question: how are fossils formed and how is the age of them determined?
comment: It wasn’t a literal cannonball, but a naturally occurring rock that resembles a cannonball. Generally speaking, fossils occur when minerals in the soil replace organic matter over very long periods of time, creating an exact replica of the original organic matter that won’t decay.
passages: Section::::Dating.:Estimating dates.:Stratigraphy.
   Section::::Fossilization processes.
  
  The process of fossilization varies according to tissue type and external conditions.
  
  Section::::Fossilization processes.:Permineralization.
   Section::::Biostratigraphic methods.
  
  Mammal zones were, like all biozones, established using geographic place names where fossil materials were obtained. The basic unit of measure is the first/last boundary statement. This shows that the first appearance event of one taxon is known to predate the last appearance event of another. If two taxa are found in the same fossil quarry or at the same stratigraphic horizon, then their age-range zones overlap.
   Section::::Types.:Derived, or reworked.
  
  A "derived", "reworked" or "remanié fossil" is a fossil found in rock that accumulated significantly later than when the fossilized animal or plant died. Reworked fossils are created by erosion exhuming (freeing) fossils from the rock formation in which they were originally deposited and their redeposition in an younger sedimentary deposit.
  
  Section::::Types.:Wood.
   Section::::Dating.
  
  Section::::Dating.:Estimating dates.
   Oldest dated rocks
  
  The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently broken down by erosion or melted, are more than 4 billion years old, formed during the Hadean Eon of Earth's geological history. Such rocks are exposed on the Earth's surface in very few places. 
   BULLET::::- Sedimentary facies:On a longer time scale, the rock record may show signs of sea level rise and fall, and features such as "fossilised" sand dunes can be identified. Scientists can get a grasp of long term climate by studying sedimentary rock going back billions of years. The division of earth history into separate periods is largely based on visible changes in sedimentary rock layers that demarcate major changes in conditions. Often, they include major shifts in climate.
  
  Section::::Reconstructing ancient climates.:Sclerochronology.
   Section::::Dating methods.:Absolute dating.
  
  Geologists also use methods to determine the absolute age of rock samples and geological events. These dates are useful on their own and may also be used in conjunction with relative dating methods or to calibrate relative methods.
   The oldest rock formation is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane, Nuvvuagittuq greenstone belt, or the Acasta Gneiss (on the Slave Craton). The difficulty in assigning the title to one particular block of gneiss is that the gneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dike. This may be difficult to find or map; hence, the oldest dates yet resolved are as much generated by luck in sampling as by understanding the rocks themselves.
   BULLET::::- Oxidation of iron from Fe to Fe by O as the former soil becomes dry and more oxygen enters the soil.
  
  BULLET::::- Drying out of hydrous ferric oxides to anhydrous oxides - again due to the presence of more available O in the dry environment.
  
  The keys to recognising fossils of various soils include:
  
  BULLET::::- Tubular structures that branch and thin irregularly downward or show the anatomy of fossilised root traces
  
  BULLET::::- Gradational alteration down from a sharp lithological contact like that between land surface and soil horizons
   Section::::Sources of evidence.:Geochemical observations.
  
  Geochemical observations may help to deduce the global level of biological activity at a certain period, or the affinity of certain fossils. For example, geochemical features of rocks may reveal when life first arose on Earth, and may provide evidence of the presence of eukaryotic cells, the type from which all multicellular organisms are built. Analyses of carbon isotope ratios may help to explain major transitions such as the Permian–Triassic extinction event.
  
  Section::::Classifying ancient organisms.
   Paleontology is the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils. The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
   Section::::Properties.:Fossils.
  
  Among the three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification.
   BULLET::::- c.3,200 Ma – Mesoarchean Era starts. Onverwacht series in South Africa form - contain some of the oldest microfossils mostly spheroidal and carbonaceous alga-like bodies.
  
  BULLET::::- c.3,200–2,600 Ma – Assembly of the Ur supercontinent to cover between 12–16% of the current continental crust. Formation of Limpopo Belt.
  
  BULLET::::- c.3,100 Ma – Fig Tree Formation: second round of fossilizations including Archaeosphaeroides barbertonensis and Eobacterium. Gneiss and greenstone belts in the Baltic Shield are laid down in Kola Peninsula, Karelia and northeastern Finland.
   Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils, examples of which are burrows, footprints, etc.
   Section::::Fossilization processes.:Casts and molds.
  
  In some cases, the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an "external mold". If this hole is later filled with other minerals, it is a "cast". An endocast, or "internal mold", is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.
  
  Section::::Fossilization processes.:Authigenic mineralization.
   BULLET::::- Cockburn examines the rare and delicate fossils preserved in silica that were discovered and rescued during the construction of the Ford engine plant at Bridgend in Wales.
  
  BULLET::::- Prof. Michael Bassett of the National Museum of Wales explains some of the myth and folklore of fossils that were eventually dismissed with the development of the science of stratigraphy.
  
  Section::::Episodes.:Episode four: "North of England".
  
  BULLET::::- Cockburn abseils down the stratified rock layers of a cliff face on the Yorkshire coast to discover the fossilised footprints of Sauropod that walked across the sand or here during the Jurassic era.
   Section::::Stratigraphic dating.:Residual and intrusive finds.
   Since the early years of the twentieth century, absolute dating methods, such as radiometric dating (including potassium/argon, argon/argon, uranium series, and, for very recent fossils, radiocarbon dating) have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that the earliest known stromatolites are over 3.4 billion years old.
  
  Section::::History of the study of fossils.:Modern era.
   Boundaries and names are established by the International Commission on Stratigraphy (ICS) of the International Union of Geological Sciences. As of 2008, the ICS is nearly finished a task begun in 1974, subdividing the Phanerozoic eonothem into internationally accepted stages using two types of benchmark. For younger stages, a Global Boundary Stratotype Section and Point (GSSP), a physical outcrop clearly demonstrates the boundary. For older stages, a Global Standard Stratigraphic Age (GSSA) is an absolute date. The benchmarks will give a much greater certainty that results can be compared with confidence in the date determinations, and such results will have farther scope than any evaluation based solely on local knowledge and conditions. 
   There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystallization, adpression, carbonization, and bioimmuration.
   This process requires a considerable degree of effort and checking of field relationships and age dates. For instance, there may be many millions of years between a bed being laid down and an intrusive rock cutting it; the estimate of age must necessarily be between the oldest cross-cutting intrusive rock in the fossil assemblage and the youngest rock upon which the fossil assemblage rests.
  
  Section::::Units.
  
  Chronostratigraphic units, with examples:
  
  BULLET::::- eonothem – Phanerozoic
  
  BULLET::::- erathem – Paleozoic
  
  BULLET::::- system – Ordovician
  
  BULLET::::- series – Upper Ordovician
  
  BULLET::::- stage – Ashgill
  
  Section::::Differences from geochronology.
   BULLET::::- Complex patterns of cracks and mineral replacements like those of soil clods ("peds") and planar "cutans".
  
  Section::::Classification.
   Nicolaus Steno, also known as Niels Stensen, was the first to observe and propose some of the basic concepts of historical geology, called the "father of geology". One of these concepts was that fossils originally came from living organisms. The other, more famous, observations are often grouped together to form the laws of stratigraphy.
   A "transitional fossil" is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation.
  
  Section::::Types.:Microfossils.
  