Boulder clay

Boulder clay is an unsorted agglomeration of clastic sediment that is unstratified and structureless and contains gravel of various sizes, shapes, and compositions distributed at random in a fine-grained matrix.

The fine-grained matrix consists of stiff, hard, pulverized clay or rock flour.

[1][2] The term boulder clay is infrequently used for gravelly sedimentary deposits of nonglacial origin.

The glacial erosion of granites, gneisses and quartzose schists often results in the formation of stony, coarse and gravelly boulder clay that, in some instances, can be hardly distinguishable from in situ decayed rock.

These physical properties result from a combination of the fine rock-flour it contains and the weight of the overlying ice while the boulder clay accumulated.