Page 35 - The Restless Earth Fossils

34 Fossils

trace Fossils: tracKs, dung, and More . . .
Tourists at a beach leave thousands of tracks in wet sand, but
most of these footprints are washed away by the tide or covered
over by other wandering tourists. Now and then, the sun’s heat
hardens tracks in soft mud before successive layers of sand and
dirt bury and preserve them. Many dinosaur tracks formed in this
way along ancient seashores that bordered a seaway that once
separated North America into two landmasses a hundred million
years ago. A number of these tracks were discovered in sandstone
in Morrison, Colorado, just west of Denver. A volunteer group
called the Friends of Dinosaur Ridge preserves them and shows
them to many visitors every year.
But sometimes, scientists do not find typical footprints.
Instead, they may find footprint casts. Here is how these casts
form: A footprint made in mud hardens, and then wet sand
fills the track depression. As the water of the seaway ebbs and
flows, many alternating layers of mud and sand will cover the
prints. Over long periods of time, the mud gets compressed to
mudstone by the weight of overlying sediments. Similarly, sand
becomes transformed into a harder rock called sandstone. Unless
protected in some way, mudstone will erode away first leaving
sandstone casts of the original footprints.
Perhaps the biggest exposed dinosaur trackway in North
America parallels the Purgatoire River in southern Colorado. In
a remote spot that has been part of a military testing range for
many years, a slab of resistant sandstone 100 yards (90 meters)
long and about the width of a two-lane highway stretches away
into the distance. A photo in the January 1993 issue of National
Geographic magazine captured dinosaur track expert Martin
Lockley examining prints produced by five giant sauropods
that had walked side by side. Each footprint was about as wide
as the end of a telephone pole. Visitors to the site can also see
the three-toed tracks of predators—perhaps Allosaurus-type the-
ropod dinosaurs. Such trackways reveal secrets about the size,

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