Pictured above is the contact between two very different rocks: the Brownstown gravel above, and the Jackfork Sandstone below. This is what geologists call an angular unconformity. The contact represents a vast period of time.
The lower sandstone was deposited in the Pennsylvanian period, around 300 million years ago, when Arkansas was at the bottom of an abyss. It was deposited in horizontal layers, but later, when the South American continent collided with the North American continent, it was deformed into mountains and raised above sea level. For about the next 200 million years it eroded and subsided, until eventually, a shallow sea covered it again. Rivers washed gravel into that sea, which became the Brownstown gravel.
This picture was taken near De Gray Lake State Park in south central Arkansas.
The above picture at first glance doesn’t look like much but from a geological perspective these rocks convey a lot of information about the history of the earth. This is what geologists call an angular unconformity. An unconformity is simply a gap in the rock record; it represents a period of time during which either erosion was taking place or there was no sediment being deposited.
We call this kind of unconformity angular because the lower rock beds are tilted at a different angle than the upper beds. We know from the differences between the fossils in each of these rocks that there was about an eighty million year gap between deposition of the lower and upper rock sequences. During that 80 million years the lower formation was exposed above sea level, eroded, and tilted by tectonic forces in the earths crust. Eventually deposition resumed and the upper unit was deposited on the eroded and deformed lower unit.
It’s amazing what you can learn from a few rocks! If you’d like to see these rocks in person, float the Buffalo National River from Tyler Bend to Gilbert.