trackways and other ichnological traces are fundamental tools used to research
fossil tracks and sedimentary structures, including ripples and tool marks.
However, commonly used methods of collecting modern footprints include the
laborious use of plaster or latex (Goodwin and Chaney, 1994). Both of these
methods are inadequate to accurately record traces on unconsolidated substrates
without crushing the print or otherwise distorting the trace or structure. In
addition, neither of these methods consistently and accurately records details
of the substrate’s sedimentology, including grain size and sorting. These
features of the sediment may influence the formation of an ichnological trace or
sedimentary structure. In order to circumvent some of the problems with these
other methods, we have developed a technique that records small-scale surficial
sedimentary structures with little, if any, distortion, requiring less labor,
and preserving more data. It is similar to the use of acetate molds in other
areas of paleontology (Darrah, 1936; Rigby and Clark, 1965).
Butvar (polyvinyl butyral, B76) in solution
Thin cellulose acetate sheeting. (Available
from plastics suppliers.)
Airbrush (that can spray acetone based
products, such as Preval® Sprayer).
Clear acrylic spray (optional).
Plastico Magic Spray Dust Hardener (Douglas
and Sturgess) (optional).
Standard spray bottle (optional).
1. Using an
airbrush, spray the sedimentary structure or track that you want to collect with
low viscosity Butvar (approximately 12:1 acetone to Butvar). The use of low
viscosity Butvar is done to penetrate the surface layer (through percolation due
to gravity) and preserve the delicate surface for collection. Clear acrylic
spray, like Krylon®, may be used instead, but it suffers from problems in field
use, in that it tends to disperse too much and does not quickly consolidate the
surface. Dust hardener also may be used for surface consolidation and can be
dispensed from a standard spray bottle. Both dust hardener and acrylic spray
require much more time to dry (1-2 hours) than does aerosol Butvar. The drying
time for the Butvar will vary with temperature since the evaporation rate of
acetone will be higher in high temperatures and lower in colder temperatures.
The freezing temperature of acetone is far below 0 Cº. The airbrush or sprayer
should not be too close to the surface because the propellants used might
disturb the surface. Generally a distance of about 30 to 40 cm is adequate.
Repeat applications of Butvar, allowing it to harden in between applications
(usually 15 to 20 minutes depending on field conditions), until the surface is
resistant to touch. Higher viscosity Butvar (approximately 10:1 acetone to
Butvar) may be used for additional applications in order to better consolidate
and harden the surface before removal.
2. Once the
surface is consolidated and relatively dry (test lightly with finger tip), lay a
sheet of cellulose acetate film over the structure. The piece of cellulose
acetate should be cut to size, leaving a margin of at least a few centimeters
around the structure to be collected. Coverage of larger structures or trackways
can be accomplished by overlapping many smaller pieces of cellulose acetate.
Spray the cellulose acetate sheet, using an airbrush filled with Butvar (low or
high viscosity will work). The acetone in the Butvar will “melt” the sheet. As
it dries, the sheet will harden and the Butvar will aid in bonding it to the
previously consolidated surface. Spray until the cellulose acetate melts and
conforms to the surface (fig.1). Application of too much Butvar may cause holes
to form in the sheet and result in reduced support. Use of too little Butvar
will leave patches of cellulose acetate that have not conformed and bonded well
to the sediment surface. Let the cellulose acetate and underlying sedimentary
surface dry. The surface of the cellulose acetate will begin to curl slightly
away from the adjacent sediment surface when dry (but may not be dry in the
layer can be collected. When removed from the substrate, the upper layer of
sediment will come away along with a three-dimensional cast of the structure
(fig.2). Acrylic spray or some other cohesive or consolidating agent (not
acetone based, like an epoxy) can be applied, in the field or later in the lab,
to the other side in order to prevent loss of too much sediment.
Fig. 1 -
Two footprints using a duck foot have been made in the sand. Steps 1 and 2 have
been completed. The darker color of the sediment indicates that the material is
not dry yet and cannot be lifted from the adjacent sediment.
Fig. 2 -
The artificial trackway after it has dried and been removed from the adjacent
sediment. It has not been trimmed or otherwise modified. Notice the retention of
the three-dimensional shape of the footprints.