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

Here's why:

* Till is unsorted sediment deposited directly by a glacier. This means the material hasn't been reworked by water or wind, so it can include a wide range of sizes, from large boulders to fine clay.

* Glacial indicates the origin of the sediment, which is from a glacier.

So, glacial till is the collective term for all the debris that a glacier leaves behind as it melts and retreats.