A Trick of the Light:
Seeing (or Not Seeing) Geologic Features Under Different Lighting Conditions

Introduction

When geologists carry out field work, variable lighting and vegetation conditions can have a dramatic effect on being able to see certain features clearly. We all know about the influence of vegetation during different seasons of the year. We can see through the woods much more easily when deciduous trees have shed their leaves in the fall. The leaf-off times of the year are a favorite with Maine geologists. Besides having fewer biting insects to contend with, it becomes much easier to spot bedrock outcrops and glacial features that are not apparent on topographic maps.

It is probably less obvious to most people that a bedrock surface or glacial landscape can look very different on a sunny day than it does when the sky is cloudy. Even if we always visit a site during sunny weather, it may vary in appearance with the sunlight coming from different directions with the changing seasons or time of day. As the sun angle changes, so do the shadows that reveal shapes of everything from large landforms to small subtle details of a rock surface. The following photos show the importance of lighting in examining some glacial features in southern Maine.

Figure 1

Figure 1

These photos show the site that inspired this article. They were taken in November, 2011, at a bedrock outcrop on Clarry Hill in Union and show glacial grooves under differing light conditions.

Figure 2

Figure 2

Closeups of the same glacial grooves at Clarry Hill in Union under differing light conditions. Here the contrast between sunlit and sunless views is even more striking.

Figure 3

Figure 3

Low-angle sunlight also helps to accentuate much larger glacial features. This photo is a late-fall view showing part of the Androscoggin Moraine complex on the ME-NH border (Thompson and Fowler, 1989).

Figure 4

Figure 4

Bedrock surfaces not only look different with varying illumination, but show certain details more clearly when they are wet. This photo shows a ledge in Warren that has been smoothed by glacial abrasion.

Figure 5

Figure 5

This photo shows an example of glacial grooves at the south end of the beach at Birch Point State Park in Owl's Head. The grooves on this ledge show much better when the rock surface is wet.

Reference

Thompson, W. B., and Fowler, B. K., 1989, Deglaciation of the upper Androscoggin River valley and northwestern White Mountains, Maine and New Hampshire, in Tucker, R. D., and Marvinney, R. G. (editors), Studies in Maine geology, Volume 6, Quaternary geology: Maine Geological Survey, p. 71-88.

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Text and photos by Woodrow B. Thompson

Originally published on the web as the June 2012 Site of the Month.

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Last updated on July 9, 2012

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