The solid geology of the Spey catchment is complex although of the catchment is dominated by slow weathering crystalline rocks such as schists and gneisses from the Cambrian period. There are a number of granite intrusions which form Cairngorm mountains and Ben Rinnes. At the northern end of the catchment the geology is dominated by sandstone with significant outcrops of limestone in the Dufftown to Tomintoul area.
The schists, gneiss and granite are hard wearing and provide little mineral input. The rivers and burns draining areas where these rocks dominate tend to be naturally low in productivity compared to those draining sandstone or limestone. This has a bearing on fish populations with some of the highest juvenile salmon densities found in the burns around Dufftown and Glenlivet.