A geologist is looking at deposits along a river below a large volcano. She finds large boulders mixed with sand and mud, and all of the material is comprised of volcanic rocks, particularly the mud which is volcanic ash. She quickly realizes these deposits represent an ancient .

lahar

Explanation:

An ancient lahar is the likely kind of volcanic deposit discovered by the geologist here considering that an ancient lahar is a mudflow on the flanks of a volcano. It usually consists of;

Thus fitting the description in the question.

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