Bedrock Geology around Dinosaur State Park
The rock formation at Dinosaur State Park is the East Berlin formation (map symbol Jeb), which is close to 200 million years in age. The formation took about 300,000 years to form, and it consists mainly of solidified mud and sand that was once in and around lakes in the Connecticut Valley. The climate went through cycles of wetter and dryer periods each lasting about 20 thousand years, so the lakes grew and shrank several times as sediments of the East Berlin formation were being deposited. Streams and rivers brought the sand and mud into the valley from the northeast and northwest. When the lakes were deep, finer mud and clay were deposited, which are dark gray because oxygen levels in these materials were too low to oxidize the iron in the sediment. When the lakes were shallow or absent, coarser sand and silt were deposited in an oxidizing environment, causing them to have red rusty colors.
East Berlin formation layers in the indoor exhibit.
You can see these alternating red and gray sedimentary layers in the small rock cut outside the building as well as in the indoor exhibit. The footprint tracks were made by animals walking across a soft wet silty-muddy surface, probably close to one of the lakes. The tracks must have dried and hardened, and then were quickly covered by new mud layers that preserved them. Coarser sand and gravel do not preserve tracks very well, although bones are sometimes found in the sandstone layers. If you trace the portion of the East Berlin formation below the Hampden basalt (Jha) around the area to major highways, you might be able to predict where similar layers with more Dinosaur tracks are exposed. Such extrapolations seem to be true, but it is neither safe nor legal to stop and dig for tracks along the highways.
A portion of the bedrock geology map of the South Hartford quadrangle.
On the geological map of the South Hartford quadrangle, several faults oriented NE-SW (heavy black lines) have broken up the earth's crust, so that the rock layers are offset or repeated across the fault lines. Although they were formed in a nearly horizontal position, the layers now dip generally downward towards the east about 10 to 15 degrees, except where they have been deformed by faults as in the region. The pink formation labeled Jha on the map is the Hampton basalt, a lava flow about 150 feet thick that originally covered a large section of southern New England. In terms of vertical separation, the East Berlin layers at the Park are less than hundred feet beneath the Hampden basalt. You can see the Hampden basalt west and south of the Park in roadcuts made for the corporate center south of West Street. The Jho formation is the Holyoke basalt, an even larger lava flow beneath the East Berlin formation. These lava flows were extremely large, so they are called "flood basalts" because of the way they poured out of the earth to cover great sections of the land. This photo of an Icelandic fissure eruption shows what these volcanic events looked like in Connecticut.
The Krafla fissure eruptions in Iceland in the late 1970's.