Travel Reference
In-Depth Information
had assumed they would he rare and so was delighted to find them in abun-
dance. In places they were almost stacked together and elsewhere they were
rare, with no apparent reason for their curious distributions. The limestone
contained the most ammonites, and it was there that I spent most of my
time, scanning rock surfaces for the telltale sign of ribs or a spiraled shape.
The most common forms had shells that looked like giant snails, rather than
the usual coiling in one pale that typified the vast majority of ammonites.
One particular species was especially common, a form called
Bostrychoceras
polyplocum
well known from Europe. Rocks containing this particular am-
monite may be exactly time-correlated with the rocks of Sucia Island, and
like the rocks on Sucia, the sediments encasing this particular ammonite
showed evidence of increasing water depth. Limestone and shale: a dance of
ancient rock, one telling of a deepening sea, the other of dropping sea level.
The Tunisian rocks were beautifully exposed; there were no missing in-
tervals due to faulting, no covered intervals due to vegetation. In this regard
they were completely unlike the rocks I had long studied in the Vancouver
Island region, where the strata are found only on seacoasts or in river gorges.
Here, in this Tunisian semi-desert, they are inescapable. They are the land
itself, the skeleton of this country with the bones exposed.
The very abundance and completeness of the rock record here enabled
me actually to
see
how important sea level change is in determining the na-
ture of sedimentary rocks. There appeared to be many different cycles of
change superimposed on one another. Long-term changes visible in thousand-
foot increments of sediment (and thus talking place over millions of years)
were made up of shorter-term fluctuations visible in hundred-foot incre-
ments. These changes could be read in the way the rocks looked, in the way
they were bedded, and in the way their enclosed fossils changed. I could see
shallow-water organisms being replaced by deeper-water assemblages as I
moved up through thousands of feet of strata in this region, and then I could
watch the opposite occur. The level of the sea had been the mastet here, dic-
tating so many aspects of the formation of sedimentary rocks long ago. In
turn, the ancient rocks themselves dictate how the land looks today, not just
in the colors of the dark shale and white limestone, but the very nature of
