Crossed the Line and Headed for a Fall

35.983675,-114.788198

Andesitic basalt atop pluton of quartz monzonite covered in alluvial gravels cemented by caliche. Every step closer to the end.

Tsmo - Older intermediate-age sedimentary rocks (middle Miocene)—Sandstone, mudstone, siltstone, limestone, gypsum, and conglomerate. Deposited in fault-controlled basins and subsequently faulted and folded. Inferred to be about 12 to 16 Ma in age. Unit lies between andesite lava and breccia of lower part of Patsy Mine Volcanics (Tpl) with an 40Ar/39Ar age of 15.72±0.03 Ma and a basalt flow of upper Mount Davis Volcanics (Tdmu) with an 40Ar/39Ar age of 12.73±0.3 Ma.

Tdmm - Mafic lavas—Modified from Anderson (1977, 1978). Varies from aphanitic to porphyritic. Outcrops previously mapped as Fortification Basalts west of Colorado River (units Tmf, Tmfc, and Tmfq of Ekren and Anderson, 1996, and unit Tmf of Anderson, 1977, 1978) have been assigned to this unit based on stratigraphic relations and geochronology. A new 40Ar/39Ar age of 13.268±0.032 Ma (appendix 1) was obtained from basalt sample collected about 1.5 miles northeast of Boulder City from outcrops that had previously been mapped as Fortification basalt (Anderson and Ekren, 1996; Anderson, 1977).

Tibu - Boulder City pluton, undifferentiated (middle Miocene)—Large composite epizonal batholith exposed mostly northeast and east of Boulder City. Bulk of pluton is quartz monzonite with lesser amounts of granite, monzonite, and syenodiorite (Ekren and Anderson, 1996). Quartz monzonite is mostly light gray, fine to medium grained, faintly to distinctly porphyritic, nonfoliated pyroxene bearing, with biotite and hornblende as important mafic constituents. Rocks either more mafic or more siliceous than quartz monzonite occur mostly as fine-grained to aphanitic dikes and border facies in Boy Scout Canyon area and in outcrops east and southeast of Boulder City. Sub-horizontal bands at about 700 m elevation, interpreted by Anderson (1969) to represent a paleohydrologic feature, are formed by strong concentrations of hematite. A new U-Pb age of 13.96±0.25 Ma was obtained from this unit; previous K/Ar analyses of biotite yielded ages of 14.17±0.6 Ma (recalculated; Anderson and others, 1972) and 14.65±0.47 Ma (recalculated; R. Fleck, U.S. Geological Survey, written commun., 2008)

Free 1977 USGS Geo Map and Google Earth KMZ:
https://ngmdb.usgs.gov/Prodesc/proddesc_10930.htm

Free Updated Geologic Map of the Black Canyon Area:
https://pubs.usgs.gov/of/2013/1267/a/

Camera: $200 Sony CX405 with slight color edit for outdoor reddish tint.

Martin Blok (Ry Cooder) Music on YouTube:
https://www.youtube.com/watch?v=IRQYdin603M