The Rosseau Creek area is underlain by basalts of the Eocene Kamloops Group. A rare zeolite, ferrierite, occurs with agate and calcite in masses 7 to 10 centimetres long. The agate may be banded or plain. Ferrierite occurs as radiating spherical or conical groups of acicular crystals up to 1 centimetre in length. The best collecting locality is marked by a rockcut on the Canadian National Railway on the north shore of Kamloops Lake near where Rosseau Creek empties into the lake. Specimens may be picked up along the lake shore in the beach gravel; weathered material is brick red. Agate is common in the bluffs overlooking the lake above the ferrierite locality.