A medial moraine is a narrow band of rock debris which usually runs down the centre of the glacier. It forms from the merging of the lateral moraines of two glaciers and often occurs in regions that have been acted upon by a past ice age.
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Material through which water flows is permeable. This material can be either natural or artificial. Naturally permeable material can be clean sand and gravel for example, or a sandstone and in the latter case the sandstone can have both porosity which is not synonymous with permeability as the pores must be connected to be permeable. An impermeable membrane could be used to line ditches for example to prevent leakage near mines.
