| Summary: | The primary aim of this research is to answer some of the questions about the role of Lesmahagow in the Caledonian orogeny. Other important considerations include how it correlates with the other inliers of the southern Midland Valley and why such an unusual fauna should be preserved here. The basin model to emerge from this study shows that the sediments of Lesmahagow accumulated in a narrow, trans-tensional basin within a largely extinct volcanic arc. The basin subsided quickly and was filled by organic rich muds. Arenaceous sediment was supplied by two turbidite sources; a fine to course grained one shed from an active fault scarp along the south-eastern margin, and a very fine grained source which entered the basin axially from the north-west. From fossil evidence it seems unlikely that the basin ever had full and open connection to the 'rump-Iapetus' ocean that remained between Laurentia and Avalonia. Whatever connection did exist became gradually more tenuous as the basin was filled and uplifted. The animals that inhabited these environments (which became progressively more lacustrine) must have had to cope with water chemistry that departed from what is regarded as 'normal' marine and this may indicate that these were the first vertebrates to colonise non-marine environments. These conclusions re-write the Silurian stratigraphy of the Midland Valley of Scotland and may have an important bearing on the history of vertebrate evolution. They also show that, during the Silurian, the stresses produced by the collision of the Laurentian and Avalonian plates were being accommodated through strike-slip processes, which ultimately led to the dismemberment of the northern margin of Iapetus and its reconstitution as a patchwork of unrelated 'suspect' terranes.
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