A Bicycle Trip Through Dating Rocks...

In response to this post:

>In all honesty, I will say that when I have tried to look at the evidence to determine what I'm going to go with as far as beliefs, I have never been impressed with the case for an old earth. There are too many unanswered assumptions. (Yes, I see there are all these layers... But how do you know it took millions of years for each layer to form?) That is just one example. I do not see them [evolutionists or non-Young Earth Christians] defending their assertions.

What follows is just a really, really simple description of one of the many reasons we can put some confidence in geology.  As with all sciences, conclusions can and should be rewritten as new data become available, but what we know reliably is based on solid data.

In geology you deal with layers of rock called "strata." Dating strata can be tricky, but scientists are generally reluctant to accept dates that are not supported by clear evidence.

For example, imagine a geological formation that consists of three strata. The bottom layer is "igneous" basalt, a volcanic rock that is formed by cooling magma. This is a rock that can be reliably dated, because cooling magma traps in it radioactive elements whose half-life is a known constant.

Let's say that we consult the "parent" and "daughter" elements of that stratum, do the math, and find that it is 5 million years old. Now we ascend up through the strata and find a layer of rhyolite, another igneous stratum. We peg that one at roughly two million years.

What we've done is called "bracketing." That is, we've determined the upward and downward ages of the intervening stratum: because those layers formed between two volcanic strata that are very reliably dated, we know that they formed somewhere between five and two million years ago.

For simplicity's sake, imagine that there is only one intervening stratum, one of sandstone. It exists between 5 million and 2 million years ago, which is pretty clear evidence that, whatever happened in that stratum, happened between those dates.

We have several techniques of collating our data with other such formations. The mathematical relationships between them give us a pretty good idea of how and when such formations come to be.

If we can rule out major trauma to the earth's crust (say, uplift or earthquake), and we generally can without much trouble because such trauma leaves discernible footprints, we've got a really nice and generally reliable snapshot of the timetables of various formations.

Again, not all geological problems are so simple and clear-cut.  But you can see that the inferences one makes are testable, that they rely on sound observations.  If you can think of objections, I urge you to pick up a really good book on geology and learn how we really do have a complex and reliable base of data to work from.