They believe that agents shaping the Earth's surface today have been working throughout the Earth's history. They also believe that the fundamental laws of chemistry, physics, and biology operate today in the same way as they did in the past. An illustration of the principle of uniformitarianism is the observation of ripples forming in sand on a beach, in a stream, or on the bottom of a lake. The ripples form because the sand is being moved along by flowing currents of water. If a slab of rock has ripple marks on its surface, geologists assume that the rock was once sand moved by a current of water.
Geologists can even tell which way the current once flowed by the shape of the ripples in the rock.
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