A geologist observing a riverbed notices that sand grains (0.5 mm) are actively moving downstream, but fine clay particles on the same streambed are not being eroded. Why might clay resist erosion at the same flow velocity that moves sand?
AClay particles are denser than sand, requiring more force to move them
BCohesive forces between clay minerals bind particles together, requiring higher erosion velocities than size alone would predict — clay's resistance to erosion is not simply a function of grain size
CClay cannot be transported by water at all; it only moves with glaciers
DThe small surface area of clay particles means drag forces cannot act on them effectively
This is the key counterintuitive result from Hjulström's diagram. For sand-sized particles, intuition holds: finer grains erode at lower velocities. But very fine particles (clay, silt) have strong cohesive electrostatic bonds between mineral surfaces, and it takes more energy to rip a clay particle off a muddy streambed than to pick up a loose sand grain. This creates the characteristic 'U-shape' minimum in the Hjulström curve around the sand fraction. However, once clay is entrained into suspension, it stays suspended at much lower velocities than were needed to erode it — this is why rivers remain turbid for days after a flood.
Question 2 Multiple Choice
A geologist finds a deposit of well-rounded, well-sorted quartz sandstone. What does this indicate about the sediment's transport history?
AThe deposit formed near its source from a brief, intense flood that winnowed fine grains
BThe grains traveled a long distance under sustained, relatively uniform current conditions — rounding by abrasion and sorting by size both require prolonged, consistent transport
CWell-sorted means the grains were deliberately arranged by grain size at the time of deposition
DThe well-rounded appearance resulted from chemical weathering, not physical transport
Rounding requires abrasion over time and distance — angular fragments lose their corners through repeated collisions during transport. Sorting reflects selective deposition: at any location, the flow velocity selects for a characteristic grain size (faster flows carry larger grains farther). A well-rounded, well-sorted deposit is the signature of sustained, relatively uniform transport — beach sand and aeolian (wind-blown) dune sand are classic examples. Contrast this with glacial till, which is unsorted (everything from clay to boulders deposited together) and angular (glacial transport doesn't produce rounding by abrasion).
Question 3 True / False
A 'well-sorted' sediment deposit means the particles are similar in size, reflecting that they were deposited under relatively uniform transport energy conditions.
TTrue
FFalse
Answer: True
Sorting in geology refers to the uniformity of grain sizes in a deposit — well-sorted means most grains are roughly the same size; poorly sorted means a wide range of sizes mixed together. It has nothing to do with arrangement or organization by human hands. Sorting is a direct record of transport energy: steady, uniform flows (beaches, dunes) sort grains effectively because only particles of a specific size range can be transported at that energy level. Variable or catastrophic flows (floods, glaciers) deposit unsorted material because the energy conditions change too rapidly for selective deposition to occur.
Question 4 True / False
Deposition of sediment is generally a slow, gradual process that occurs over years to centuries — catastrophic rapid deposition does not produce recognizable sedimentary structures.
TTrue
FFalse
Answer: False
Turbidity currents are underwater avalanches of sediment-laden water that flow down continental slopes at speeds up to 100 km/h, depositing entire graded beds (coarse-to-fine upward) in a matter of hours. Each bed records a single catastrophic event. The 1929 Grand Banks earthquake triggered a turbidity current that deposited graded beds over a vast area of the Atlantic seafloor in hours, snapping submarine telegraph cables in sequence. Recognizing the difference between gradual and catastrophic deposition is essential for interpreting the geological record correctly.
Question 5 Short Answer
Why does Hjulström's diagram show that very fine clay particles require higher erosion velocities than medium sand grains, even though the clay particles are far smaller and lighter?
Think about your answer, then reveal below.
Model answer: Clay minerals have strong cohesive electrostatic bonds between their flat, sheet-like surfaces. On a muddy streambed, clay particles are bound to each other and to the substrate by these forces, and the flow must overcome them before particles can be detached. Sand grains, by contrast, are loose — only their weight resists entrainment. Because sand is heavy enough that a moderate flow velocity provides sufficient drag and lift, and light enough that cohesion is irrelevant, it erodes at lower velocities than either gravel (too heavy) or clay (too cohesive). This creates the Hjulström diagram's minimum erosion velocity around medium sand.
This question tests whether students understand that 'harder to erode' is not simply a function of grain size or weight — cohesion introduces a completely different resistance mechanism. The practical implication is important: rivers running across soft clay floodplains don't erode them as easily as one might expect, but once the clay is disturbed and entrained, it stays in suspension far longer than the flow conditions that initiated transport would predict. Understanding this asymmetry between erosion and deposition thresholds for fine particles explains why fine-grained rivers take so long to 'run clear' after a disturbance.