6.1 Clastic Sedimentary Rocks
A clast is a fragment of rock or mineral, ranging in size from less than a micron[1] (too small to see) to as big as an apartment block. Various types of clasts are shown in Figure 5.12 and in Exercise 5.3. The smaller ones tend to be composed of a single mineral crystal, and the larger ones are typically composed of pieces of rock. 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. Most clasts that are smaller than sand size (<1/16 mm) are made of clay minerals. Most clasts larger 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.
Grain-Size Classification
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).
| Description | Size Range in mm | ||||
| from | to | ||||
| Boulder | large | 1,024 | no limit | ||
| medium | 512 | 1024 | |||
| small | 256 | 512 | |||
| Cobble | large | 128 | 256 | ||
| small | 64 | 128 | |||
| Pebble (Granule) |
very coarse | 32 | 64 | ||
| coarse | 16 | 32 | |||
| medium | 8 | 16 | |||
| fine | 4 | 8 | Size in microns | ||
| very fine | 2 | 4 | from | to | |
| Sand | very coarse | 1 | 2 | 1,000 | 2,000 |
| coarse | 0.5 | 1 | 500 | 1,000 | |
| medium | 0.25 | 0.5 | 250 | 500 | |
| fine | 0.125 | 0.25 | 125 | 250 | |
| very fine | 0.063 | 0.125 | 63 | 125 | |
| Silt | very coarse | 32 | 63 | ||
| coarse | 16 | 32 | |||
| medium | 8 | 16 | |||
| fine | 4 | 8 | |||
| v. fine | 2 | 4 | |||
| Clay | clay | 0 | 2 |
Table 6.1 The Udden-Wentworth grain-size scale for classifying sediments and the grains that make up sedimentary rocks
There are six main grain-size categories; five are broken down into subcategories, with clay being the exception. The diameter limits for each successive subcategory are twice as large as the one beneath it. In general, a boulder is bigger than a toaster and difficult to lift. There is no upper limit to the size of boulder.[2] A small cobble will fit in one hand, a large one in two hands. A pebble is something that you could throw quite easily. The smaller ones — known as granules — are gravel size, but still you could throw one. But you can’t really throw a single grain of sand. Sand ranges from 2 mm down to 0.063 mm, and its key characteristic is that it feels “sandy” or gritty between your fingers — even the finest sand grains feel that way. Silt is essentially too small for individual grains to be visible, and while sand feels sandy to your fingers, silt feels smooth to your fingers but gritty in your mouth. Clay is so fine that it feels smooth even in your mouth.
Exercises
Exercise 6.1 Describe the Sediment on a Beach
Providing that your landscape isn’t covered in deep snow at present, visit a beach somewhere nearby — a