Geology Reference
In-Depth Information
Fig. 19.9
Geomorphic characteristics of the tidal fl ats of west
Andros Island southeast of Williams Island. (
a
-
b
) False-color
(NIR-G-B) remote sensing images from tidal fl ats of southwestern
Andros Island. (
a
) Overview of part of the coastline, illustrating
the irregular shoreline morphology (cf. Fig.
19.5f
), abundant
creeks, ponds, and hammocks. (
b
-
c
) Remote-sensing image
(
b
) and interpretive sketch (
c
) of an illustrative area (Remote
sensing images copyright GeoEye.com)
analogy with the characteristics of parts of the present
coast (see Fig.
19.8b
), these features can be reasonably
interpreted to represent the positions of relict shore-
lines and channel levees. Gebelein (
1974
) interpreted a
succession of these features to represent a succession of
progradational shorelines. The distinct hammocks that
preserve relict shorelines suggest that the shoreline
prograded in a series of pulses, rather than through pro-
gressive accretion at the shoreface, although the strati-
graphic succession of this area is not known in detail.
(Figs.
19.6f
and
19.9
). Many promontories include
bedrock exposed at or near the low-tide level, whereas
bedrock outcrops are absent in most of the recesses,
illustrating that bedrock highs protect shorelines from
erosion.
Unlike the Three Creeks area, which has generally
similar depths (~2-3 m) to bedrock across most of the
channeled belt, or the southwest-facing margin, which
has deeper bedrock (>5 m; Gebelein
1974
) , the facies
patterns in this area are strongly infl uenced by the
irregular bedrock topography. Many of the larger ham-
mocks (red in Fig.
19.9a
) include bedrock highs, some
of which reach a meter or so above high tide. Similarly,
even many smaller hammocks (e.g. Fig.
19.9b, c
) have
bedrock highs near the surface, whereas subtidal chan-
nels between these features reach 3 m depth. Many of
these channels, and trends in the hammocks, are
Northwest-facing margin
. The northwest-facing mar-
gin of the western Andros system includes some of the
most complex subfacies patterns on the tidal fl ats of
Andros Island. As in the Three Creeks area, the shore-
line is irregular (and likely erosional) at several scales,
including several km-scale promontories and recesses
