Unconformity

Four basic steps create angular unconformities. In step one, sediment weathered from land and carried to the sea accumulates on the sea floor and over millions of years turns to rock layers. Then the collision of plates, giant sections of the earth's crust that constantly shift, lift and tilt the layers until the layers rise above sea level and then weather and erode. They erode for millions of years until the edges of the tilted layers become a flattened plane (a "peneplain" is a broad land surface flattened by erosion). Finally, in step four, sea level rises or land sinks. Sediments wash down, forming new horizontal layers that cover the submerged, tilted layers. These four steps could take hundreds of millions of years to complete.

The Colorado River at the Grand Canyon exposed one of the best angular unconformities in the world. From even miles away on the rim of the canyon, observers can see tilted layers of rock truncated roughly 550 million years ago by a horizontal sedimentary layer called the Tapeats Sandstone. The inner gorge of the Grand Canyon provides a great example of angular unconformity formation. Except, instead of four steps, the rocks tell of seven: (1) Over two billion years ago, layers of sediment accumulated and turned to rock. (2) Around two billion years ago, plate collisions lifted mountains and turned the sedimentary rocks into the Vishnu Schist, a metamorphic rock. (3) A half a billion years later, the mountains eroded into a peneplain. (4) The land subsided or sea level rose to deposit new layers (known as the Grand Canyon Series) on the old Vishnu Schist. (5) New plate collisions tilted and uplifted the Grand Canyon Series. (6) Erosion truncated the tilted Series and created another peneplain. The erosional episode lasted almost a billion years. (7) The land subsided eventually and the Tapeats Sandstone accumulated on the tilted Grand Canyon Series. In some places in the canyon, the Tapeats lies not on the angled Series but directly on the metamorphic Vishnu Schist—making this a nonconformity.

Another famous angular nonconformity is Scotland's Siccar Point, a site which played a part in the development of modern geology. In the eighteenth century, most people believed the earth to be only 6,000 years old, a figure arrived at earlier by Bishop Ussher, a prominent theologian who added the ages of Biblical characters and thus concluded the world was created in 4004 B.C. Scottish scientist James Hutton, however, realized that thousand-year-old Roman ruins in Great Britain were barely touched by weathering and erosion. He therefore wondered how long it takes for whole mountains, like those in Scotland, to wear down.

The angular unconformity he discovered at Siccar Point in Scotland provided dramatic evidence for his time "expansion." He saw nearly horizontal sandstone resting on nearly vertical graywacke (a sedimentary rock similar to sandstone) and marveled at how long it took to deposit the graywacke, tilt it, erode it, and then lay sandstone across it. As his friend, John Playfair, wrote "The mind seemed to grow giddy by looking so far into the abyss of time."