The concentration and burial of dinosaur bones is only the beginning of
the fossil story. The combination of circumstances which operated here
was a common one and yet fossil quarries are rare. Why? The bones have
to be preserved and this seldom happens. The bones that are buried in
one flood are frequently unearthed and scattered by the next. Those that
are exposed to the weather usually disintegrate completely in a few
years. The bones in the Dinosaur Quarry did not.
Sometime after they were buried, the organic minerals of the bones were
more or less completely replaced by minerals of inorganic origin such as
silica. No one knows exactly why or how this happened, but it did. Most
geologists think this replacement process occurs when subsurface or
ground water containing soluble and colloidal minerals dissolves a
molecule of the bone and immediately replaces it with a new mineral.
Roughly such a process is like removing red bricks from a house and
substituting yellow. When the substitution is complete, the house still
has the same dimensions but it is composed of different materials. The
replacement was a faithful one, too, because microscopic structure of
the original bone was faithfully reproduced by the replacing minerals.
[Illustration: ROAD MAP
DINOSAUR
NATIONAL MONUMENT
UTAH - COLORADO
High-resolution Map]
Following Morrison time, thousands of feet of younger sediments were
deposited on the sandbar that contained the dinosaur bones. The whole
sequence of sediments was compacted into rock and some bones were
crushed and distorted.