Complex sentence: The lithology underlying the dam is a metamorphic rock called gneiss, this rock type is known to be relatively impermeable meaning that there is no significant groundwater flow within the rock unit also it does not allow water to penetration the ground.
Split into simpler sentences: The lithology underlying the dam is a metamorphic rock called gneiss. This rock type is known to be relatively impermeable meaning that there is no significant groundwater flow within the rock unit, and it does not allow water to penetrate the ground.