Deposits made, directly or indirectly, by the agency of ice are very characteristic, and though some are formed on land and some under water, it is desirable to consider them together in a single section. The peculiar features of ice formations may be much obscured by the action of water, either at the time of their deposition or at some subsequent period. Ice deposits play but a small part quantitatively in the construction of the earth's crust, but the light which they throw upon changes of climate and similar questions, lends them an unusual degree of interest. Only very recently has the great importance of the part played by glaciers in former ages of the earth's history been appreciated.