Sentence:  When sandstone is coarse-grained, it is usually called "grit". If the grains are rounded, and large enough to be called pebbles, it becomes a <WSD>"conglomerate</WSD>", or "pudding-stone", which may consist of pieces of one or of many different kinds of rock. A <WSD>conglomerate</WSD>, therefore, is simply gravel bound together by a cement. 
Meaning of the word marked with <WSD> tags:  A rock consisting of gravel or pebbles embedded in a matrix.