Passage V

Moving water can shape sediment into bedforms, such as ripples (narrow ridges separated by wider troughs). A geologist performed 3 studies to examine bedforms.

Study 1

Samples were taken from 4 layers of an exposed section of sandstone and the average sample grain diameter, in millimeters (mm), was measured. Any bedforms preserved in the sections were recorded. The results are shown in Table 1.

Study 2

A sediment sample was taken at 5 sites across the stream shown in Figure 1. The average grain diameter of each sample was determined and any bedforms at the site were noted. The water depth and velocity, in centimeters per second (cm/sec), were measured at Sites 3, 4, and 5. The results are shown in Table 2.

Study 3

A stream channel was built that contained 20 cm deep flowing water. Three experiments were done, each using a flat layer of a different sediment grain diameter. The bed-forms created by flowing water of various velocities were recorded for each experiment. The results are shown in Table 3. "Flat bed" means sediment movement but no ripples. "NM" means no sediment movement.

27. Closer examination of Section 1 in Study 1 revealed that the largest grains (up to 20 mm in diameter) were at the bottom of the section. According to the studies, which of the following statements best explains why such large grains were found at the bottom of the section?

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

The fundamental reason why large particles sink to the bottom is because the small particles have been washed away, and only the largest particles cannot be washed away. The difference in depth is due to the rapid flow of the small particles, rather than the large particles remaining behind.