In this case a horizontal line would be a chronostratigraphic horizon. In other words, it would be comparable to the surface of the earth at a given time, or a transect of the depositional zones in Walther's Law. The Mauv and the Bright Angel are being deposited at the same time, not in different time periods.
If you are trying to communicate with Faith you should avoid at least 4 of those words which aren't helping.
From Message 831 we see 35 meters deposited over 42,000 years at Lake Suigetsu. That's 0.8333 mm per year, and over a million years that would result in sediment 833.33 km thick. We know this time because of the annual layers of the diatom/clay varves, confirmed by 14C dating calibrations.
35 meters in 42,00o years is 0.833 mm per year fine. But 1 million times that is only 833,300 mm which is 833 meters not kms.
The continents hadn't fully formed and, as I vaguely recall, the tectonics hadn't gotten rolling. This is in the first billion years or 2 but all of this is from memory which is worth just as much as you think it is .