As with other continents, the mounds and pyramids of North America vary greatly. It could be that humankind has a primal need to build fake mountains, and that there are absolutely no connections between these sites. Or perhaps size and shape are irrelevant, and location is everything, and the guidelines for their placement was once universally known.
Monk's Mound at Cahokia, USA
Just east of St. Louis, near Collinsville, Illinois is the largest earth mound in the western hemisphere. It is 30 metres high and dates back to 1100-1400 AD.
"The largest of these mounds, Monk's Mound covers 16 acres; it rests on a base 1,037 feet long and 790 feet wide, with a total volume of approximately 21,690,000 cubic feet, a base and total volume greater than that of the pyramid of Khufu, the largest in Egypt. In all the world, only the pyramids at Cholula and Teotihuacan in central Mexico surpass the Cahokia pyramid in size and total volume. No other structure in the United States approached the size of the Cahokia pyramid until the building of airplane hangars, the Pentagon, and skyscrapers in the twentieth century." 
There are more than one hundred other, smaller mounds at Cahokia - as well as Woodhenge, which is of course a wooden counterpart to England's Stonehenge.
According to the Cahokia website, around March 1998 something unexpected happened:
"During the process of installing horizontal drains to relieve the internal water in Monks Mound that had contributed to several severe slumping episodes along the west side (Second Terrace), the drilling rig encountered stones about 140 feet in and 40 feet below the surface of the Second Terrace. The operator said it felt like "soft stone," probably limestone or sandstone, and that it was mostly cobbles or slabs at least six inches in diameter. The drill went through about 32 feet of stones and the drill bit broke off. We have no idea what it is, what shape or size it is, or why it is there. It should not be there. No other cores or excavations have revealed stone in Monks Mound or any other mound at the site, or, as far as we know, at other Mississippian mound sites. We do not know its vertical thickness or the extent of it horizontally, other than the 32 feet that the drill went through."
Etowah Mounds of Cartersville, Georgia, USA
These were made during the same Mississippian Temple Mound Building Period, as were mounds at Moundville (near Tuscaloosa, Alabama) and at Cahokia - roughly 700 AD to 1400 AD.
The six flat-topped earthen knolls and a plaza were used for rituals by several thousand Native Americans between 1000 and 1500 A.D. The largest mound has a height of 63 feet. Only nine percent of this site has been excavated, but we already know that the mounds have caves underneath them as do some Mayan and Giza pyramids.
It may also just be a coincidence, but there is a Limonite mine at Etowah. Limonite is a iron-bearing ore with a very special use - as radiation shielding for atomic bomb tests, nuclear reactors and space stations. It is also what gives Mars its red colour.
Poverty Point, Louisiana, USA
Poverty Point combines mounds with an aspect of ancient Rome - an amphitheatre. Consisting of concentric ridges 5-10 feet high and 150 wide, the construction has a diameter of ¾ of a mile, five times the diameter of the Colosseum in Rome. The ridges were built with 530,000 cubic yards of earth (over 35 times the cubic amount of the Great Pyramid of Giza). Of the earth mounds, one has a base of 700 feet by 800 feet and is 70 feet high. It is shaped like a bird.
Miamisburg Mound, Ohio, USA
This mound is conical, like Silbury Hill in England. Archaeologists believe that it was constructed by the Adena Indians (800 BC - 100 AD). The mound sits on a 100 foot high bluff, and measures 877 feet in circumference. Originally it attained a height of 70 feet.
- See also the Conus Mounds of NE USA - they are not very big, but they are many
It is hard to determine how many mounds were built in North America, for many have been destroyed by modern civilization - but there were many. One hundred and fifty years ago, for example, there were approximately 20,000 Indian mounds in Wisconsin alone, with a large portion of them alongside the Mississippi River. Now, after a century of plowing, town construction and urban sprawl  , there are less than 2,000 mounds remaining in the state.
As anyone who was in the area in 1927 would tell you: the Mississippi floods. In that year over one thousand people drowned, almost a million people were forced from their homes, and over five million acres of farmland were ruined. Regular flooding replenishes the soil in this region and makes it rich. This is the same reason the ancient Egyptians lived alongside the Nile. In fact, the similarities between the Mississippi and the Nile (floods and pyramids) caused the original settlers of Illinois to name new towns after places in Egypt, such as Goshen, Cairo and Thebes.
This region has many, many pyramids. Here I choose to merely describe a handful of them.
There are three stepped pyramids within this ancient Mayan ceremonial centre, located in the Mexican state of Chiapas. The more famous of these is the Pyramid of Inscriptions. Its height of 20 metres consists of eight stepped storeys, not counting the temple on top. Inside the temple, two large vaulted chambers house three glyphic panels which collectively make the second longest known ancient Mayan inscription . Deciphered in the 1970s, the inscriptions revealed that the funerary crypt within the pyramid belonged to Lord Pacal, whose reign ended with his death in 683 AD. The crypt is found via a secret staircase below a slab in the temple floor. Within this crypt was found a five-ton slab of carved rock lying on top of Pacal's sarcophagus. Much has been made of this lid, especially by Erich von Daniken, because of its bizarre imagery - easily taken to be a man in a spaceship. See it here
Central American pyramids are closely related to the ziggurats of Sumer, Babylonia and Assyria: they are step pyramids with a central staircase, and a shrine at the top. Size and geometric perfection seem to be less important than religious and mathematical function.
Pyramid of Inscriptions (left)
Near Mexico's current capital, this city of ruins was thought to have been home to 125,000 people in 600 AD. Teotihuacan was founded in 100 BC, and deserted by 750 AD, which sounds tragic yet few civilizations manage to last 850 years.
Founded by who? Archaeologists have been trying to figure this out and have few clues to work with. The civilization was smart enough to create this massive city, but no trace has yet been found of a writing system - there is the occasional pictograph, but no hieroglyphs or words have been found.
Whoever they were, they were succeeded by the Toltecs, and then by the Aztecs. The Aztecs named the place ("Place of the Gods") and its major structures.
From the air, it looks not unlike a computer circuit board, containing two large processor chips, the Pyramid of the Sun and the Pyramid of the Moon, and 600 smaller pyramids. Once again, there are parallels with Egypt - the construction of satellite pyramids. The Pyramid of the Sun has sides of 225m, giving it a similar base area to the Great Pyramid of Giza, although it is only half the height. Like the Great Pyramid, it incorporates the mathematical ratio of "pi". The perimeter of the base of the Pyramid of the Sun is 4pi times its height, whereas the Great pyramid of Giza's base perimeter is 2pi times its height.
Another Giza connection: the tops of the Pyramids of the Moon and the Sun are level, because the smaller Pyramid of the Moon is built on an elevated location. The same relationship exists between the two largest pyramids at Giza.
And another. Bottom middle of the red layout below is The Citadel, which many consider to be the base of a pyramid the size of the Pyramid of the Sun, that was never built. Had it been, the three main pyramids would've had a similar Orion layout as Giza.
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In 1971 it was discovered that the Pyramid of the Sun was built over a natural cave with four chambers, which had been enlarged from a natural lava tube (the region is very volcanic). The cave was obviously a sacred place for it contained remains of offerings and rituals from a period much earlier than the pyramids.
It is common for ancient sites to have been built in stages, reinforcing the idea that it is the location that is most important. The Pyramid of the Moon went through five distinct phases of building successively larger pyramids on top of previous ones, often partially destroying the previous pyramid in the process. If the unknown civilization had continued for a few hundred years more, perhaps even larger pyramids would have been built on top of the existing ones. Anyone looking for meaning in the angles and lengths of the pyramids should perhaps take into account the prior phases buried beneath the last layer of construction.An interesting fact highlighted by Graham Hancock in Fingerprints of the Gods is that some of the pyramids contain broad, thick layers of mica, which had to be transported over 2000 miles from Brazil. These sheets are ninety feet square and were not visible, but merely another layer in the construction. Perhaps its purpose was radiation shielding? Sheets and rods of mica bonded with glass can tolerate extreme temperatures, radiation, high voltage, and moisture. One of the modern uses of mica is windows for microwave ovens. It is also used in Geiger counters and cosmic ray detectors. (see here )
The Pyramid of Kukulkan is 30 metres in height, with sides that measure 55.3 metres. Graham Hancock explains the effects generated by it's positioning in Chapter XXX.
Most of the many other Mayan pyramids are of the same basic shape.
Los Guachimontones in Mexico, the only place you can find circular, stepped pyramids.
Many of Peru's ancient structures have lasted well, but not the pyramids. Most of them have eroded so much they are unrecognisable. Below is a restored adobe pyramid from around 800BC, located in the middle of the high-rise district of San Isidro.
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Covering over 540 acres and including 26 major pyramids the Tucume site dates to about 1100 AD. The largest of the adobe brick pyramids, Huaca Larga, is 2300 ft long, 910 ft wide and 65 ft high, although it may have originally been three times this height.
Is it so surprising that, with so many pyramids, the Americas also had mummies? This one was found within a pyramid at Huaca Huallamarca.
 Weatherford, Jack, Native Roots. How the Indians Enriched America. Fawcett Columbine, New York, 1991. Page 9
 by Robert "Ernie" Boszhardt, Mississippi Valley Archaeology Center, http://www.epa.gov/reg5ogis/t_histry.htm