Wednesday, June 13, 2012
White Rock Lake in Dallas Was Once an Ancient Sea
Did you know that White Rock Lake in Dallas, Texas was once an ancient sea?
North America was literally divided into two landmasses by a large inland sea called the Western Interior Seaway (also called the Cretaceous Seaway and the North American Inland Sea) that existed during the Mid-to-Late Cretaceous Period.
Western Interior Seaway - 100 million years ago
This ancient, shallow sea had abundant marine life and stretched from the Gulf of Mexico in the south and through the middle of what we now know as the United States and Canada, meeting with the Arctic Ocean to the north. This sea was 600 miles wide and over 2,000 miles long. All of Texas would have been covered by this ancient sea.
The Cretaceous period succeeded the Jurassic Period approximately 144 million years ago and lasted to approximately 65 million years ago. The word Cretaceous comes from the word for "chalk," a soft, white, porous sedimentary rock that is a form of limestone. Indeed, if you walk around White Rock Lake, you will see intermittent outcrops of white limestone, and this is how the lake got its name: White Rock Lake.
If you look carefully at these limestone outcrops, you may be lucky enough to identify crustacean shells dating back to the Cretaceous Period, such as these found at Winfrey Point.
Shells and other items found in the limestone at Winfrey Point
The sea began to wane about 70 million years ago and the ancient sea bed gradually became exposed over time through erosion and other upheavals. This ancient sea bed is the white rock that we now see exposed around the lake, especially the area below the spillway where large tracts of limestone are clearly visible, and known locally as Austin stone.
It is during the Cretaceous Period that we find the first fossils of many insect groups, modern mammal and bird groups, and the first flowering plants (called angiosperms). Around White Rock Lake, the most common fossils to be found are organisms such as clams and oysters. Shark teeth, fish and marine reptile remains have also been found at the lake on rare occasions.
A closer view of the shells
At the end of the Cretaceous Period, 65 million years ago, an asteroid hit Earth in the Yucatan Peninsula, Mexico. This event resulted is what we now call the Chicxulub impact crater, buried under the Yucatan Peninsula. The crater is more than 110 miles in diameter.
It has been estimated that half of the world's species went extinct at about this time (including the Dinasaurs), either from the impact of the asteroid or other reasons, such as environmental changes. Whatever the cause of this extinction, this asteroid event marks the end of the Cretaceous Period.
The next time you take a walk around White Rock Lake, keep your eyes peeled. That stone you are kicking may just turn out to be a 60 million-year-old fossil.