There are still a few ancient earthfast buildings on the East Coast, but I am not sure just how old they are. I doubt this type of building would normally last much more than 50 years in the East.
Some trivia for you; earthfast posts must be the modern archeaologists name for piles, an ancient technique for building on soft ground, even underpinning stone foundations! At Plimouth Plantation they are called puncheon posts. In your area, there may still be some box houses, also known as single-wall, California cottages, or board-and-batten houses. With these houses it was standard practice to build them on posts set into the ground. The Dells seems to be an area where these were used in the mid to late 19th century, maybe later.
Take care;
Jim
