| 21. | Hobnailed boots were formerly much used for mountaineering to grip on sloping rock surfaces.
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| 22. | Crustose lichens may also be used for dating rock surfaces, through a technique called lichenometry.
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| 23. | The expansion sets up stresses which cause fractures parallel to the rock surface to form.
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| 24. | The eruptions from the field were accompanied by a slight sinking of the older rock surface.
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| 25. | Petoglyphs are rock drawings made by pecking directly on the rock surface with a stone chisel.
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| 26. | Rockfowl is a reference to the species'habit of building mud nests on rock surfaces and caves.
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| 27. | It is common in deserts on vertical rock surfaces that are partially shade protected near seepages.
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| 28. | Usually attached to open rock surfaces on wave-exposed shores in the mid to low intertidal zone.
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| 29. | Some microbial communities lived under rock surfaces, others bound sand together, and some sat atop soils.
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| 30. | It is formed by colonies of bacteria living on the rock surface for thousands of years.
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