region.Christensen, P. R., Bandfield, J. L., Clark, R. N., Edgett, K. S., Hamilton, V. E., Hoefen, T., et al., 2000, "Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water." J. Geophys. Res.: Planets, 105, 9623–9642. https://doi.org/10.1029/1999JE001093 The dried river valleys are easily seen in thermal inertia images taken in orbit by Mars Odyssey and reproduced in Figure 4 (click on it for higher resolution). The river valleys seen in Figure 4 terminate abruptly as they flow into the Meridiani's massive formation of sediments. Formation of Today's Sediments & Embedded Spherules From around the late-Noachian/early-Hesperian to sometime around 3.5 billion years ago, the layered sediments deposited in the earlier Noachian epoch were transformed. This transformation probably included a significant additional deposition of high-sulfur-content material of volcanic origin. The change certainly included aqueous geochemistry that was acidic and salty, as well as rising & falling water levels: Features providing evidence include cross-bedded sediments, the presence of vugs (cavities), and embedded hematite spherules that cut across sediment layers, additionally the presence of large amounts of magnesium sulfate and other sulfate-rich minerals such as jarosite and chlorides.Clark, B.C., Morris, R.V., McLennan, S.M., Gellert, R., Jolliff, B., Knoll, A.H., et al., 2005, "Chemistry and mineralogy of outcrops at Meridiani Planum," Earth and Planetary Sci. Let., 240 73 – 94. https://doi.or/10.1016j.epsl.2005.09.040Tosca, N.J., McLennan, S.M., Clark, B.C., Grotzinger, J.P., Hurowitz, J. A., Knoll, A.H., Schröder, C., and Squyres, S.W., 2005, "Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum," Earth and Planetary Sci. Let., 240(1), 122–148. https://doi.org/10.1016/j.epsl.2005.09.042Marion, G.M., Catling, D.C., Zahnle, K.J., and Claire, M.W. (2010). "Modeling aqueous perchlorate chemistries with applications to Mars". Icarus. 207 (2): 675–685. https://doi.org/10.1016/j.icarus.2009.12.003. Jarosite formation requires aqueous acidic conditions below pH 3. Figures 5 and 6 show Microscopic Imager close-ups of the sediment rock matrix that appeared in a prestigious paper. Figure 5 illustrates the four physical constituents of sediment outcrop: (i) the sedimentary layers containing a lot of basaltic sand particles; (ii) the embedded hematite spherules; (iii) fine-grained, sulfate-rich cement (in most parts of the outcrop); (iv) vug cavities (that are thought to be molds for crystals of, for example, hydrated sulfates). Figure 6 images a similar sediment outcrop surface to Figure 5. However, Opportunity's Rock Abrasion Tool abraded this surface. Such abrasions showed that (a) the sediment layers are very soft and easy to cut, and