Because it is so solid, shale is a great rock to create a foundation on in the building industry. Furthermore, unlike clay, it is compact enough to withstand a structural pile without splitting. Its weakness lies in its susceptibility to heat and water, which can cause it to break down over time. The most common type of shale found in the United States is sandstone, but there are also deposits of siltstone, mudstone, and coal. Shale is a form of sedimentary rock that was once part of a sea floor. As the name implies, it is made up of large sheets of hardened sediment called slates. Over time, these sediments were deposited in shallow seas or oceans where they were compressed into rock by tectonic plates. Shales can be very thin or very thick; there are even cases where layers of shale have been stacked on top of each other like sheets of paper. Thicknesses range from a few inches to more than 10 feet (300 mm). There are several varieties of shale. Those used for building purposes must be quite dense since the weight of a structure depends on its mass per unit area. Denser shales contain more glassy particles that result in stronger rocks. On the other hand, less dense shales contain more pores that allow water to seep through them.