This investigation is concerned with the sedimentation, petrography, and stratigraphy of Lower Old. Red Sandstone (Devonian) rocks of the north-western Midland Valley and North Argyll areas of Scotland. During Lower Old ted Sandstone sedimentation the western end of the Scottish Midland Valley was a trough being infilled with detritus from the north. The thickness of the sequence developed varies from C. 3000 m. in the north-east of the area (Loch Lomond) to c. 1200 m. in the south-west (Kintyre) and to only c. 170 m. in a small inlier further north-west (North Argyll). These sequences are incorporated in a lithostratigraphical framework for this area which is similar to, and can be correlated with, an earlier lithostratigraphy proposed for the northern margin of the Midland Valley east of the River Clyde. The rocks in these sequences are coarse conglomerates, breccias, sandstones, siltstones, mudstones and minor amounts of concretionary carbonate which together form a red-bed sequence of probably alluvial origin. As in many similar continental basin sequences two main types of alluvial sediment are recognised, and thought to be the product of different physiographic environments. Alluvial fan and flood-plain deposits are distinguished mainly on the basis of grain size, sedimentary textures and structures, and the presence, or absence, of fining-upward cycles. Alluvial fan deposits consist of conglomerates and sandstones in which three major facies can be recognised on the basis of association of lithologies, types of conglomerate textures, the degree of sorting, sedimentary structures, nature of basal erosion surface and the geometry of the sedimentary unit (lithosome). By comparison with modern sediments these facies have been interpreted as being of streamflood, stream, or braided stream origin. Floodplain deposits consisting of sandstones, silt tones and mudstones, are principally distinguished by their relatively fine grained nature and their close association with, or organisation into, fining-upward cycles. They have been interpreted as the products of low sinuosity rivers on floodplains of varying geomorphological diversity, in which vertical sequences of both channel and overbank deposits were developed and preserved. Within each facies the cross-strata, subdivided, on the basis of their morphology, lithology and associated sedimentary structures, are assigned to bedforms which are known in recent alluvium to give rise to comparable structures. Palaeoaurrent data can be related to specific bedforms and flow conditions. Different associations of bedforms are recognised, and are the basis on which different types of braided systems are proposed to have been responsible for the basin infill on both fans and floodplains. The detrital composition of conglomerates and sandstones is comparable with Dalradian rock types in the Highlands with a broad horizon enriched in igneous material in the middle of the stratigraphic sequence. This material, probably of first cycle origin, predominates in North Argyll and Kintyre where it could have been derived from contemporaneous volcanic activity (e.. Lorne Plateau Lavas). The abundance of high grade quartzite clasts, (Lower Dalradian aspect), at the base of the sequence resting unconformably on low grade schists/phyllites (Upper Dalradian aspect) is anomalous. However northerly directed palaeocurrents at the base of the Kintyre sequence may fatrour an origin for these polycyclic clasts within the Midland Valley; they having been transported into the Highlands and then retransported southwards into the basin. Within the lithostratigraphical framework estabished, the time-area changes in sedimentation can be elucidated and the basin evolution discussed in relation to structural control along the inferred basin margins. The thickness of the basinal sequence comprising a stratigraphic alternation and lateral intertonguing of fining-upward fan and floodplain sequences, implies a pulsed nature to continuous basin subsidence, which probably resulted from faulting rather than folding. By comparison with other basinal sequences, the geophysical avidencel the sedimentation time-trends, and the inability to locate any basin margin fault scarps, this faulting is thought to have occurred along a number of small normal faults shifting their location with time. This, together with the presence of water lain fan deposits, implying low fan profiles and low rates of source uplift in these western areas is in contrast with the nature of the basin margin envisaged east of Loch Lomond. West from Loch Lomond a basin with a southwesterly dipping regional palaeoslope, except close to the inferred basin margins, spread laterally over the projected line of the Highland Boundary. In the early stages of evolution this basin may have been linked west of Kintyre with deposition in the North Argyll basinal area, a situation which may have continued until a major volcanic episode resulted in the filling up of the basinal area in North Argyll. Following this, sedimentation continued south of an upland block in a basin extending, from Kintyre eastwards to Callander and Aberfoyle, and in which the basin margin migrated northwards with time.