Major progradations of clastic sediments are recorded in the sedimentary record of the Famennian and the Visean-Namurian of the United Kingdom and surrounding waters. We have determined 40Ar/39Ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 Ma, spanning both progradations. Detrital mica ages are dominated by a peak at 415 Ma, with minor peaks at 440 Ma and 390 Ma. The 415 Ma muscovites are derived from the unroofing of the Scandian nappes during the compressional phase of the Caledonian orogeny in Scandinavia. The 440 Ma muscovites record pre-Scandian orogenic activity, which is rarely preserved in the orogenic record. Thermochronological evidence suggests that episodic postorogenic uplift, and exhumation events kept the Scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations. The near total absence of detrital muscovites with ages <415 Ma suggests that the Scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension. The river(s) which supplied the sediments probably ran parallel to the strike of the major Scandian thrusts, along the length of the Caledonian orogen, in a manner analogous to the major river systems of contemporary orogenic highlands. ¿ 2001 American Geophysical Union