-alpine vegetation growing at its most southerly limit. Facilities are limited to toilets and walking trails.