CTgeo

Making your own cast of a dinosaur footprint


    This is a popular activity at Dinosaur State Park, and on busy days you should be prepared to wait.  There are several tracks to select among, both as molds (raised tracks) and casts (indented tracks).  You can choose to make a cast of a mold, or a mold of a cast.  The tracks are single large Eubrontes footprints in fine-grained sandstone (siltstone).  These tracks were made by meat-eating theropod-type dinosaurs, similar in size to the Dilophosaurus model inside the exhibit building.  Geologists make similar plaster track casts from natural outcrops to study back at the lab, so you are practicing an actual scientific field technique.  

    You need to bring your own cooking oil, plaster of paris, rags, and bucket, so read the instruction sign carefully and assemble your materials beforehand.  Plaster of paris (10 pounds) can be obtained from most home supply stores, and any cooking oil can be used (a pint is enough).  A large 5-gallon plastic bucket is ideal, which is also available at home centers.  You can bring a large mixing stick, or use your hand to mix the plaster with water at the site.  Allow at least an hour for your visit, because the plaster must harden before you can lift it, and you must clean up your cast location afterwards -- this can be messy!

casting area The casting area is fenced and not available at all times.  There are metal rings, a water spigot, and cleaning brushes for public use, plus a few buckets and rags at the far end near the instruction sign.
instruction sign Read the sign carefully so you will know all the steps.  Click on this image for a larger version.
cleaning before starting Select a track and sweep it clean if needed.
oil the track Spread the cooking oil over the track.  This will keep the plaster from sticking to the rock.
materials at the track Some of the materials, ready to start.  Water is in the bucket, and plaster is in the bag, in correct proportions to mix.  Set the ring over the track and pack wet rags (shown in the next photo) around the base of the ring to keep excess wet plaster from running out.
mix the plaster The ring is set and plaster is being mixed.
pour the plaster The plaster has been poured into the ring, but some is escaping under it.  The plaster mix tends to be soupy, so you should allow time for it to thicken a little before you pour it.  This will be fairly quick on a warm day, but slow on a cool day like this one.
plaster has set After about 20 minutes or so, the plaster has set hard enough to remove the ring.  The time needed for this will depend on the temperature.  Be careful with this step, as you can easily break the soft plaster.  You can scrape the outside of the cast to make it smooth and round, and flat on the back.
finished track The track cast is finished.  The next step is to clean up the casting site, using a brush and water, so that the next visitor can get right to work.  Do not leave a mess!

This is a big track, about 16 inches long, and you can see many details of the dinosaur's foot. It makes a fine display specimen, the equal of a "real" track that might cost over $100 from a fossil dealer.