 EPOD - a service of USRA

The Earth Science Picture of the Day (EPOD) highlights the diverse processes
and phenomena which shape our planet and our lives. EPOD will collect and
archive photos, imagery, graphics, and artwork with short explanatory
captions and links exemplifying features within the Earth system. The
community is invited to contribute digital imagery, short captions and
relevant links.


 Point Gratiot Disconformity

   December 14, 2020

    PointGratiotEastEPODReduced

   Photographer:  James R. Ebert
   Summary Author:  James R. Ebert

    Point Gratiot, a rocky  headland in Dunkirk, New York, protrudes
   one kilometer into  Lake Erie, one of the  Laurentian Great
   Lakes. The point is underlain by the  Upper Devonian Dunkirk Shale,
   a black  shale deposited under  anoxic conditions in the
    Appalachian Foreland Basin during the  Acadian Orogeny. Here,
   the Dunkirk Shale is overlain by  Pleistocene glacial deposits
   producing a  disconformity that represents the loss of approximately
   375 million years of Earth’s history.
   The eastern side of Point Gratiot (upper picture) is usually difficult
   to access. However, during fall 2020, a small beach formed at the base
   of the cliff enabling access. The shale clearly shows two sets of
    joints (stress release fractures) that are nearly perpendicular to
   each other.
   The lower photo shows the disconformity between the Devonian shale and
   the Pleistocene glacial sediments. The top of the shale is marked with
    glacial striations and grooves accented by sand blown from the
   adjacent beach. This erosional surface is overlain by 10-20 cm of
   cobble-bearing  glacial till deposited directly by the ice. Above
   the till is over one meter of well-sorted  silt that displays
   spectacular contorted bedding. Contorted bedding results from the rapid
   dewatering of sediment as it compacts. This silt was deposited in the
    Pleistocene  precursors of Lake Erie. Photos taken October 13,
   2020.
     * Point Gratiot, New York Coordinates:  42.4882, -79.35304

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Geology Links

     *  Earthquakes
     *  Geologic Time
     *  Geomagnetism
     *  General Dictionary of Geology
     *  Mineral and Locality Database
     *  Mohs Scale of Mineral Hardness
     *  This Dynamic Earth
     *  USGS
     *  USGS Ask a Geologist
     *  USGS/NPS Geologic Glossary
     *  USGS Volcano Hazards Program

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   Space Research Association.

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