I'll take a crack at it:
1. If a global flood happened some 4,500 years ago, then we should expect to find one very large layer of sedimentary rock (much thicker than any other layer), and we should be able to find within it the remains of virtually every life form that has ever existed. We should find this layer everywhere around the planet, except in areas where erosion may have removed it.
2. Instead, what we find around the globe are many distinct layers of different types of rock (igneous metamorphic and sedimentary). Furthermore, each sedimentary layer corresponds to a different depositional environment, and the fossils contained within each layer are specific to that layer's corresponding environment.
For example, the Grand Canyon is composed of many different layers of different types of rock. One of those layers is the "Kaibab Limestone," which was laid down by an advancing warm, shallow sea. Shark teeth have been found in this formation as well as abundant fossils of marine invertebrates such as brachiopods, corals, mollusks, sea lilies, and worms. A separate layer is known as the "Hermit Formation," and it contains the fossilized remains of winged insects, cone-bearing plants, and ferns as well as tracks of vertebrate animals. So what we have are different formations of different composition. Each of these layers requires a different depositional environment, and each of these layers contain fossils that are specific to that layer's requisite depositional environment.
3. Therefore, it is impossible that all of these layers could have been deposited in a single event.