You call it proof when it's nothing but the usual speculative guesswork. If things were appreciably different before the Flood, as we believe, then your assumptions don't hold water. As it were.
You are entirely correct.
IF Nature operated in a different manner yesterday than today, then the assumptions geologists make today about the rocks would be invalid.
However, it is not an unreasonable assumption for geologists to make that Nature operates in a very similar manner today as it did yesterday. Our evidence for this are the many rock formations, rock textures, rock mineralogies, rock types, etc. present in the rock record that have modern sedimentary analogs. Examples of these I've mentioned many times in the past include, sand dunes, stream channels, beaches, etc.
An unreasonable assumption to make is that Nature operates substantially different today than it did yesterday when there is no evidence to support it. If you do make that assumption, then YOU have the obligation to provide the evidence in support of this assertion. Until you do so, you are presenting nothing but personal opinion and the rest of the world has the right to ignore it. Therefore, a global flood must still follow the rules of Nature.
Making the reasonable assumption that Nature behaved similarly in the past, the first stages of the global flood, an extremely high-energy environment, would erode and deposit in the same manner of floods today -- except at a much more massive and extensive level. We should -- AT THE VERY LEAST -- see in the rock record evidence of :
-- mass wasting and landslide deposits
-- deeply carved canyons (MANY!) in many parts of the earth (particularly at the bases of large mountain chains or high altitude plains) -- mass graves containing a multitude of KNOWN animal skeletons (not dinosaurs, unless there was a secret valley where all these strange and unknown animals lived) -- and immense marine deltas (with many lobes due to intense upriver erosion) where eroded material shedding off the continents was deposited in the marine basins
These deposits would form the base of the flood rock record and any Creationist should be able to find this sequence of rocks and .
Below this sequence of rocks, you should be able to find and put your finger on the rocks upon which man lived. Above these deposits, would be evidence of the flood waters deepening, some quiescence, and then recedence of the waters. Each of these three hydrologic regimes will deposit sediment in distinct manners because of how the water is behaving. These rocks should be in the rock record and Creationists should have already identified, described, and explained them in minute detail.
When the Creationists can point out all these rocks and explain EVERY SINGLE ROCK TYPE, then and only then will they be taken seriously. To date, NONE of you have been able to do so. In fact, I am still waiting to hear from you, Faith, how flood waters deposit carbonate when today, carbonate depositing environments require very specific water chemistry, water temperature, environmental, and sunlight conditions.
You are correct in suggesting that modern geology requires a lot of speculation. Geologists are notorious for disagreeing with each other. We agree on what constitutes a sandstone, granite, gabbro, fault, limestone, a coral fossil, a tectonic plate, what a sand dune or stream deposits look like, and anything that has been specifically defined or identified, but we often disagree on individual interpretations of how landscapes evolved, timing of faults, types of faults, why rocks are deformed, how gold or other metals end up where they are found, and thousands of other things.
Geology is hard. There is a lot of guessing, assuming, speculating, hypothesizing, and arguing about the past in order to figure out why the rocks are the way they are today. That even happens in my group of 8 geologists on rocks that have been studied continuously, in detail, over the last 100+ years. But that's why we research geochemistry, mineralogy, isotopes, mineral assemblages, and why we go out into the field and map the geologic units in as much detail as makes sense. We can never know exactly how the past happened, but we can construct a very nice picture of it using the data we collect.
Creationist assertions that the rock record is unknowable, untestable, and unpredictable are completely unfounded, and spoken from a place of abject ignorance. This is refuted by the sheer number of books and technical papers detailing the research upon which our interpretations are based. And of course refuted by the fact that we can predict the rocks, fossils, ore deposits, and a multitude of other things that we will find in the subsurface.