The once deeply buried old igneous rock bodies are actually compressed by the weight of the overlying rock. As erosion removes the overlying rock, the once buried underlying rock expands (strain release or unloading) parallel to the ground surface (technically the surface of lowest confining pressure). These unloading fractures form sheets of rock that are near horizontal under gently sloping landscapes (Photo 1 above), and steeper under more steeply sloping mountain sides.
These two mechanisms have resulted in intersecting horizontal and vertical fractures that naturally break the bedrock into rectangular blocks (Photo 2 below). These fractures also produce surfaces for weathering to act upon, breaking up the rocks further. The closer together the fractures, the smaller the resulting blocks of rock are. The closer spacing also helps to speed up weathering and erosion of the bedrock. So inland valleys and bays along shore form where there are more closely space fractures. The road cuts along the northern end of Somes Sound show some of these more closely spaced fractures. Granite quarrying takes advantage of these natural vertical and horizontal fractures to help guide the breaking out of blocks from the quarry face.