The boulders, sand, clay, and silt that drop from a glacier when it retreats are called glacial till.

Here's a breakdown of why:

* Till: This is the general term for unsorted sediment deposited directly by a glacier.

* Unsorted: This means the sediment is a mixture of different sizes, from large boulders to fine clay particles, unlike sediment deposited by rivers or wind, which is usually sorted by size.

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