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evidence that support the idea that this artifact layer is not of a fluvial origin.
Examination of the slope of the layer shows that it dips to the southwest at approx-
imately 0.5 which is consistent with the slope of the present-day site topography. In
addition, if this were a fluvial lag deposit then one might expect to observe significant
channelization and relief along the artifact layer. Cross sections normal to the slope
of the layer show little relief along the surface that is consistently less than 1 m.
Examination of the FCR within the artifact layer does not reveal evidence of any
imbrication. In fluvial deposits with a significant coarse fraction of gravel size
clasts, currents may arrange the clasts in a shingled fabric inclined in the upstream
direction. The organization of the FCR is random and thus does not show any sort of
organized fabric that would be consistent with aqueous deposition. If the FCR was
subjected to any significant fluvial transport, the clasts would also exhibit a more
rounded shape. The very angular nature of the FCR suggests little fluvial reworking
and transport. The sediment size and sedimentary structures of the overlying
sediment are consistent with deposition by low velocity flow that would not have
been sufficient to transport artifacts of the size observed. The sediments that overlie
the artifact layer are primarily sandy silts that are rippled and cross-laminated.
Using a Hjulström diagram (Sundborg 1956 ) allows the determination of the
minimum flow velocity required to entrain particles of a particular size. The very
fine sand fraction would require a flow velocity of 20 cm/s to entrain and transport
material of that size compared to a flow velocity of 100-180 cm/s needed to move
large pieces of FCR and other artifacts that are 2-6 cm in diameter. Furthermore, the
sedimentary structures that were observed are primarily ripples which according to
Southard and Boguchwal ( 1990 ) are formed in sediments that have a flow velocity
of less than 100 cm/s. Therefore, the grain size and structures of the sediments
overlying the artifact layer are consistent with low flow velocities that would be
insufficient to entrain and transport larger artifacts. If the flow velocities were
sufficient to move these materials, then the sedimentary structures present would be
characterized by 2D and 3D aqueous dunes instead of ripples. Although it is evident
that fluvial deposition is responsible for burial of the artifact layer beneath a suc-
cession of silt and sand, the lack of temporal control and detailed climatic studies
hamper the correlation of this burial event with specific climatic events.
Conclusions
The artifact layer that underlies parts of the La Playa site is a cumulative
palimpsest that mixes fire cracked rock, groundstone, projectile points, pottery,
and human skeletal remains from the Early Agricultural period with those of
subsequent occupations into the Trincheras period. Eolian deflation and sheet flood
processes are responsible for erosion of the landscape to allow for exposure of
older artifacts. This erosion surface was then buried by later fluvial deposits
resulting in a succession of interbedded rippled and cross laminated silty sands and
laminated silt.
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