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Fig. 10 Results of laser
particle size analysis of the
fine fraction from four
samples of the La Playa silt.
The fine fraction is dominated
by very fine silt with clay
focusing on the fine fraction of those same four samples, were measured with a
Malvern laser particle size analyzer. The results of that analysis indicate a median
particle size of 6.7-8.5 lm (very fine silt). The clay content for the mud fraction
ranges from 28 to 35% (Fig.
10
).
Modern Rio Boquillas Bar Sands: Qrbbs
These fine to medium-grained sands within the deeply incised Rio Boquillas are
adjacent to the coarser channel deposits (Qrbcd). They are distinguished from
channel deposits by the lack of coarse clasts, the abundance of vegetation, and
ridge and swale topography which reflects growth and migration of the laterally
migrating barforms (Miall
2010
).
Modern Rio Boquillas Channel Deposits: Qrbcd
These deposits occupy the main channel of the Rio Boquillas. The channel is
incised into Holocene and Pleistocene deposits and contains sand and cobble-size
materials composed of multiple lithologies that include sedimentary, volcanic,
plutonic, and metamorphic rocks. The in-channel deposits reflects a mix of bajada
gravels, reworked Pleistocene gravels, and modern deposits. Comparison of aerial
photography from May 1996 and the October 2008 Quickbird imagery show that
the river channel is fairly stable and only exhibits significant lateral migration in
areas where Rio Boquillas bar sands (Qrbbs) are present.
Fire-Cracked Rock: Qfcr
Fire-cracked rock or FCR is cultural material remaining from utilization and occu-
pation of the site and may represent a palimpsest produced by deflation and win-
nowing of the finer-grained silt (Qs). Such a deposit would represent comingling of
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