These rocks provide an important constraint on models of how the current geometry arose during the Caledonian Orogeny.
[1][2] The main lithologies found within the HBO are serpentinite, often in the form of ophicarbonate, with associated sedimentary rocks, including sandstone, limestone, chert and black mudstone (now black slate).
[2] The Trossachs Group as interpreted ranges in age from Lower Cambrian to Lower Ordovician, based on trilobite fossils found in the Leny Limestone and conodont fossils found in the Margie Limestone.
In this model, the HBO represents pieces of exhumed mantle from the non-volcanic margin of Laurentia, transported onto the Dalradian Supergroup with its sedimentary cover.
[1] The HBO has been correlated with the Deerpark Complex of western Ireland, which consists of fault-bounded lenses of serpentinite, juxtaposed with amphibolites and metasediments.