The Old Red Sandstone lavas of the Perthshire Sidlaws are basalts with some andesite-basalts, andesites, and rare trachyandesite. The basalts comprise chiefly olivine-bearing flows that include feldspar-phyric and aphyric varieties; olivine-free basalts with two pyroxenes are of restricted distribution.
The significance of augite textures and their relationship to augite content in the basalts is discussed.
The lava pile is apparently composed of interdigitating lenticular flows that vary from a few feet up to about 80 feet in thickness.
Unlike most other Scottish Old Red Sandstone lava suites the Sidlaw suite is fairly fresh and, like the Eastern Forfarshire lavas, it is relatively basic.