A pebble is something that you could throw quite easily. A small cobble will fit in one hand, a large one in two hands. There is no upper limit to the size of boulder. In general, a boulder is bigger than a toaster and difficult to lift. The diameter limits for each successive subcategory are twice as large as the one beneath it. There are six main grain-size categories five are broken down into subcategories, with clay being the exception. Table 6.1 The Udden-Wentworth grain-size scale for classifying sediments and the grains that make up sedimentary rocks Description Geologists that study sediments and sedimentary rocks use the Udden-Wentworth grain-size scale for describing the sizes of the grains in these materials (Table 6.1). Most clasts that are smaller than sand size (2 mm) are actual fragments of rock, and commonly these might be fine-grained rock like basalt or andesite, or if they are bigger, coarse-grained rock like granite or gneiss. As we’ve seen in Chapter 5, most sand-sized clasts are made of quartz because quartz is more resistant to weathering than any other common mineral. The smaller ones tend to be composed of a single mineral crystal, and the larger ones are typically composed of pieces of rock. Various types of clasts are shown in Figure 5.12 and in Exercise 5.3. Chapter 6 Sediments and Sedimentary RocksĪ clast is a fragment of rock or mineral, ranging in size from less than a micron (too small to see) to as big as an apartment block.
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