Unconformities are the geologic features which convinced Hutton and Lyell that the earth was old. An unconformity is a period of erosion found between two rock layers. The evidence of erosion can be severe or mild, but the interval does represent a significant period of time between the deposition of the older (lower) rocks and younger(upper rocks).
Siccar Point is a famous outcrop on the Scottish coast south of Edinburgh. Here is a picture of it.
The lower (now vertical) rocks are Ordovician shales. There is no way that these shales could have been deposited vertically. They had to be deposited horizontally. After they had solidified, they were deformed by crustal collision. The collision was the Silurian collision of Avelonia (part of Britain) with North America. the deformed (now vertical) shales were eroded. At the unconformity (the intersection of the vertical and horizontal rocks in the picture) there are pebbles of eroded Ordovician shale. Then at some later point, the land sank beneath the waves again and the Devonian Old Red Sandstone was deposited on top of the eroded Ordovician surface. These rocks were initially horizontal when they were deposited but gentle tilting of the land has now tilted them slightly. This sequence of actions takes time. (And for another day, I will note, this sequence of deformation shows why runaway subduction can't explain the pattern of continental collisions.)
Now, this is not the only type of erosional unconformity of its kind. Raymond Moore, in Historical Geology, 1933 McGraw-Hill, p. 160 shows a similar type of unconformity between the Ordovician and Silurian of western Pennsylvania. Here is the picture
The Ordovician strata to the left of the picture was initially horizontal when it was deposited. It was then deformed to the vertical (like the lower rocks at Siccar point), and eroded. But these Ordovician rocks were also heated to the point where they are partically metamorphosed. At this time, the line marked 'unconformity' was slightly tilted and there are pebbles of the metamorphosed Ordovician rocks at the surface of the unconformity.
At this point, the Medina sandstone was deposited, once again, it was deposited horizontally. After the Medina sandstone lithified, it too was deformed, making it stand vertically but by coincidence bringing the lower Ordovician rocks almost to horizontality again.
The evidences of time in this unconformity are the erosional pebbles at the unconformity surface, as well as the fact that the Ordovician rocks were heated and the Medina sandstone wasn't. This proves that the Medina sandstone couldn't be there during the heating event. It takes a long time for the rocks to cool down after heating. David Tyler will probably claim that water convection can reduce the time (and he is correct), it still takes time for larger unfractured rocks to cool down because conduction of heat is so slow.
Now some of the unconformities can be quite large as my favorite unconformity taken from a seismic line along the Alabama-Mississippi border.
For a full explanation of this unconformity see:
One young-earth creationist, David Tyler made the claim "Large unconformities are evidences of large scale erosive episodes. Unconformities that are laterally extensive, cut across a variety of sediment types, and leave essentially no relict topography are actually evidence for non-uniformitarian processes operating in the Earth's past." on the web pagehttp://www.theologyweb.com/forum/showthread.php?s=&threadid=16992&perpage=16&pagenumber=1
He is very wrong. In the picture above, I have marked a hill and valley at the unconformity. That is topography. Topography also exists at the unconformity which sits above Hibernia oil field. Here is a picture of that unconformity made from a 3D seismic section.. One can see the topographic relief.
Here is a picture of the Permian Powwow conglomerate lying above Precambrian metamorphics. The conglomerates fill the topographic lows.
The YECs out there might be tempted to claim that this represents preflood topography, this can't be. The cross section shows that the Powwow conglomerate lies atop the Precambrian metamorphics in the SW but it lies atop Paleozoic strata to the NE. That means that the unconformity on top of the Precambrian is of late Paleozoic age and that means that this is in the middle of post flood recolonization or in the middle of the flood according to other YEC models.
Do any YECs have explanations for this data? Do you think you should ask your leaders why they don't show this kind of geologic data in the YEC literature?