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overall distribution of the artifact layer at La Playa and to identify sediment
thickness trends. The measured stratigraphic sections from the field notes were
used to construct digital geologic profiles. These sections were then correlated as
part of three cross-sections that illustrate the distribution of the artifact layer and
sediment types.
Results
Mapping the distribution of the artifact layer at La Playa indicates it covers
approximately 0.4 km
2
(99 acres), situated between the Rio Boquillas and the
edge of the bajada (alluvial fans) in the El Canal and Los Monticulos areas of the
La Playa site (Fig.
4
). The artifact layer overlies the main La Playa silt from which
many artifacts have been identified and excavated. The artifact layer occurs in a
1 km long, northeast-southwest oriented zone that is approximately 500 m in
width. Within this area, its occurrence is intermittent due to the extensive arroyo
incision and erosion that has served both to expose and also erode the interval of
interest, resulting in a landscape of badland-style topography. Up to 2.7 m thick of
the Holocene sedimentary succession is exposed in the northern parts of Los
Monticulos, while only up to 60 cm of it is present in the less deeply-eroded
western zone of artifact layer distribution.
The overall slope of the artifact layer is approximately 0.5 to the southwest
from an elevation of approximately 524-518 m (Fig.
5
). Cross-section A-A
0
(Fig.
6
) shows that in the northern area of Los Monticulos, the slope of the layer is
0.7 whereas further south, the slope flattens out to essentially horizontal (0.1).
Cross sections normal to A-A
0
(B-B
0
and C-C
0
) (Fig.
6
) show minor relief on the
artifact layer not exceeding 1 m.
The thickness of the artifact layer ranges from 2 to 6 cm and is largely
controlled by the sizes of the material present. The artifact layer is dominated
by pebble- to cobble-size angular fragments of fire-cracked rock (FCR) (Fig.
7
).
FCR is present over the entire surface of the La Playa site and is occasionally
clustered into large circular hornos (roasting pits) or linear concentrations. The
composition of the FCR is dominated by fine- to medium-grained sandstone
that is sourced from the Cretaceous rocks of the adjacent Boquillas Hills.
In addition to FCR, the artifact layer contains groundstone (manos) that are
typically composed of plutonic igneous rocks that are from a non-local
source. Ceramics are present and are dominated by Trincheras period
(A.D. 150-1450) Purple-on-Red and Purple-on-Brown pottery (Watson
2005
).
Human skeletal remains have been identified and excavated from within the
artifact layer.
The sediments that overlie the artifact layer reflect an overall lensoid
geometry and range in thickness from 12 cm to 1.4 m with an average thickness
of 58 cm (Fig.
8
). These sediments are distinct from the underlying La Playa silt
in
both
grain-size
characteristics
and
in
the
assemblage
of
sedimentary
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