The USGS maps of coastal California label alluvial deposits as Qyf and Qof — the “younger” and “older” fans of coarse sediments. But in areas of the earth that never saw Quaternary glaciation (like California), does that distinction make any sense?

More generally, in lower latitudes, how is “younger,” meaning Holocene, any different geologically from “Older,” meaning Pleistocene?

  • $\begingroup$ Re "areas of the earth that never saw Quaternary glaciation (like California)": Certain parts of California, such as the Sierra Nevada, were glaciated during the Quaternary. $\endgroup$
    – njuffa
    Oct 7 '20 at 6:01
  • $\begingroup$ Thank you! As a non-professional I was under the impression that “Quaternary glaciation” refers to the ice sheets, not disconnected alpine glaciers. Glad to be corrected! $\endgroup$ Oct 8 '20 at 19:50

They seem to be differentiated according to their morphology, Qof deposits being highly dissected and more consolidated than Qyf, thus older. From a random pamphlet of the map series:

  • Qyf Alluvial fan deposits (Holocene)—Unconsolidated, heterogeneous layers of sand, silt, and gravel; relatively undissected; deposited by streams emanating from canyons onto alluvial plains; identified primarily by fan morphology and topographic expression. Internal contacts delineate individual alluvial fans
  • Qof Alluvial fan deposits (Pleistocene)—Discontinuous or highly dissected deposits of semiconsolidated, moderately to poorly sorted layers of silty clay, silt, sand, and gravel

The pamphlet also states that "unit ages [...] reflect local stratigraphic relations", so they are relative ages, not absolute dating.


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