Final answer:
The relative sea level rise along the U.S. East Coast roughly 20,000 years ago was primarily caused by the melting of vast ice sheets from the last glacial maximum and the subsequent release of water into the oceans, as well as land rebound and minor contributions from thermal expansion of ocean waters.
Step-by-step explanation:
Approximately 20,000 years ago, during the last glacial maximum, the relative sea level along the East Coast of the U.S. rose due to the end of the glaciation period. The main cause of the rise in sea level was the melting of massive ice sheets that had previously covered large areas of what is now North America and other parts of the Northern Hemisphere.
As temperatures increased and the ice melted, vast quantities of water that had been stored as ice on land flowed into the oceans, contributing to a global sea level rise. This process not only increased the volume of water in the oceans but also caused the previously ice-burdened land to rebound and rise, in some cases, further altering relative sea levels. Moreover, the thermal expansion of the warming ocean water also contributed to the rising sea levels, albeit to a lesser extent during this time period compared to the effect of the melting ice sheets.
The historical data indicates that at its peak, the glaciation resulted in sea levels that were about 120 meters lower than today's levels. As the climate continued to warm past the last glacial maximum, the melting glaciers and ice sheets played a significant role in the rising sea levels, shaping the coastlines that we are familiar with today.