text: Henry Cowles pioneered the study of ecological succession on structures made of this substance.
full_question: Henry Cowles pioneered the study of ecological succession on structures made of this substance. Unlike regs and hamadas, ergs are covered with this material. Large fields of this substance mixed with clay and bitumen are located in Athabasca, Canada. This term describes particles that are larger in diameter than silts and clay but smaller than gravel. In many regions, this substance is primarily composed of silicon dioxide. Marram grass helps stabilize coastal hills of this material that have accumulated from winds blowing it onshore. For 10 points, name this material that forms dunes and is abundant in the Sahara desert.
first_sentence: Henry Cowles pioneered the study of ecological succession on structures made of this substance.
raw_answer: sand particles [prompt on "soil"; prompt on "silicon dioxide" or "silica" until "silicon" is mentioned]