Rocks on the surface of the ground sometimes can form something called desert pavement and the surfaces of those rocks may also become covered with desert varnish.
The origins of desert varnish have long defied full explanation.
Varnish microlamination compositional differences are principally influenced by regional climate conditions.
Pediments are gravel-covered, gently-sloping, low-relief planation surfaces incised into bedrock; their processes and formation have been a source of debate for more than a century.
History of Study: biological origin proposed in 1921.
Recent advances based on electron beam microprobe (Hooke et al., 1979), which demonstrated layering of varnish. file=/NATURE/journal/v392/n6673/full/392218a0_September 25, 2003.
plays a fully developed basal layering unit of LU-4 (WP6), suggesting a minimum age of 60 ka for the flow ( Fig. Since the ‘‘Undated’’ flow is morphologically and also stratigraphically older than the I V flow, as observed at the sampling locality (see Fig.Hillers; UTM: Z17, -2124969m E, 4640124m N) and pediment level L8 (west of Mt. VML exposure ages were derived using the varnish microlamination record for the western USA. This date correlates to wet period (WP) 11 in layering unit (LU)-6, which is correlative with MIS-6 (Late Illinoian Stage or Bull Lake Glaciation).An age of ~105 ka was obtained for pediment L8, which corresponds to LU-5 (WP8) and is coeval with MIS-5d (a cooler, wetter MIS-5 interval).2C), the age of the former should predate the age of the latter.Therefore, the age of the I V flow most likely falls between 46 and 60 ka.It is made up of clays and iron and manganese oxides.As such the chemical composition of the varnish is different from that of the rocks on which they form and thus the components, especially manganese, of the varnish must be derived from air-borne dust and rainwater.Microlamination layers precipitated during periods of wet climate are black on ultra-thin sections under transmitted light microscope and Mn and Ba achieve their highest concentrations in these dark layers.Microlamination stratigraphy and mineralogy indicate that gravel emplacement/pediment formation occurred during periods of cold and wet climate wherein enhanced moisture levels may have promoted gravel transport.One of the most common places to find rock varnish is in deserts were this varnish is called desert varnish.Desert varnish appears on the vertical surface of rocks but it may also appear on very stable horizontal surfaces.