Fig. 205. Unconformity showing Buried Valleys
lm, limestone; sh, shale; r, r´, and r´´, river silts filling eroded valleys in the limestone. The upper surface of the limestone is evidently a land surface developed by erosion. The valleys which trench it are narrow and steep-sided; hence the land surface had not reached maturity. The sands and muds, now hardened to firm rock, which fill these valleys, r, r´, and r´´, contain no relics of the sea, but Instead the remains of land animals and plants. They are river deposits, and we may infer that owing to a subsidence the young rivers ceased to degrade their channels and slowly filled their gorges with sands and silts. The overlying shale records a further depression which brought the lanes below the level of the sea. A section similar to this is to be seen in the coal mines of Bernissant, Belgium, where a gorge twice as deep as that of Niagara was discovered within whose ancient river deposits were found entombed the skeletons of more than a score of the huge reptiles characteristic of the age when the gorge was cut and filled
Fig. 206. Unconformity showing Buried Mountains, Scotland
gn, ancient crystalline rocks; ss, marine sandstones. The surface bb of the ancient crystalline rocks is mountainous, with peaks rising to a height of as much as three thousand feet. It is one of the most ancient land surfaces on the planet and is covered unconformably with pre-Cambrian sandstones thousands of feet in thickness, in which the Torridonian Mountains of Scotland have been carved. What has been the history of the region since the mountainous surface bb was produced by erosion?
Unconformities in the Colorado Canyon, Arizona. How geological history may be read in unconformities is further illustrated in Figures [207] and [208]. The dark crystalline rocks a at the bottom of the canyon are among the most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstones b, whose total thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip. Both a and b rise to a common level nn´ and upon them rest the horizontal sea-laid strata c, in which the upper portion of the canyon has been cut.