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