Travel Reference
In-Depth Information
itself. There is very little bedding or any other type of sedimentary structure
visible in the conglomerates. They make up an imposing coastline. As we
continue our walk, we eventually come to a transition, where the conglom-
erate is overlain by another sediment type, one dominated by sandstone.
This gradual transition is best seen in the rocky sides of the cliffs lining the
beach. Over several meters of thickness, the rocks in this conglomerate grow
finer, changing from cobble, to gravel, to very coarse sand-sized material.
This transition in grain size is one bit of evidence that the ancient environ-
ment of deposition that created this part of our stratigraphic section was
rapidly changing. When environments remain the same, so do the rocks de-
posited in them. Any change in rock type tells us that the environments,
which control rock type, must themselves have somehow changed.
In these oldest of Sucia strata, we find only small shards of fossil
shells. They are so fragmented and abraded that they can't be assigned to
any genus or species. Nevertheless, they can still tell us something about
the ancient environments where these coarse, basal strata were deposited.
The fact that there are fossils at all in the conglomerates and basal sand-
stone, and the fact that they are all fragmented, are important clues. They
tell us that the basal Sucia strata accumulated in a high-energy environ-
ment at the edge of the sea and that these fossils were transported by cur-
rents and broken by heavy wave action. This biological evidence is consis-
tent with the clues given by the nature of the sediments themselves, for the
gravel and coarse sandstone show unmistakable signs of water movement.
The pebbles of the conglomerates are all stacked in a rough order—a
process called imbrication that occurs only when small rocks are deposited
in moving water. The sandstone too gives evidence of having been de-
posited in moving water; it shows the sedimentary structures called cross
beds, the fossil remains of ancient sand dunes or ripples, both of which can
form only in moving water. From this evidence, we thus arrive at a con-
vincing picture of a seashore where strong waves act on gravel and sand.
The fossil material was probably brought in from deeper water by this wave
action.
