Some ca. 85-68 Ma sandstone samples from the Insular Superterrane (IST) contains large populations of Proterozoic detrital zircon (e.g., uppermost Nanaimo basin BC, Swakane metasandstone WA, Yakutat AK). If this detrital zircon (DZ) was sourced from SW Laurentia (SWL), it would favor the Baja-BC hypothesis, whereas if sources were in Idaho, it would disfavor BBC. Many of these samples exhibit sharp, prominent DZ age peaks at 1.38 Ga, plus DZ ages spread from ca. 1.65 to 1.80 Ga. 1.38 Ga plutons are rare in SWL and, where present, occur close to 1.40-1.45 Ga anorogenic granites and so tend to generate diffuse, mixed DZ age peaks. Sharp 1.38 Ga peaks are absent in southern California forearc basin samples, evidence against major 1.38 Ga source plutons in SWL. In Idaho near Shoup, 1.38 Ga bimodal plutonic rocks (Aleinikoffet al 2012) intruded Proterozoic strata of the Lemhi subbasin of the Belt Supergroup basin, strata which host 1.65-1.80 Ga DZ. Mapping of twelve 7.5’ quadrangles east of the Shoup reveals giant folds with amplitudes exceeding 10 km (Lonn et al 2016), suggesting intense Late Cretaceous uplift and erosion of roughly 5000 cubic kilometers of rock from the region. Much of the detritus was apparently transported by the Kione paleoriver SSW to the Great Valley forearc basin in northern California, where it was the main source for a ca. 82-80 Ma, 600-m-thick package within the Kione-Forbes delta-slope system (Dumitru et al 2016). Considerable detritus transited the forearc basin and was deposited in the Franciscan trench in central California. Limited detritus apparently also travel by sediment gravity flows SSE along the trench axis for ~300 km and sourced small portions of the Rand and related schists in southern California. I infer that additional paleorivers transported detritus W from the Lemhi subbasin, plus from other parts of the western Belt basin, to nearby coastal Oregon and Washington, partly sourcing some of the 85-68 Ma Insular sandstone units. Only limited Hf isotopic data are available from Belt rocks (Gaschnig et al 2013, Fig 7) but are an excellent match to DZ Hf data available from Yakutat sandstone (Garver and Davidson 2015). Therefore, Idaho is the mostly likely source area for Proterozoic DZ seen in some Insular sandstone, favoring moderate northward translation models for the IST and disfavoring BBC models.