The Ordovician and Silurian successions between Falahill and Galashiels encompass six flysch-dominated formations: the Upper Ordovician Portpatrick and Shinnel Formations representing the Leadhills Group, the Llandovery Mindork, Garheugh, and Buckholm Formations together comprising the Gala Group, and a formation indeterminate of age within the Hawick Group. Southward ensialic andesitic volcanic arc and northward low- to medium-grade sialic sources contributed sediment, whilst ophiolitic and subduction-related sources made minor contributions. Deposition took place firstly, in a SE-migrating back-arc basin bordering the northerly source, the Laurentian continent. Subsequent NW-directed underthrusting led to formation out of the back-arc basin of an imbricate thrust stack which migrated southeastwards. Ultimately a foreland successor basin formed ahead of the rising thrust stack.

Flysch units are typically associated with linear outcrops of Moffat Shales which are the loci of major steep SE-translating reverse faults, two of which participate in a late-stage sinistral strike–slip duplex with large-scale imbrication. The faults divide the succession into a sequence of tectonostratigraphic blocks, successively younger to the SE. At least six of the ten blocks customarily recognised in the Southern Uplands, Blocks 3–8, are represented, some of which coincide with single or complete formations.