Читать книгу One Best Hike: Grand Canyon. Everything You Need to Know to Successfully Hike from the Rim to the River—and Back онлайн

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No records remain from the following 120 million years, probably because decreasing sea level at the end of this period allowed the upper sediment layers to erode. One intermediate layer, the Temple Butte Limestone, is present as a thick stratum in the western Grand Canyon, where waters were deeper, but along the Bright Angel and South Kaibab trails it exists only in eroded channels in the Muav Limestone. Note that each of the Grand Canyon’s rock layers above the Muav Limestone is separated by an unconformity; some gaps in the rock record are brief, but others correspond to the removal of considerable sediment.

When rock strata abutting one another do not represent a continuous time sequence, the surface between the two layers is referred to as an unconformity. This gap in time indicates that sediment was eroded from atop the lower stratum before the upper stratum was deposited.

By 340 million years ago sea level was again rising, and much of the Colorado Plateau region was submerged beneath a large, shallow sea. Rivers transported little sediment to the region, creating the clear water environment that promoted the deposition of the thick layer known as the Redwall Limestone. The sea then retreated, eroding the top of this layer.