RESULTS

Anaglyph Photography of Tracks in the Field

During the short Greenlandic summer, track-bearing localities above the Arctic Circle offer the rare opportunity to photograph most footprints under sunlight 24 hours a day. Such freedom allows a single print to be naturally lit from all possible directions while still in situ (Figure 1). Despite this flexibility, many of our track photographs suffer from the commonly encountered flaws of excessive contrast (Figure 1.2-1.4), misleading or concealing shadows (Figure 1.3-1.4), confusing color artifacts, or morphological ambiguity due to uniform illumination (Figure 1.5). Even when multiple images are captured of the same track under different lighting conditions, the topology of the sediment’s surface may not be obvious. Morphological description, artistic illustration, and scientific interpretation can be hampered by this variable fidelity, particularly if viewers are unfamiliar with the original material.

Over the last 10 years, we have documented tracks in the field by taking sets of two to five photographs from slightly different perspectives. To avoid bulky hardware, we use a simple 35 mm single lens reflex camera with a zoom lens rather than specialized cameras or multi-camera configurations. Exposure, focus, and focal length are set manually and kept constant throughout a series. In lieu of a tripod, which casts undesirable shadows, the camera is hand-held using our legs and body to maintain a constant height above the track. We orient the specimen’s anteroposterior axis along the width of the film frame, typically by standing to the side of the track furthest away from the sun to keep our own shadow out of the field. The first picture is taken while leaning forward (weight on toes) with the camera approximately 10 cm past a position directly above the center of the track. One to four additional pictures are then taken in quick succession before the cloud cover can appreciably change. We keep our feet planted, but progressively shift weight to our heels to move the camera backward in ca. 10 cm increments. Before each shot we center the camera on the same point in the track and maintain a correct focal distance by fine-tuning camera height until the target is focused crisply in the viewfinder.

We organize our 35 mm slides on a light table using a stereo viewer. Slides are lined up from leftmost to rightmost and labeled sequentially. We then choose the two most effective pictures to create a stereo pair, keeping the lowest numbered slide on the left at all times. Typically these are the images shot most orthogonally and an adjacent slide, but different pairs can be substituted if needed. Slides are scanned into a computer at 2400 dpi. Digital files are aligned and cropped to form stereo pairs and anaglyphs in Adobe Photoshop (Figure 2).

Following ichnological tradition, we initially tried to photograph only under relatively cloud-free conditions so that a track’s shape would be well defined by the primary light source. However, as examples in Figure 1 show, direct sun frequently creates harsh contrast that conceals portions of the track. Ripple marks that are clearly discernable in some situations (Figure 1.1, 1.3) seem to disappear when lighting parallels crests and troughs (Figure 1.2, 1.4). Even relatively minor differences in sun position (compare Figure 1.3 and Figure 1.4) can have a dramatic effect on cast shadows, which may obscure or overly emphasize specific areas. The sun’s elevation is also important. Tracks in the Fleming Fjord Formation of Greenland typically exhibit features elevated well above the bedding plane. When incident light strikes a track at a very low angle, even small surface irregularities can cast long, distracting shadows. These mounds and crests can eclipse large portions of the track and surrounding rock (Figure 1.2-1.4). In some circumstances, strong directional lighting creates shading cues that cause concave structures to appear convex and vice versa (Figure 3).

At the other extreme, photographs taken under overcast or hazy skies are particularly difficult to interpret as monocular images (Figure 1.5). But when viewed in pairs, the relatively shadow-free illumination produces superior anaglyphs (Figure 2). We now prefer to collect images on days with relatively uniform, ambient lighting, even if the track’s structure appears indistinct when seen through the camera’s viewfinder and as a single slide.

Anaglyph Microscopy of Tracks in the Laboratory

Our Greenlandic track collection includes over three dozen specimens preserving skin impressions in the form of dimples, pimples, ridges, valleys, and striations (Gatesy 2001; Gatesy et al. 2003). Such minute, finely detailed textures are quite shallow (ca. 0.2 mm or less), making them extremely difficult to photograph in the field.

Under laboratory conditions, accurately documenting skin impressions presents two main challenges. First, adjacent regions of impressions may appear different because of uneven lighting. Microstructural features are best seen under a binocular dissecting microscope using low-angle, grazing illumination. Unfortunately, skin impressions are most often found lining the concave or undulating depressions made by the digital pads. Such areas are impossible to light uniformly even with flexible-necked fiber-optic lamps, resulting in apparent textural variation across any non-planar surface (Figure 4). Second, the same region of skin impressions can appear quite different under the binocular microscope as the angle of incident light is altered. For example, a small patch of skin impression may look like an array of concave dimples when lit from one direction (lower region of Figure 4.2) but shift in appearance to valleys of interconnected dimples when lit from another (Figure 4.1, 4.3). This raises the question of whether a single primary light source is the best method for revealing track microtopography. As with whole tracks lit by the sun on a clear day, strong directional lighting from a single lamp casts crisp shadows that make textures stand out, but these high-contrast patterns may be misleading about surface geometry. As with whole tracks, concave structures can sometimes flip to appear convex (Figure 4.4), and vice versa.

We capture pairs of images for anaglyphs by mounting a digital camera (Olympus Camedia C-50) on a tripod and then sequentially positioning its lens in front of the microscope’s left and right eyepieces. Shutter speeds are set manually to achieve the proper exposure. Specimens are lit with four arms from two fiber-optic lamps to provide a relatively diffuse illumination without strong shadows. Digital JPEG files are brought directly into Adobe Photoshop and combined into anaglyphs (Figure 5).

If the region of interest is relatively inaccessible to viewing and/or illumination, we make silicone peels. Peels offer more freedom for lighting, reduce the risk of damaging original material, and provide a homogeneous color that accentuates shape. We cast small areas of skin impression using silicone putty (Knead-A-Mold, A2Z Solutions) that does not require a separator. After thorough mixing, we smear small (ca. 1 mm diameter) balls of putty into the cleaned rock surface to minimize the possibility of entrapping air bubbles, gradually building up layers to create a peel 2-3 mm thick. Before the putty hardens we mark the back of each peel with the specimen number, digit number, and orientation with respect to the track’s main axis.

Peels can be trimmed and mounted for viewing at higher magnifications by Scanning Electron Microscopy (SEM). We used a Hitachi 2700 SEM to collect images at magnifications of ca. 50X. Despite the high resolution, we found that microtopography was sometimes ambiguous in monocular images. In particular, our interpretation of an SEM image of a silicone peel often changes from concave to convex, or vice versa, if the image is reoriented on the page (Figure 6). Such ambiguity can be avoided by using stereopsis to resolve visual conflicts caused by directional illumination. Capturing multiple images of the same region while incrementally tilting the stage allows us to assemble anaglyphs quite easily. Figure 7.1 shows the true orientation of the peel, with hill-like pimples. However, we can intentionally reverse the illusion of depth by rotating the completed anaglyph about a vertical axis. In Figure 7.2 these convexities now appear concave. Such “virtual” casts of silicone molds foster direct comparison with the original footprint.