An Ancient Land of Martian Lakes Examples of layered unit outcrop expressions. If life existed on Mars during this time period, researchers believe that the fossil remnants of that past life may be sandwiched within the sedimentary rock layers, just as they are on Earth. The martian outcrops, in some cases a few kilometers thick, appear to be made of fine-grained materials deposited in horizontal layers, the hallmark of sedimentary rock. These outcrops are found inside craters, between craters, and within chasms, said Michael C. Malin and Kenneth S. Edgett of Malin Space Science Systems in San Diego, California.
Massive units appear as one bulky rock layer with no clearly defined horizontal bedding.
In a few cases, these types appear together, with the massive unit always perched on top of the bedded unit like a thick, indistinct coat of frosting on a layer cake. Thin mesa units, with surfaces ranging from smooth to pitted to ridged and grooved, are almost always found on top of eroded massive or layered sedimentary rock. Under this scenario, sediments may have been transported into the lakes in regular, swift pulses, building up thin layer units. Massive units may have been deposited when the lake became stagnant or deep enough to cause sediments to sift down through the water over longer intervals. The sedimentary units show no telltale signs of wind deposition, and the researchers concluded that explosive volcanic eruptions and impact cratering probably could not have produced enough sediment to create the large-scale and geographically widespread outcrops seen on the martian surface.