CompuServe Messages

Kamron: >>Since you seem familiar with glaciers have you inspected the moraine often? I have an unsettled problem with the formation of gravel. Isn't the gravel very rough surfaced in the moraine?<< That depends on the moraine! Moraines in alpine glaciers generally have rough, angular clasts, since most of the gravel is ground from rock surfaces on the valley walls, or consists of fragments that have fallen onto the surface of the glacier from overhanging cliffs or rock-slides. In any case, the clasts have not been transported very far from their origin, and thus haven't been rounded. As well, you sometimes see grooves or striations on these clasts, from their being ground over bedrock and/or larger boulders. On the other hand, the big continental glaciers and large valley glaciers have been bulldozing everything in their path, including old soil and gravel deposits that were originally rounded and reworked by pre-existing rivers. These moraines contain a mixture of rounded and rough clasts, as well as a great deal of very fine (to clay-sized) material. The distinctive feature of moraine deposits is their very poor sorting–everything from house-sized boulders, down to clay, all mixed together. >>I have seen those in National Geographic. Is there more of an explanation…<< The explanation I got from my geomorphology prof. was roughly as follows: as a big continental glacier begins to retreat due to melting at the toe of the glacier, masses of stagnant (ie melting) ice begin to break up into discrete blocks. Some of these blocks happen to be buried under a thin layer of till (gravel), which begins to slump off on all sides of the ice block. The block eventually melts completely, leaving an empty pit, surrounded by a ring of gravel. I don't blame you if you're not totally convinced by this explanation <G>.