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Since we cannot see either your pebble or the photo, we cannot answer the question for you.
I am not exactly sure but I have a good scientific guess. No because the crater is always bigger than the meteor so it starts forming a circle when it comes out because the pressure is so hard that it will tear the shape a part as it forms.
Four main causes: 1) Erosion due to weather, etc. 2) Concealment by sedimentary deposits. 3) Most of the Earth's surface is water. 4) Plate tectonics changes the Earth's surface. _________________________________ One other significant factor is that some of the impact craters are so enormous that we fail to recognize the typical "crater" shape until we can see it from space, or at least from high altitude. There's a new generation of meteor crater hunters who never leave their desks, who search Google Earth for the telltale signs of "ring" mountains . Of course, some meteor craters are as obvious as can be; the classic Barringer Meteor Crater near Winslow, Arizona, for example.
It has no particular shape. It is a rock
Breccia is made up of pebbles, boulders, cobbles or gravel that are angular in shape. Conglomerate is similar to Breccia but have a rounded shape instead of angular shape.
Since we cannot see either your pebble or the photo, we cannot answer the question for you.
Crater
This is known as an impact crater. Please see the related link.
Craters are depressions in a planet's or moon's surface caused when a meteor hits the surface. On the moon craters remain undisturbed because there are no environmental forces like wind and rain to disturb resulting shape of the crater.
Assuming the question refers to the crater of an astrobleme, as opposed to a volcanic crater or a man-made one, the mass, velocity and angle of the the impacting object.
There are a number of factors that determine the size and shape of a crater. The two most significant are the mass of the impactor, and the speed. Other influential factors would include the composition of the object (solid rock or more aggregate, like pebbles? Ice?) the shape of the object, and the composition of the impact site. If it is on dry land, it will be more likely to leave a visible crater, while a water or marshy impact site would be quickly erased. If a water impact, the depth of the water and the topography of the surrounding seas would be vastly important. A strike in a deep ocean basin might be relatively mild, while a water strike in the South China Sea or Gulf of Mexico, with the constricted water flow, might result in catastrophic tsunamis inundating the surrounding areas.
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