Moraines are composed of jumbled piles of very poorly sorted rock fragments ranging in size from boulder to clay that fall out near the edges of glaciers. Why are the larger particles (boulders, etc.) not broken down during transport in the glacier

The larger particles (boulders, etc.) not broken down during transport in the glacier because of the slow speed of glacier and abrasion.

Explanation:

When a mountain glacier start moving from mountain towards valley it creates some sorts of landforms by following some methods like abrasion, attrition etc. Moraine formed as a depositional land form in front, back, parallel and side pf a glacier. It refers to the accumulation of sediments ranging from large boulders to small particles at the front or at the end of a glacier.

The large boulders do not break down in smaller partcles as the followed the process of abrasion where friction takes place between glacier and its valley floor. Unless and untill the collision between two large boulders occur the boulders would not broke down.